[Java][okhttp-gson] better support for additional properties (#11964)

* add support for additiional properties in okhttp-gson java client

* deserialize with additional properties

* better handling of non-primitive type

* use entry set instead

* support disallowAdditionalPropertiesIfNotPresent

* remove additional properties in serialization

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AdditionalPropertiesClass.java

@@ -40,6 +40,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -302,6 +303,42 @@ public class AdditionalPropertiesClass {
     this.mapWithUndeclaredPropertiesString = mapWithUndeclaredPropertiesString;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public AdditionalPropertiesClass putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -319,7 +356,8 @@ public class AdditionalPropertiesClass {
         Objects.equals(this.mapWithUndeclaredPropertiesAnytype2, additionalPropertiesClass.mapWithUndeclaredPropertiesAnytype2) &&
         Objects.equals(this.mapWithUndeclaredPropertiesAnytype3, additionalPropertiesClass.mapWithUndeclaredPropertiesAnytype3) &&
         Objects.equals(this.emptyMap, additionalPropertiesClass.emptyMap) &&
-        Objects.equals(this.mapWithUndeclaredPropertiesString, additionalPropertiesClass.mapWithUndeclaredPropertiesString);
+        Objects.equals(this.mapWithUndeclaredPropertiesString, additionalPropertiesClass.mapWithUndeclaredPropertiesString)&&
+        Objects.equals(this.additionalProperties, additionalPropertiesClass.additionalProperties);
   }
 
   private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b) {
@@ -328,7 +366,7 @@ public class AdditionalPropertiesClass {
 
   @Override
   public int hashCode() {
-    return Objects.hash(mapProperty, mapOfMapProperty, anytype1, mapWithUndeclaredPropertiesAnytype1, mapWithUndeclaredPropertiesAnytype2, mapWithUndeclaredPropertiesAnytype3, emptyMap, mapWithUndeclaredPropertiesString);
+    return Objects.hash(mapProperty, mapOfMapProperty, anytype1, mapWithUndeclaredPropertiesAnytype1, mapWithUndeclaredPropertiesAnytype2, mapWithUndeclaredPropertiesAnytype3, emptyMap, mapWithUndeclaredPropertiesString, additionalProperties);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
@@ -350,6 +388,7 @@ public class AdditionalPropertiesClass {
     sb.append("    mapWithUndeclaredPropertiesAnytype3: ").append(toIndentedString(mapWithUndeclaredPropertiesAnytype3)).append("\n");
     sb.append("    emptyMap: ").append(toIndentedString(emptyMap)).append("\n");
     sb.append("    mapWithUndeclaredPropertiesString: ").append(toIndentedString(mapWithUndeclaredPropertiesString)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -399,13 +438,6 @@ public class AdditionalPropertiesClass {
           throw new IllegalArgumentException(String.format("The required field(s) %s in AdditionalPropertiesClass is not found in the empty JSON string", AdditionalPropertiesClass.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!AdditionalPropertiesClass.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `AdditionalPropertiesClass` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -423,6 +455,23 @@ public class AdditionalPropertiesClass {
            @Override
            public void write(JsonWriter out, AdditionalPropertiesClass value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -430,7 +479,25 @@ public class AdditionalPropertiesClass {
            public AdditionalPropertiesClass read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             AdditionalPropertiesClass instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Animal.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -107,6 +108,42 @@ public class Animal {
     this.color = color;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Animal putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -118,12 +155,13 @@ public class Animal {
     }
     Animal animal = (Animal) o;
     return Objects.equals(this.className, animal.className) &&
-        Objects.equals(this.color, animal.color);
+        Objects.equals(this.color, animal.color)&&
+        Objects.equals(this.additionalProperties, animal.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(className, color);
+    return Objects.hash(className, color, additionalProperties);
   }
 
   @Override
@@ -132,6 +170,7 @@ public class Animal {
     sb.append("class Animal {\n");
     sb.append("    className: ").append(toIndentedString(className)).append("\n");
     sb.append("    color: ").append(toIndentedString(color)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Apple.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -104,6 +105,42 @@ public class Apple {
     this.origin = origin;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Apple putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -115,12 +152,13 @@ public class Apple {
     }
     Apple apple = (Apple) o;
     return Objects.equals(this.cultivar, apple.cultivar) &&
-        Objects.equals(this.origin, apple.origin);
+        Objects.equals(this.origin, apple.origin)&&
+        Objects.equals(this.additionalProperties, apple.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(cultivar, origin);
+    return Objects.hash(cultivar, origin, additionalProperties);
   }
 
   @Override
@@ -129,6 +167,7 @@ public class Apple {
     sb.append("class Apple {\n");
     sb.append("    cultivar: ").append(toIndentedString(cultivar)).append("\n");
     sb.append("    origin: ").append(toIndentedString(origin)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -172,13 +211,6 @@ public class Apple {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Apple is not found in the empty JSON string", Apple.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Apple.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Apple` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -196,6 +228,23 @@ public class Apple {
            @Override
            public void write(JsonWriter out, Apple value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -203,7 +252,25 @@ public class Apple {
            public Apple read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Apple instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AppleReq.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -105,6 +106,7 @@ public class AppleReq {
   }
 
 
+
   @Override
   public boolean equals(Object o) {
     if (this == o) {
@@ -173,6 +175,7 @@ public class AppleReq {
           throw new IllegalArgumentException(String.format("The required field(s) %s in AppleReq is not found in the empty JSON string", AppleReq.openapiRequiredFields.toString()));
         }
       }
+
       Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
       // check to see if the JSON string contains additional fields
       for (Entry<String, JsonElement> entry : entries) {

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayOfArrayOfNumberOnly.java

@@ -39,6 +39,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -88,6 +89,42 @@ public class ArrayOfArrayOfNumberOnly {
     this.arrayArrayNumber = arrayArrayNumber;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ArrayOfArrayOfNumberOnly putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -98,12 +135,13 @@ public class ArrayOfArrayOfNumberOnly {
       return false;
     }
     ArrayOfArrayOfNumberOnly arrayOfArrayOfNumberOnly = (ArrayOfArrayOfNumberOnly) o;
-    return Objects.equals(this.arrayArrayNumber, arrayOfArrayOfNumberOnly.arrayArrayNumber);
+    return Objects.equals(this.arrayArrayNumber, arrayOfArrayOfNumberOnly.arrayArrayNumber)&&
+        Objects.equals(this.additionalProperties, arrayOfArrayOfNumberOnly.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(arrayArrayNumber);
+    return Objects.hash(arrayArrayNumber, additionalProperties);
   }
 
   @Override
@@ -111,6 +149,7 @@ public class ArrayOfArrayOfNumberOnly {
     StringBuilder sb = new StringBuilder();
     sb.append("class ArrayOfArrayOfNumberOnly {\n");
     sb.append("    arrayArrayNumber: ").append(toIndentedString(arrayArrayNumber)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -153,13 +192,6 @@ public class ArrayOfArrayOfNumberOnly {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ArrayOfArrayOfNumberOnly is not found in the empty JSON string", ArrayOfArrayOfNumberOnly.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ArrayOfArrayOfNumberOnly.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ArrayOfArrayOfNumberOnly` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -177,6 +209,23 @@ public class ArrayOfArrayOfNumberOnly {
            @Override
            public void write(JsonWriter out, ArrayOfArrayOfNumberOnly value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -184,7 +233,25 @@ public class ArrayOfArrayOfNumberOnly {
            public ArrayOfArrayOfNumberOnly read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ArrayOfArrayOfNumberOnly instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayOfNumberOnly.java

@@ -39,6 +39,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -88,6 +89,42 @@ public class ArrayOfNumberOnly {
     this.arrayNumber = arrayNumber;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ArrayOfNumberOnly putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -98,12 +135,13 @@ public class ArrayOfNumberOnly {
       return false;
     }
     ArrayOfNumberOnly arrayOfNumberOnly = (ArrayOfNumberOnly) o;
-    return Objects.equals(this.arrayNumber, arrayOfNumberOnly.arrayNumber);
+    return Objects.equals(this.arrayNumber, arrayOfNumberOnly.arrayNumber)&&
+        Objects.equals(this.additionalProperties, arrayOfNumberOnly.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(arrayNumber);
+    return Objects.hash(arrayNumber, additionalProperties);
   }
 
   @Override
@@ -111,6 +149,7 @@ public class ArrayOfNumberOnly {
     StringBuilder sb = new StringBuilder();
     sb.append("class ArrayOfNumberOnly {\n");
     sb.append("    arrayNumber: ").append(toIndentedString(arrayNumber)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -153,13 +192,6 @@ public class ArrayOfNumberOnly {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ArrayOfNumberOnly is not found in the empty JSON string", ArrayOfNumberOnly.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ArrayOfNumberOnly.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ArrayOfNumberOnly` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -177,6 +209,23 @@ public class ArrayOfNumberOnly {
            @Override
            public void write(JsonWriter out, ArrayOfNumberOnly value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -184,7 +233,25 @@ public class ArrayOfNumberOnly {
            public ArrayOfNumberOnly read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ArrayOfNumberOnly instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayTest.java

@@ -39,6 +39,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -158,6 +159,42 @@ public class ArrayTest {
     this.arrayArrayOfModel = arrayArrayOfModel;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ArrayTest putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -170,12 +207,13 @@ public class ArrayTest {
     ArrayTest arrayTest = (ArrayTest) o;
     return Objects.equals(this.arrayOfString, arrayTest.arrayOfString) &&
         Objects.equals(this.arrayArrayOfInteger, arrayTest.arrayArrayOfInteger) &&
-        Objects.equals(this.arrayArrayOfModel, arrayTest.arrayArrayOfModel);
+        Objects.equals(this.arrayArrayOfModel, arrayTest.arrayArrayOfModel)&&
+        Objects.equals(this.additionalProperties, arrayTest.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(arrayOfString, arrayArrayOfInteger, arrayArrayOfModel);
+    return Objects.hash(arrayOfString, arrayArrayOfInteger, arrayArrayOfModel, additionalProperties);
   }
 
   @Override
@@ -185,6 +223,7 @@ public class ArrayTest {
     sb.append("    arrayOfString: ").append(toIndentedString(arrayOfString)).append("\n");
     sb.append("    arrayArrayOfInteger: ").append(toIndentedString(arrayArrayOfInteger)).append("\n");
     sb.append("    arrayArrayOfModel: ").append(toIndentedString(arrayArrayOfModel)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -229,13 +268,6 @@ public class ArrayTest {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ArrayTest is not found in the empty JSON string", ArrayTest.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ArrayTest.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ArrayTest` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -253,6 +285,23 @@ public class ArrayTest {
            @Override
            public void write(JsonWriter out, ArrayTest value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -260,7 +309,25 @@ public class ArrayTest {
            public ArrayTest read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ArrayTest instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Banana.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -78,6 +79,42 @@ public class Banana {
     this.lengthCm = lengthCm;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Banana putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -88,12 +125,13 @@ public class Banana {
       return false;
     }
     Banana banana = (Banana) o;
-    return Objects.equals(this.lengthCm, banana.lengthCm);
+    return Objects.equals(this.lengthCm, banana.lengthCm)&&
+        Objects.equals(this.additionalProperties, banana.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(lengthCm);
+    return Objects.hash(lengthCm, additionalProperties);
   }
 
   @Override
@@ -101,6 +139,7 @@ public class Banana {
     StringBuilder sb = new StringBuilder();
     sb.append("class Banana {\n");
     sb.append("    lengthCm: ").append(toIndentedString(lengthCm)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class Banana {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Banana is not found in the empty JSON string", Banana.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Banana.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Banana` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -167,6 +199,23 @@ public class Banana {
            @Override
            public void write(JsonWriter out, Banana value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -174,7 +223,25 @@ public class Banana {
            public Banana read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Banana instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/BananaReq.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -106,6 +107,7 @@ public class BananaReq {
   }
 
 
+
   @Override
   public boolean equals(Object o) {
     if (this == o) {
@@ -174,6 +176,7 @@ public class BananaReq {
           throw new IllegalArgumentException(String.format("The required field(s) %s in BananaReq is not found in the empty JSON string", BananaReq.openapiRequiredFields.toString()));
         }
       }
+
       Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
       // check to see if the JSON string contains additional fields
       for (Entry<String, JsonElement> entry : entries) {

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/BasquePig.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class BasquePig {
     this.className = className;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public BasquePig putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class BasquePig {
       return false;
     }
     BasquePig basquePig = (BasquePig) o;
-    return Objects.equals(this.className, basquePig.className);
+    return Objects.equals(this.className, basquePig.className)&&
+        Objects.equals(this.additionalProperties, basquePig.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(className);
+    return Objects.hash(className, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class BasquePig {
     StringBuilder sb = new StringBuilder();
     sb.append("class BasquePig {\n");
     sb.append("    className: ").append(toIndentedString(className)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class BasquePig {
           throw new IllegalArgumentException(String.format("The required field(s) %s in BasquePig is not found in the empty JSON string", BasquePig.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!BasquePig.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `BasquePig` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : BasquePig.openapiRequiredFields) {
@@ -174,6 +206,23 @@ public class BasquePig {
            @Override
            public void write(JsonWriter out, BasquePig value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -181,7 +230,25 @@ public class BasquePig {
            public BasquePig read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             BasquePig instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Capitalization.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -212,6 +213,42 @@ public class Capitalization {
     this.ATT_NAME = ATT_NAME;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Capitalization putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -227,12 +264,13 @@ public class Capitalization {
         Objects.equals(this.smallSnake, capitalization.smallSnake) &&
         Objects.equals(this.capitalSnake, capitalization.capitalSnake) &&
         Objects.equals(this.scAETHFlowPoints, capitalization.scAETHFlowPoints) &&
-        Objects.equals(this.ATT_NAME, capitalization.ATT_NAME);
+        Objects.equals(this.ATT_NAME, capitalization.ATT_NAME)&&
+        Objects.equals(this.additionalProperties, capitalization.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(smallCamel, capitalCamel, smallSnake, capitalSnake, scAETHFlowPoints, ATT_NAME);
+    return Objects.hash(smallCamel, capitalCamel, smallSnake, capitalSnake, scAETHFlowPoints, ATT_NAME, additionalProperties);
   }
 
   @Override
@@ -245,6 +283,7 @@ public class Capitalization {
     sb.append("    capitalSnake: ").append(toIndentedString(capitalSnake)).append("\n");
     sb.append("    scAETHFlowPoints: ").append(toIndentedString(scAETHFlowPoints)).append("\n");
     sb.append("    ATT_NAME: ").append(toIndentedString(ATT_NAME)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -292,13 +331,6 @@ public class Capitalization {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Capitalization is not found in the empty JSON string", Capitalization.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Capitalization.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Capitalization` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -316,6 +348,23 @@ public class Capitalization {
            @Override
            public void write(JsonWriter out, Capitalization value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -323,7 +372,25 @@ public class Capitalization {
            public Capitalization read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Capitalization instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Cat.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -80,6 +81,42 @@ public class Cat extends Animal {
     this.declawed = declawed;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Cat putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -90,13 +127,14 @@ public class Cat extends Animal {
       return false;
     }
     Cat cat = (Cat) o;
-    return Objects.equals(this.declawed, cat.declawed) &&
+    return Objects.equals(this.declawed, cat.declawed)&&
+        Objects.equals(this.additionalProperties, cat.additionalProperties) &&
         super.equals(o);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(declawed, super.hashCode());
+    return Objects.hash(declawed, super.hashCode(), additionalProperties);
   }
 
   @Override
@@ -105,6 +143,7 @@ public class Cat extends Animal {
     sb.append("class Cat {\n");
     sb.append("    ").append(toIndentedString(super.toString())).append("\n");
     sb.append("    declawed: ").append(toIndentedString(declawed)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -150,13 +189,6 @@ public class Cat extends Animal {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Cat is not found in the empty JSON string", Cat.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Cat.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Cat` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : Cat.openapiRequiredFields) {
@@ -181,6 +213,23 @@ public class Cat extends Animal {
            @Override
            public void write(JsonWriter out, Cat value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -188,7 +237,25 @@ public class Cat extends Animal {
            public Cat read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Cat instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/CatAllOf.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class CatAllOf {
     this.declawed = declawed;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public CatAllOf putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class CatAllOf {
       return false;
     }
     CatAllOf catAllOf = (CatAllOf) o;
-    return Objects.equals(this.declawed, catAllOf.declawed);
+    return Objects.equals(this.declawed, catAllOf.declawed)&&
+        Objects.equals(this.additionalProperties, catAllOf.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(declawed);
+    return Objects.hash(declawed, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class CatAllOf {
     StringBuilder sb = new StringBuilder();
     sb.append("class CatAllOf {\n");
     sb.append("    declawed: ").append(toIndentedString(declawed)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -142,13 +181,6 @@ public class CatAllOf {
           throw new IllegalArgumentException(String.format("The required field(s) %s in CatAllOf is not found in the empty JSON string", CatAllOf.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!CatAllOf.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `CatAllOf` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -166,6 +198,23 @@ public class CatAllOf {
            @Override
            public void write(JsonWriter out, CatAllOf value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -173,7 +222,25 @@ public class CatAllOf {
            public CatAllOf read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             CatAllOf instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Category.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -104,6 +105,42 @@ public class Category {
     this.name = name;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Category putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -115,12 +152,13 @@ public class Category {
     }
     Category category = (Category) o;
     return Objects.equals(this.id, category.id) &&
-        Objects.equals(this.name, category.name);
+        Objects.equals(this.name, category.name)&&
+        Objects.equals(this.additionalProperties, category.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(id, name);
+    return Objects.hash(id, name, additionalProperties);
   }
 
   @Override
@@ -129,6 +167,7 @@ public class Category {
     sb.append("class Category {\n");
     sb.append("    id: ").append(toIndentedString(id)).append("\n");
     sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -173,13 +212,6 @@ public class Category {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Category is not found in the empty JSON string", Category.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Category.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Category` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : Category.openapiRequiredFields) {
@@ -204,6 +236,23 @@ public class Category {
            @Override
            public void write(JsonWriter out, Category value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -211,7 +260,25 @@ public class Category {
            public Category read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Category instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ClassModel.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -78,6 +79,42 @@ public class ClassModel {
     this.propertyClass = propertyClass;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ClassModel putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -88,12 +125,13 @@ public class ClassModel {
       return false;
     }
     ClassModel classModel = (ClassModel) o;
-    return Objects.equals(this.propertyClass, classModel.propertyClass);
+    return Objects.equals(this.propertyClass, classModel.propertyClass)&&
+        Objects.equals(this.additionalProperties, classModel.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(propertyClass);
+    return Objects.hash(propertyClass, additionalProperties);
   }
 
   @Override
@@ -101,6 +139,7 @@ public class ClassModel {
     StringBuilder sb = new StringBuilder();
     sb.append("class ClassModel {\n");
     sb.append("    propertyClass: ").append(toIndentedString(propertyClass)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class ClassModel {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ClassModel is not found in the empty JSON string", ClassModel.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ClassModel.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ClassModel` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -167,6 +199,23 @@ public class ClassModel {
            @Override
            public void write(JsonWriter out, ClassModel value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -174,7 +223,25 @@ public class ClassModel {
            public ClassModel read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ClassModel instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Client.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class Client {
     this.client = client;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Client putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class Client {
       return false;
     }
     Client client = (Client) o;
-    return Objects.equals(this.client, client.client);
+    return Objects.equals(this.client, client.client)&&
+        Objects.equals(this.additionalProperties, client.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(client);
+    return Objects.hash(client, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class Client {
     StringBuilder sb = new StringBuilder();
     sb.append("class Client {\n");
     sb.append("    client: ").append(toIndentedString(client)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -142,13 +181,6 @@ public class Client {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Client is not found in the empty JSON string", Client.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Client.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Client` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -166,6 +198,23 @@ public class Client {
            @Override
            public void write(JsonWriter out, Client value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -173,7 +222,25 @@ public class Client {
            public Client read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Client instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ComplexQuadrilateral.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -106,6 +107,42 @@ public class ComplexQuadrilateral {
     this.quadrilateralType = quadrilateralType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ComplexQuadrilateral putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -117,12 +154,13 @@ public class ComplexQuadrilateral {
     }
     ComplexQuadrilateral complexQuadrilateral = (ComplexQuadrilateral) o;
     return Objects.equals(this.shapeType, complexQuadrilateral.shapeType) &&
-        Objects.equals(this.quadrilateralType, complexQuadrilateral.quadrilateralType);
+        Objects.equals(this.quadrilateralType, complexQuadrilateral.quadrilateralType)&&
+        Objects.equals(this.additionalProperties, complexQuadrilateral.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(shapeType, quadrilateralType);
+    return Objects.hash(shapeType, quadrilateralType, additionalProperties);
   }
 
   @Override
@@ -131,6 +169,7 @@ public class ComplexQuadrilateral {
     sb.append("class ComplexQuadrilateral {\n");
     sb.append("    shapeType: ").append(toIndentedString(shapeType)).append("\n");
     sb.append("    quadrilateralType: ").append(toIndentedString(quadrilateralType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -176,13 +215,6 @@ public class ComplexQuadrilateral {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ComplexQuadrilateral is not found in the empty JSON string", ComplexQuadrilateral.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ComplexQuadrilateral.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ComplexQuadrilateral` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : ComplexQuadrilateral.openapiRequiredFields) {
@@ -207,6 +239,23 @@ public class ComplexQuadrilateral {
            @Override
            public void write(JsonWriter out, ComplexQuadrilateral value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -214,7 +263,25 @@ public class ComplexQuadrilateral {
            public ComplexQuadrilateral read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ComplexQuadrilateral instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/DanishPig.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class DanishPig {
     this.className = className;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public DanishPig putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class DanishPig {
       return false;
     }
     DanishPig danishPig = (DanishPig) o;
-    return Objects.equals(this.className, danishPig.className);
+    return Objects.equals(this.className, danishPig.className)&&
+        Objects.equals(this.additionalProperties, danishPig.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(className);
+    return Objects.hash(className, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class DanishPig {
     StringBuilder sb = new StringBuilder();
     sb.append("class DanishPig {\n");
     sb.append("    className: ").append(toIndentedString(className)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class DanishPig {
           throw new IllegalArgumentException(String.format("The required field(s) %s in DanishPig is not found in the empty JSON string", DanishPig.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!DanishPig.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `DanishPig` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : DanishPig.openapiRequiredFields) {
@@ -174,6 +206,23 @@ public class DanishPig {
            @Override
            public void write(JsonWriter out, DanishPig value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -181,7 +230,25 @@ public class DanishPig {
            public DanishPig read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             DanishPig instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/DeprecatedObject.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -79,6 +80,42 @@ public class DeprecatedObject {
     this.name = name;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public DeprecatedObject putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -89,12 +126,13 @@ public class DeprecatedObject {
       return false;
     }
     DeprecatedObject deprecatedObject = (DeprecatedObject) o;
-    return Objects.equals(this.name, deprecatedObject.name);
+    return Objects.equals(this.name, deprecatedObject.name)&&
+        Objects.equals(this.additionalProperties, deprecatedObject.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(name);
+    return Objects.hash(name, additionalProperties);
   }
 
   @Override
@@ -102,6 +140,7 @@ public class DeprecatedObject {
     StringBuilder sb = new StringBuilder();
     sb.append("class DeprecatedObject {\n");
     sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -144,13 +183,6 @@ public class DeprecatedObject {
           throw new IllegalArgumentException(String.format("The required field(s) %s in DeprecatedObject is not found in the empty JSON string", DeprecatedObject.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!DeprecatedObject.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `DeprecatedObject` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -168,6 +200,23 @@ public class DeprecatedObject {
            @Override
            public void write(JsonWriter out, DeprecatedObject value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -175,7 +224,25 @@ public class DeprecatedObject {
            public DeprecatedObject read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             DeprecatedObject instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Dog.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -80,6 +81,42 @@ public class Dog extends Animal {
     this.breed = breed;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Dog putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -90,13 +127,14 @@ public class Dog extends Animal {
       return false;
     }
     Dog dog = (Dog) o;
-    return Objects.equals(this.breed, dog.breed) &&
+    return Objects.equals(this.breed, dog.breed)&&
+        Objects.equals(this.additionalProperties, dog.additionalProperties) &&
         super.equals(o);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(breed, super.hashCode());
+    return Objects.hash(breed, super.hashCode(), additionalProperties);
   }
 
   @Override
@@ -105,6 +143,7 @@ public class Dog extends Animal {
     sb.append("class Dog {\n");
     sb.append("    ").append(toIndentedString(super.toString())).append("\n");
     sb.append("    breed: ").append(toIndentedString(breed)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -150,13 +189,6 @@ public class Dog extends Animal {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Dog is not found in the empty JSON string", Dog.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Dog.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Dog` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : Dog.openapiRequiredFields) {
@@ -181,6 +213,23 @@ public class Dog extends Animal {
            @Override
            public void write(JsonWriter out, Dog value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -188,7 +237,25 @@ public class Dog extends Animal {
            public Dog read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Dog instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/DogAllOf.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class DogAllOf {
     this.breed = breed;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public DogAllOf putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class DogAllOf {
       return false;
     }
     DogAllOf dogAllOf = (DogAllOf) o;
-    return Objects.equals(this.breed, dogAllOf.breed);
+    return Objects.equals(this.breed, dogAllOf.breed)&&
+        Objects.equals(this.additionalProperties, dogAllOf.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(breed);
+    return Objects.hash(breed, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class DogAllOf {
     StringBuilder sb = new StringBuilder();
     sb.append("class DogAllOf {\n");
     sb.append("    breed: ").append(toIndentedString(breed)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -142,13 +181,6 @@ public class DogAllOf {
           throw new IllegalArgumentException(String.format("The required field(s) %s in DogAllOf is not found in the empty JSON string", DogAllOf.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!DogAllOf.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `DogAllOf` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -166,6 +198,23 @@ public class DogAllOf {
            @Override
            public void write(JsonWriter out, DogAllOf value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -173,7 +222,25 @@ public class DogAllOf {
            public DogAllOf read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             DogAllOf instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Drawing.java

@@ -24,9 +24,7 @@ import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
 import org.openapitools.client.model.Fruit;
 import org.openapitools.client.model.NullableShape;
 import org.openapitools.client.model.Shape;
@@ -45,6 +43,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -56,7 +55,7 @@ import org.openapitools.client.JSON;
  * Drawing
  */
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
-public class Drawing extends HashMap<String, Fruit> {
+public class Drawing {
   public static final String SERIALIZED_NAME_MAIN_SHAPE = "mainShape";
   @SerializedName(SERIALIZED_NAME_MAIN_SHAPE)
   private Shape mainShape;
@@ -176,6 +175,7 @@ public class Drawing extends HashMap<String, Fruit> {
   }
 
 
+
   @Override
   public boolean equals(Object o) {
     if (this == o) {
@@ -188,8 +188,7 @@ public class Drawing extends HashMap<String, Fruit> {
     return Objects.equals(this.mainShape, drawing.mainShape) &&
         Objects.equals(this.shapeOrNull, drawing.shapeOrNull) &&
         Objects.equals(this.nullableShape, drawing.nullableShape) &&
-        Objects.equals(this.shapes, drawing.shapes) &&
-        super.equals(o);
+        Objects.equals(this.shapes, drawing.shapes);
   }
 
   private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b) {
@@ -198,7 +197,7 @@ public class Drawing extends HashMap<String, Fruit> {
 
   @Override
   public int hashCode() {
-    return Objects.hash(mainShape, shapeOrNull, nullableShape, shapes, super.hashCode());
+    return Objects.hash(mainShape, shapeOrNull, nullableShape, shapes);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
@@ -212,7 +211,6 @@ public class Drawing extends HashMap<String, Fruit> {
   public String toString() {
     StringBuilder sb = new StringBuilder();
     sb.append("class Drawing {\n");
-    sb.append("    ").append(toIndentedString(super.toString())).append("\n");
     sb.append("    mainShape: ").append(toIndentedString(mainShape)).append("\n");
     sb.append("    shapeOrNull: ").append(toIndentedString(shapeOrNull)).append("\n");
     sb.append("    nullableShape: ").append(toIndentedString(nullableShape)).append("\n");
@@ -262,6 +260,7 @@ public class Drawing extends HashMap<String, Fruit> {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Drawing is not found in the empty JSON string", Drawing.openapiRequiredFields.toString()));
         }
       }
+
       Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
       // check to see if the JSON string contains additional fields
       for (Entry<String, JsonElement> entry : entries) {

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/EnumArrays.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -208,6 +209,42 @@ public class EnumArrays {
     this.arrayEnum = arrayEnum;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public EnumArrays putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -219,12 +256,13 @@ public class EnumArrays {
     }
     EnumArrays enumArrays = (EnumArrays) o;
     return Objects.equals(this.justSymbol, enumArrays.justSymbol) &&
-        Objects.equals(this.arrayEnum, enumArrays.arrayEnum);
+        Objects.equals(this.arrayEnum, enumArrays.arrayEnum)&&
+        Objects.equals(this.additionalProperties, enumArrays.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(justSymbol, arrayEnum);
+    return Objects.hash(justSymbol, arrayEnum, additionalProperties);
   }
 
   @Override
@@ -233,6 +271,7 @@ public class EnumArrays {
     sb.append("class EnumArrays {\n");
     sb.append("    justSymbol: ").append(toIndentedString(justSymbol)).append("\n");
     sb.append("    arrayEnum: ").append(toIndentedString(arrayEnum)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -276,13 +315,6 @@ public class EnumArrays {
           throw new IllegalArgumentException(String.format("The required field(s) %s in EnumArrays is not found in the empty JSON string", EnumArrays.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!EnumArrays.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `EnumArrays` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -300,6 +332,23 @@ public class EnumArrays {
            @Override
            public void write(JsonWriter out, EnumArrays value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -307,7 +356,25 @@ public class EnumArrays {
            public EnumArrays read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             EnumArrays instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/EnumTest.java

@@ -41,6 +41,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -537,6 +538,42 @@ public class EnumTest {
     this.outerEnumIntegerDefaultValue = outerEnumIntegerDefaultValue;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public EnumTest putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -555,7 +592,8 @@ public class EnumTest {
         Objects.equals(this.outerEnum, enumTest.outerEnum) &&
         Objects.equals(this.outerEnumInteger, enumTest.outerEnumInteger) &&
         Objects.equals(this.outerEnumDefaultValue, enumTest.outerEnumDefaultValue) &&
-        Objects.equals(this.outerEnumIntegerDefaultValue, enumTest.outerEnumIntegerDefaultValue);
+        Objects.equals(this.outerEnumIntegerDefaultValue, enumTest.outerEnumIntegerDefaultValue)&&
+        Objects.equals(this.additionalProperties, enumTest.additionalProperties);
   }
 
   private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b) {
@@ -564,7 +602,7 @@ public class EnumTest {
 
   @Override
   public int hashCode() {
-    return Objects.hash(enumString, enumStringRequired, enumInteger, enumIntegerOnly, enumNumber, outerEnum, outerEnumInteger, outerEnumDefaultValue, outerEnumIntegerDefaultValue);
+    return Objects.hash(enumString, enumStringRequired, enumInteger, enumIntegerOnly, enumNumber, outerEnum, outerEnumInteger, outerEnumDefaultValue, outerEnumIntegerDefaultValue, additionalProperties);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
@@ -587,6 +625,7 @@ public class EnumTest {
     sb.append("    outerEnumInteger: ").append(toIndentedString(outerEnumInteger)).append("\n");
     sb.append("    outerEnumDefaultValue: ").append(toIndentedString(outerEnumDefaultValue)).append("\n");
     sb.append("    outerEnumIntegerDefaultValue: ").append(toIndentedString(outerEnumIntegerDefaultValue)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -638,13 +677,6 @@ public class EnumTest {
           throw new IllegalArgumentException(String.format("The required field(s) %s in EnumTest is not found in the empty JSON string", EnumTest.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!EnumTest.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `EnumTest` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : EnumTest.openapiRequiredFields) {
@@ -669,6 +701,23 @@ public class EnumTest {
            @Override
            public void write(JsonWriter out, EnumTest value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -676,7 +725,25 @@ public class EnumTest {
            public EnumTest read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             EnumTest instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/EquilateralTriangle.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -106,6 +107,42 @@ public class EquilateralTriangle {
     this.triangleType = triangleType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public EquilateralTriangle putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -117,12 +154,13 @@ public class EquilateralTriangle {
     }
     EquilateralTriangle equilateralTriangle = (EquilateralTriangle) o;
     return Objects.equals(this.shapeType, equilateralTriangle.shapeType) &&
-        Objects.equals(this.triangleType, equilateralTriangle.triangleType);
+        Objects.equals(this.triangleType, equilateralTriangle.triangleType)&&
+        Objects.equals(this.additionalProperties, equilateralTriangle.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(shapeType, triangleType);
+    return Objects.hash(shapeType, triangleType, additionalProperties);
   }
 
   @Override
@@ -131,6 +169,7 @@ public class EquilateralTriangle {
     sb.append("class EquilateralTriangle {\n");
     sb.append("    shapeType: ").append(toIndentedString(shapeType)).append("\n");
     sb.append("    triangleType: ").append(toIndentedString(triangleType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -176,13 +215,6 @@ public class EquilateralTriangle {
           throw new IllegalArgumentException(String.format("The required field(s) %s in EquilateralTriangle is not found in the empty JSON string", EquilateralTriangle.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!EquilateralTriangle.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `EquilateralTriangle` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : EquilateralTriangle.openapiRequiredFields) {
@@ -207,6 +239,23 @@ public class EquilateralTriangle {
            @Override
            public void write(JsonWriter out, EquilateralTriangle value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -214,7 +263,25 @@ public class EquilateralTriangle {
            public EquilateralTriangle read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             EquilateralTriangle instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FileSchemaTestClass.java

@@ -39,6 +39,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -115,6 +116,42 @@ public class FileSchemaTestClass {
     this.files = files;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public FileSchemaTestClass putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -126,12 +163,13 @@ public class FileSchemaTestClass {
     }
     FileSchemaTestClass fileSchemaTestClass = (FileSchemaTestClass) o;
     return Objects.equals(this._file, fileSchemaTestClass._file) &&
-        Objects.equals(this.files, fileSchemaTestClass.files);
+        Objects.equals(this.files, fileSchemaTestClass.files)&&
+        Objects.equals(this.additionalProperties, fileSchemaTestClass.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(_file, files);
+    return Objects.hash(_file, files, additionalProperties);
   }
 
   @Override
@@ -140,6 +178,7 @@ public class FileSchemaTestClass {
     sb.append("class FileSchemaTestClass {\n");
     sb.append("    _file: ").append(toIndentedString(_file)).append("\n");
     sb.append("    files: ").append(toIndentedString(files)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -183,13 +222,6 @@ public class FileSchemaTestClass {
           throw new IllegalArgumentException(String.format("The required field(s) %s in FileSchemaTestClass is not found in the empty JSON string", FileSchemaTestClass.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!FileSchemaTestClass.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `FileSchemaTestClass` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
       // validate the optional field `file`
       if (jsonObj.getAsJsonObject("file") != null) {
         ModelFile.validateJsonObject(jsonObj.getAsJsonObject("file"));
@@ -218,6 +250,23 @@ public class FileSchemaTestClass {
            @Override
            public void write(JsonWriter out, FileSchemaTestClass value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -225,7 +274,25 @@ public class FileSchemaTestClass {
            public FileSchemaTestClass read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             FileSchemaTestClass instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Foo.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class Foo {
     this.bar = bar;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Foo putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class Foo {
       return false;
     }
     Foo foo = (Foo) o;
-    return Objects.equals(this.bar, foo.bar);
+    return Objects.equals(this.bar, foo.bar)&&
+        Objects.equals(this.additionalProperties, foo.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(bar);
+    return Objects.hash(bar, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class Foo {
     StringBuilder sb = new StringBuilder();
     sb.append("class Foo {\n");
     sb.append("    bar: ").append(toIndentedString(bar)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -142,13 +181,6 @@ public class Foo {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Foo is not found in the empty JSON string", Foo.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Foo.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Foo` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -166,6 +198,23 @@ public class Foo {
            @Override
            public void write(JsonWriter out, Foo value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -173,7 +222,25 @@ public class Foo {
            public Foo read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Foo instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FormatTest.java

@@ -41,6 +41,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -497,6 +498,42 @@ public class FormatTest {
     this.patternWithDigitsAndDelimiter = patternWithDigitsAndDelimiter;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public FormatTest putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -522,12 +559,13 @@ public class FormatTest {
         Objects.equals(this.uuid, formatTest.uuid) &&
         Objects.equals(this.password, formatTest.password) &&
         Objects.equals(this.patternWithDigits, formatTest.patternWithDigits) &&
-        Objects.equals(this.patternWithDigitsAndDelimiter, formatTest.patternWithDigitsAndDelimiter);
+        Objects.equals(this.patternWithDigitsAndDelimiter, formatTest.patternWithDigitsAndDelimiter)&&
+        Objects.equals(this.additionalProperties, formatTest.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(integer, int32, int64, number, _float, _double, decimal, string, Arrays.hashCode(_byte), binary, date, dateTime, uuid, password, patternWithDigits, patternWithDigitsAndDelimiter);
+    return Objects.hash(integer, int32, int64, number, _float, _double, decimal, string, Arrays.hashCode(_byte), binary, date, dateTime, uuid, password, patternWithDigits, patternWithDigitsAndDelimiter, additionalProperties);
   }
 
   @Override
@@ -550,6 +588,7 @@ public class FormatTest {
     sb.append("    password: ").append(toIndentedString(password)).append("\n");
     sb.append("    patternWithDigits: ").append(toIndentedString(patternWithDigits)).append("\n");
     sb.append("    patternWithDigitsAndDelimiter: ").append(toIndentedString(patternWithDigitsAndDelimiter)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -611,13 +650,6 @@ public class FormatTest {
           throw new IllegalArgumentException(String.format("The required field(s) %s in FormatTest is not found in the empty JSON string", FormatTest.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!FormatTest.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `FormatTest` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : FormatTest.openapiRequiredFields) {
@@ -642,6 +674,23 @@ public class FormatTest {
            @Override
            public void write(JsonWriter out, FormatTest value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -649,7 +698,25 @@ public class FormatTest {
            public FormatTest read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             FormatTest instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/GrandparentAnimal.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -79,6 +80,42 @@ public class GrandparentAnimal {
     this.petType = petType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public GrandparentAnimal putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -89,12 +126,13 @@ public class GrandparentAnimal {
       return false;
     }
     GrandparentAnimal grandparentAnimal = (GrandparentAnimal) o;
-    return Objects.equals(this.petType, grandparentAnimal.petType);
+    return Objects.equals(this.petType, grandparentAnimal.petType)&&
+        Objects.equals(this.additionalProperties, grandparentAnimal.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(petType);
+    return Objects.hash(petType, additionalProperties);
   }
 
   @Override
@@ -102,6 +140,7 @@ public class GrandparentAnimal {
     StringBuilder sb = new StringBuilder();
     sb.append("class GrandparentAnimal {\n");
     sb.append("    petType: ").append(toIndentedString(petType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/HasOnlyReadOnly.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -96,6 +97,42 @@ public class HasOnlyReadOnly {
 
 
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public HasOnlyReadOnly putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -107,12 +144,13 @@ public class HasOnlyReadOnly {
     }
     HasOnlyReadOnly hasOnlyReadOnly = (HasOnlyReadOnly) o;
     return Objects.equals(this.bar, hasOnlyReadOnly.bar) &&
-        Objects.equals(this.foo, hasOnlyReadOnly.foo);
+        Objects.equals(this.foo, hasOnlyReadOnly.foo)&&
+        Objects.equals(this.additionalProperties, hasOnlyReadOnly.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(bar, foo);
+    return Objects.hash(bar, foo, additionalProperties);
   }
 
   @Override
@@ -121,6 +159,7 @@ public class HasOnlyReadOnly {
     sb.append("class HasOnlyReadOnly {\n");
     sb.append("    bar: ").append(toIndentedString(bar)).append("\n");
     sb.append("    foo: ").append(toIndentedString(foo)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -164,13 +203,6 @@ public class HasOnlyReadOnly {
           throw new IllegalArgumentException(String.format("The required field(s) %s in HasOnlyReadOnly is not found in the empty JSON string", HasOnlyReadOnly.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!HasOnlyReadOnly.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `HasOnlyReadOnly` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -188,6 +220,23 @@ public class HasOnlyReadOnly {
            @Override
            public void write(JsonWriter out, HasOnlyReadOnly value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -195,7 +244,25 @@ public class HasOnlyReadOnly {
            public HasOnlyReadOnly read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             HasOnlyReadOnly instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/HealthCheckResult.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -79,6 +80,42 @@ public class HealthCheckResult {
     this.nullableMessage = nullableMessage;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public HealthCheckResult putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -89,7 +126,8 @@ public class HealthCheckResult {
       return false;
     }
     HealthCheckResult healthCheckResult = (HealthCheckResult) o;
-    return Objects.equals(this.nullableMessage, healthCheckResult.nullableMessage);
+    return Objects.equals(this.nullableMessage, healthCheckResult.nullableMessage)&&
+        Objects.equals(this.additionalProperties, healthCheckResult.additionalProperties);
   }
 
   private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b) {
@@ -98,7 +136,7 @@ public class HealthCheckResult {
 
   @Override
   public int hashCode() {
-    return Objects.hash(nullableMessage);
+    return Objects.hash(nullableMessage, additionalProperties);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
@@ -113,6 +151,7 @@ public class HealthCheckResult {
     StringBuilder sb = new StringBuilder();
     sb.append("class HealthCheckResult {\n");
     sb.append("    nullableMessage: ").append(toIndentedString(nullableMessage)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -155,13 +194,6 @@ public class HealthCheckResult {
           throw new IllegalArgumentException(String.format("The required field(s) %s in HealthCheckResult is not found in the empty JSON string", HealthCheckResult.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!HealthCheckResult.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `HealthCheckResult` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -179,6 +211,23 @@ public class HealthCheckResult {
            @Override
            public void write(JsonWriter out, HealthCheckResult value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -186,7 +235,25 @@ public class HealthCheckResult {
            public HealthCheckResult read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             HealthCheckResult instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/InlineResponseDefault.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -78,6 +79,42 @@ public class InlineResponseDefault {
     this.string = string;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public InlineResponseDefault putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -88,12 +125,13 @@ public class InlineResponseDefault {
       return false;
     }
     InlineResponseDefault inlineResponseDefault = (InlineResponseDefault) o;
-    return Objects.equals(this.string, inlineResponseDefault.string);
+    return Objects.equals(this.string, inlineResponseDefault.string)&&
+        Objects.equals(this.additionalProperties, inlineResponseDefault.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(string);
+    return Objects.hash(string, additionalProperties);
   }
 
   @Override
@@ -101,6 +139,7 @@ public class InlineResponseDefault {
     StringBuilder sb = new StringBuilder();
     sb.append("class InlineResponseDefault {\n");
     sb.append("    string: ").append(toIndentedString(string)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class InlineResponseDefault {
           throw new IllegalArgumentException(String.format("The required field(s) %s in InlineResponseDefault is not found in the empty JSON string", InlineResponseDefault.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!InlineResponseDefault.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `InlineResponseDefault` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
       // validate the optional field `string`
       if (jsonObj.getAsJsonObject("string") != null) {
         Foo.validateJsonObject(jsonObj.getAsJsonObject("string"));
@@ -171,6 +203,23 @@ public class InlineResponseDefault {
            @Override
            public void write(JsonWriter out, InlineResponseDefault value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -178,7 +227,25 @@ public class InlineResponseDefault {
            public InlineResponseDefault read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             InlineResponseDefault instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/IsoscelesTriangle.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -107,6 +108,7 @@ public class IsoscelesTriangle {
   }
 
 
+
   @Override
   public boolean equals(Object o) {
     if (this == o) {
@@ -176,6 +178,7 @@ public class IsoscelesTriangle {
           throw new IllegalArgumentException(String.format("The required field(s) %s in IsoscelesTriangle is not found in the empty JSON string", IsoscelesTriangle.openapiRequiredFields.toString()));
         }
       }
+
       Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
       // check to see if the JSON string contains additional fields
       for (Entry<String, JsonElement> entry : entries) {

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/MapTest.java

@@ -39,6 +39,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -240,6 +241,42 @@ public class MapTest {
     this.indirectMap = indirectMap;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public MapTest putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -253,12 +290,13 @@ public class MapTest {
     return Objects.equals(this.mapMapOfString, mapTest.mapMapOfString) &&
         Objects.equals(this.mapOfEnumString, mapTest.mapOfEnumString) &&
         Objects.equals(this.directMap, mapTest.directMap) &&
-        Objects.equals(this.indirectMap, mapTest.indirectMap);
+        Objects.equals(this.indirectMap, mapTest.indirectMap)&&
+        Objects.equals(this.additionalProperties, mapTest.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(mapMapOfString, mapOfEnumString, directMap, indirectMap);
+    return Objects.hash(mapMapOfString, mapOfEnumString, directMap, indirectMap, additionalProperties);
   }
 
   @Override
@@ -269,6 +307,7 @@ public class MapTest {
     sb.append("    mapOfEnumString: ").append(toIndentedString(mapOfEnumString)).append("\n");
     sb.append("    directMap: ").append(toIndentedString(directMap)).append("\n");
     sb.append("    indirectMap: ").append(toIndentedString(indirectMap)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -314,13 +353,6 @@ public class MapTest {
           throw new IllegalArgumentException(String.format("The required field(s) %s in MapTest is not found in the empty JSON string", MapTest.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!MapTest.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `MapTest` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -338,6 +370,23 @@ public class MapTest {
            @Override
            public void write(JsonWriter out, MapTest value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -345,7 +394,25 @@ public class MapTest {
            public MapTest read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             MapTest instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/MixedPropertiesAndAdditionalPropertiesClass.java

@@ -42,6 +42,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -145,6 +146,42 @@ public class MixedPropertiesAndAdditionalPropertiesClass {
     this.map = map;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public MixedPropertiesAndAdditionalPropertiesClass putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -157,12 +194,13 @@ public class MixedPropertiesAndAdditionalPropertiesClass {
     MixedPropertiesAndAdditionalPropertiesClass mixedPropertiesAndAdditionalPropertiesClass = (MixedPropertiesAndAdditionalPropertiesClass) o;
     return Objects.equals(this.uuid, mixedPropertiesAndAdditionalPropertiesClass.uuid) &&
         Objects.equals(this.dateTime, mixedPropertiesAndAdditionalPropertiesClass.dateTime) &&
-        Objects.equals(this.map, mixedPropertiesAndAdditionalPropertiesClass.map);
+        Objects.equals(this.map, mixedPropertiesAndAdditionalPropertiesClass.map)&&
+        Objects.equals(this.additionalProperties, mixedPropertiesAndAdditionalPropertiesClass.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(uuid, dateTime, map);
+    return Objects.hash(uuid, dateTime, map, additionalProperties);
   }
 
   @Override
@@ -172,6 +210,7 @@ public class MixedPropertiesAndAdditionalPropertiesClass {
     sb.append("    uuid: ").append(toIndentedString(uuid)).append("\n");
     sb.append("    dateTime: ").append(toIndentedString(dateTime)).append("\n");
     sb.append("    map: ").append(toIndentedString(map)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -216,13 +255,6 @@ public class MixedPropertiesAndAdditionalPropertiesClass {
           throw new IllegalArgumentException(String.format("The required field(s) %s in MixedPropertiesAndAdditionalPropertiesClass is not found in the empty JSON string", MixedPropertiesAndAdditionalPropertiesClass.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!MixedPropertiesAndAdditionalPropertiesClass.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `MixedPropertiesAndAdditionalPropertiesClass` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -240,6 +272,23 @@ public class MixedPropertiesAndAdditionalPropertiesClass {
            @Override
            public void write(JsonWriter out, MixedPropertiesAndAdditionalPropertiesClass value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -247,7 +296,25 @@ public class MixedPropertiesAndAdditionalPropertiesClass {
            public MixedPropertiesAndAdditionalPropertiesClass read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             MixedPropertiesAndAdditionalPropertiesClass instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Model200Response.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -105,6 +106,42 @@ public class Model200Response {
     this.propertyClass = propertyClass;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Model200Response putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -116,12 +153,13 @@ public class Model200Response {
     }
     Model200Response _200response = (Model200Response) o;
     return Objects.equals(this.name, _200response.name) &&
-        Objects.equals(this.propertyClass, _200response.propertyClass);
+        Objects.equals(this.propertyClass, _200response.propertyClass)&&
+        Objects.equals(this.additionalProperties, _200response.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(name, propertyClass);
+    return Objects.hash(name, propertyClass, additionalProperties);
   }
 
   @Override
@@ -130,6 +168,7 @@ public class Model200Response {
     sb.append("class Model200Response {\n");
     sb.append("    name: ").append(toIndentedString(name)).append("\n");
     sb.append("    propertyClass: ").append(toIndentedString(propertyClass)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -173,13 +212,6 @@ public class Model200Response {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Model200Response is not found in the empty JSON string", Model200Response.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Model200Response.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Model200Response` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -197,6 +229,23 @@ public class Model200Response {
            @Override
            public void write(JsonWriter out, Model200Response value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -204,7 +253,25 @@ public class Model200Response {
            public Model200Response read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Model200Response instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ModelApiResponse.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -131,6 +132,42 @@ public class ModelApiResponse {
     this.message = message;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ModelApiResponse putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -143,12 +180,13 @@ public class ModelApiResponse {
     ModelApiResponse _apiResponse = (ModelApiResponse) o;
     return Objects.equals(this.code, _apiResponse.code) &&
         Objects.equals(this.type, _apiResponse.type) &&
-        Objects.equals(this.message, _apiResponse.message);
+        Objects.equals(this.message, _apiResponse.message)&&
+        Objects.equals(this.additionalProperties, _apiResponse.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(code, type, message);
+    return Objects.hash(code, type, message, additionalProperties);
   }
 
   @Override
@@ -158,6 +196,7 @@ public class ModelApiResponse {
     sb.append("    code: ").append(toIndentedString(code)).append("\n");
     sb.append("    type: ").append(toIndentedString(type)).append("\n");
     sb.append("    message: ").append(toIndentedString(message)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -202,13 +241,6 @@ public class ModelApiResponse {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ModelApiResponse is not found in the empty JSON string", ModelApiResponse.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ModelApiResponse.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ModelApiResponse` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -226,6 +258,23 @@ public class ModelApiResponse {
            @Override
            public void write(JsonWriter out, ModelApiResponse value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -233,7 +282,25 @@ public class ModelApiResponse {
            public ModelApiResponse read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ModelApiResponse instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ModelFile.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -78,6 +79,42 @@ public class ModelFile {
     this.sourceURI = sourceURI;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ModelFile putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -88,12 +125,13 @@ public class ModelFile {
       return false;
     }
     ModelFile _file = (ModelFile) o;
-    return Objects.equals(this.sourceURI, _file.sourceURI);
+    return Objects.equals(this.sourceURI, _file.sourceURI)&&
+        Objects.equals(this.additionalProperties, _file.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(sourceURI);
+    return Objects.hash(sourceURI, additionalProperties);
   }
 
   @Override
@@ -101,6 +139,7 @@ public class ModelFile {
     StringBuilder sb = new StringBuilder();
     sb.append("class ModelFile {\n");
     sb.append("    sourceURI: ").append(toIndentedString(sourceURI)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class ModelFile {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ModelFile is not found in the empty JSON string", ModelFile.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ModelFile.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ModelFile` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -167,6 +199,23 @@ public class ModelFile {
            @Override
            public void write(JsonWriter out, ModelFile value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -174,7 +223,25 @@ public class ModelFile {
            public ModelFile read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ModelFile instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ModelList.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class ModelList {
     this._123list = _123list;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ModelList putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class ModelList {
       return false;
     }
     ModelList _list = (ModelList) o;
-    return Objects.equals(this._123list, _list._123list);
+    return Objects.equals(this._123list, _list._123list)&&
+        Objects.equals(this.additionalProperties, _list.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(_123list);
+    return Objects.hash(_123list, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class ModelList {
     StringBuilder sb = new StringBuilder();
     sb.append("class ModelList {\n");
     sb.append("    _123list: ").append(toIndentedString(_123list)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -142,13 +181,6 @@ public class ModelList {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ModelList is not found in the empty JSON string", ModelList.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ModelList.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ModelList` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -166,6 +198,23 @@ public class ModelList {
            @Override
            public void write(JsonWriter out, ModelList value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -173,7 +222,25 @@ public class ModelList {
            public ModelList read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ModelList instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ModelReturn.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -78,6 +79,42 @@ public class ModelReturn {
     this._return = _return;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ModelReturn putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -88,12 +125,13 @@ public class ModelReturn {
       return false;
     }
     ModelReturn _return = (ModelReturn) o;
-    return Objects.equals(this._return, _return._return);
+    return Objects.equals(this._return, _return._return)&&
+        Objects.equals(this.additionalProperties, _return.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(_return);
+    return Objects.hash(_return, additionalProperties);
   }
 
   @Override
@@ -101,6 +139,7 @@ public class ModelReturn {
     StringBuilder sb = new StringBuilder();
     sb.append("class ModelReturn {\n");
     sb.append("    _return: ").append(toIndentedString(_return)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class ModelReturn {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ModelReturn is not found in the empty JSON string", ModelReturn.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ModelReturn.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ModelReturn` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -167,6 +199,23 @@ public class ModelReturn {
            @Override
            public void write(JsonWriter out, ModelReturn value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -174,7 +223,25 @@ public class ModelReturn {
            public ModelReturn read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ModelReturn instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Name.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -151,6 +152,42 @@ public class Name {
 
 
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Name putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -164,12 +201,13 @@ public class Name {
     return Objects.equals(this.name, name.name) &&
         Objects.equals(this.snakeCase, name.snakeCase) &&
         Objects.equals(this.property, name.property) &&
-        Objects.equals(this._123number, name._123number);
+        Objects.equals(this._123number, name._123number)&&
+        Objects.equals(this.additionalProperties, name.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(name, snakeCase, property, _123number);
+    return Objects.hash(name, snakeCase, property, _123number, additionalProperties);
   }
 
   @Override
@@ -180,6 +218,7 @@ public class Name {
     sb.append("    snakeCase: ").append(toIndentedString(snakeCase)).append("\n");
     sb.append("    property: ").append(toIndentedString(property)).append("\n");
     sb.append("    _123number: ").append(toIndentedString(_123number)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -226,13 +265,6 @@ public class Name {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Name is not found in the empty JSON string", Name.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Name.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Name` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : Name.openapiRequiredFields) {
@@ -257,6 +289,23 @@ public class Name {
            @Override
            public void write(JsonWriter out, Name value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -264,7 +313,25 @@ public class Name {
            public Name read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Name instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/NullableClass.java

@@ -44,6 +44,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -55,7 +56,7 @@ import org.openapitools.client.JSON;
  * NullableClass
  */
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
-public class NullableClass extends HashMap<String, Object> {
+public class NullableClass {
   public static final String SERIALIZED_NAME_INTEGER_PROP = "integer_prop";
   @SerializedName(SERIALIZED_NAME_INTEGER_PROP)
   private Integer integerProp;
@@ -431,6 +432,7 @@ public class NullableClass extends HashMap<String, Object> {
   }
 
 
+
   @Override
   public boolean equals(Object o) {
     if (this == o) {
@@ -451,8 +453,7 @@ public class NullableClass extends HashMap<String, Object> {
         Objects.equals(this.arrayItemsNullable, nullableClass.arrayItemsNullable) &&
         Objects.equals(this.objectNullableProp, nullableClass.objectNullableProp) &&
         Objects.equals(this.objectAndItemsNullableProp, nullableClass.objectAndItemsNullableProp) &&
-        Objects.equals(this.objectItemsNullable, nullableClass.objectItemsNullable) &&
-        super.equals(o);
+        Objects.equals(this.objectItemsNullable, nullableClass.objectItemsNullable);
   }
 
   private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b) {
@@ -461,7 +462,7 @@ public class NullableClass extends HashMap<String, Object> {
 
   @Override
   public int hashCode() {
-    return Objects.hash(integerProp, numberProp, booleanProp, stringProp, dateProp, datetimeProp, arrayNullableProp, arrayAndItemsNullableProp, arrayItemsNullable, objectNullableProp, objectAndItemsNullableProp, objectItemsNullable, super.hashCode());
+    return Objects.hash(integerProp, numberProp, booleanProp, stringProp, dateProp, datetimeProp, arrayNullableProp, arrayAndItemsNullableProp, arrayItemsNullable, objectNullableProp, objectAndItemsNullableProp, objectItemsNullable);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
@@ -475,7 +476,6 @@ public class NullableClass extends HashMap<String, Object> {
   public String toString() {
     StringBuilder sb = new StringBuilder();
     sb.append("class NullableClass {\n");
-    sb.append("    ").append(toIndentedString(super.toString())).append("\n");
     sb.append("    integerProp: ").append(toIndentedString(integerProp)).append("\n");
     sb.append("    numberProp: ").append(toIndentedString(numberProp)).append("\n");
     sb.append("    booleanProp: ").append(toIndentedString(booleanProp)).append("\n");
@@ -541,6 +541,7 @@ public class NullableClass extends HashMap<String, Object> {
           throw new IllegalArgumentException(String.format("The required field(s) %s in NullableClass is not found in the empty JSON string", NullableClass.openapiRequiredFields.toString()));
         }
       }
+
       Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
       // check to see if the JSON string contains additional fields
       for (Entry<String, JsonElement> entry : entries) {

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/NumberOnly.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -78,6 +79,42 @@ public class NumberOnly {
     this.justNumber = justNumber;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public NumberOnly putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -88,12 +125,13 @@ public class NumberOnly {
       return false;
     }
     NumberOnly numberOnly = (NumberOnly) o;
-    return Objects.equals(this.justNumber, numberOnly.justNumber);
+    return Objects.equals(this.justNumber, numberOnly.justNumber)&&
+        Objects.equals(this.additionalProperties, numberOnly.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(justNumber);
+    return Objects.hash(justNumber, additionalProperties);
   }
 
   @Override
@@ -101,6 +139,7 @@ public class NumberOnly {
     StringBuilder sb = new StringBuilder();
     sb.append("class NumberOnly {\n");
     sb.append("    justNumber: ").append(toIndentedString(justNumber)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class NumberOnly {
           throw new IllegalArgumentException(String.format("The required field(s) %s in NumberOnly is not found in the empty JSON string", NumberOnly.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!NumberOnly.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `NumberOnly` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -167,6 +199,23 @@ public class NumberOnly {
            @Override
            public void write(JsonWriter out, NumberOnly value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -174,7 +223,25 @@ public class NumberOnly {
            public NumberOnly read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             NumberOnly instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ObjectWithDeprecatedFields.java

@@ -40,6 +40,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -176,6 +177,42 @@ public class ObjectWithDeprecatedFields {
     this.bars = bars;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ObjectWithDeprecatedFields putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -189,12 +226,13 @@ public class ObjectWithDeprecatedFields {
     return Objects.equals(this.uuid, objectWithDeprecatedFields.uuid) &&
         Objects.equals(this.id, objectWithDeprecatedFields.id) &&
         Objects.equals(this.deprecatedRef, objectWithDeprecatedFields.deprecatedRef) &&
-        Objects.equals(this.bars, objectWithDeprecatedFields.bars);
+        Objects.equals(this.bars, objectWithDeprecatedFields.bars)&&
+        Objects.equals(this.additionalProperties, objectWithDeprecatedFields.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(uuid, id, deprecatedRef, bars);
+    return Objects.hash(uuid, id, deprecatedRef, bars, additionalProperties);
   }
 
   @Override
@@ -205,6 +243,7 @@ public class ObjectWithDeprecatedFields {
     sb.append("    id: ").append(toIndentedString(id)).append("\n");
     sb.append("    deprecatedRef: ").append(toIndentedString(deprecatedRef)).append("\n");
     sb.append("    bars: ").append(toIndentedString(bars)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -250,13 +289,6 @@ public class ObjectWithDeprecatedFields {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ObjectWithDeprecatedFields is not found in the empty JSON string", ObjectWithDeprecatedFields.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ObjectWithDeprecatedFields.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ObjectWithDeprecatedFields` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
       // validate the optional field `deprecatedRef`
       if (jsonObj.getAsJsonObject("deprecatedRef") != null) {
         DeprecatedObject.validateJsonObject(jsonObj.getAsJsonObject("deprecatedRef"));
@@ -278,6 +310,23 @@ public class ObjectWithDeprecatedFields {
            @Override
            public void write(JsonWriter out, ObjectWithDeprecatedFields value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -285,7 +334,25 @@ public class ObjectWithDeprecatedFields {
            public ObjectWithDeprecatedFields read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ObjectWithDeprecatedFields instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Order.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -262,6 +263,42 @@ public class Order {
     this.complete = complete;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Order putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -277,12 +314,13 @@ public class Order {
         Objects.equals(this.quantity, order.quantity) &&
         Objects.equals(this.shipDate, order.shipDate) &&
         Objects.equals(this.status, order.status) &&
-        Objects.equals(this.complete, order.complete);
+        Objects.equals(this.complete, order.complete)&&
+        Objects.equals(this.additionalProperties, order.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(id, petId, quantity, shipDate, status, complete);
+    return Objects.hash(id, petId, quantity, shipDate, status, complete, additionalProperties);
   }
 
   @Override
@@ -295,6 +333,7 @@ public class Order {
     sb.append("    shipDate: ").append(toIndentedString(shipDate)).append("\n");
     sb.append("    status: ").append(toIndentedString(status)).append("\n");
     sb.append("    complete: ").append(toIndentedString(complete)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -342,13 +381,6 @@ public class Order {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Order is not found in the empty JSON string", Order.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Order.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Order` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -366,6 +398,23 @@ public class Order {
            @Override
            public void write(JsonWriter out, Order value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -373,7 +422,25 @@ public class Order {
            public Order read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Order instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/OuterComposite.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -132,6 +133,42 @@ public class OuterComposite {
     this.myBoolean = myBoolean;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public OuterComposite putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -144,12 +181,13 @@ public class OuterComposite {
     OuterComposite outerComposite = (OuterComposite) o;
     return Objects.equals(this.myNumber, outerComposite.myNumber) &&
         Objects.equals(this.myString, outerComposite.myString) &&
-        Objects.equals(this.myBoolean, outerComposite.myBoolean);
+        Objects.equals(this.myBoolean, outerComposite.myBoolean)&&
+        Objects.equals(this.additionalProperties, outerComposite.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(myNumber, myString, myBoolean);
+    return Objects.hash(myNumber, myString, myBoolean, additionalProperties);
   }
 
   @Override
@@ -159,6 +197,7 @@ public class OuterComposite {
     sb.append("    myNumber: ").append(toIndentedString(myNumber)).append("\n");
     sb.append("    myString: ").append(toIndentedString(myString)).append("\n");
     sb.append("    myBoolean: ").append(toIndentedString(myBoolean)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -203,13 +242,6 @@ public class OuterComposite {
           throw new IllegalArgumentException(String.format("The required field(s) %s in OuterComposite is not found in the empty JSON string", OuterComposite.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!OuterComposite.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `OuterComposite` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -227,6 +259,23 @@ public class OuterComposite {
            @Override
            public void write(JsonWriter out, OuterComposite value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -234,7 +283,25 @@ public class OuterComposite {
            public OuterComposite read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             OuterComposite instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ParentPet.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -52,6 +53,42 @@ public class ParentPet extends GrandparentAnimal {
   public ParentPet() { 
     this.petType = this.getClass().getSimpleName();
   }
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ParentPet putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -66,7 +103,7 @@ public class ParentPet extends GrandparentAnimal {
 
   @Override
   public int hashCode() {
-    return Objects.hash(super.hashCode());
+    return Objects.hash(super.hashCode(), additionalProperties);
   }
 
   @Override
@@ -74,6 +111,7 @@ public class ParentPet extends GrandparentAnimal {
     StringBuilder sb = new StringBuilder();
     sb.append("class ParentPet {\n");
     sb.append("    ").append(toIndentedString(super.toString())).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -117,13 +155,6 @@ public class ParentPet extends GrandparentAnimal {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ParentPet is not found in the empty JSON string", ParentPet.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ParentPet.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ParentPet` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : ParentPet.openapiRequiredFields) {
@@ -148,6 +179,23 @@ public class ParentPet extends GrandparentAnimal {
            @Override
            public void write(JsonWriter out, ParentPet value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -155,7 +203,25 @@ public class ParentPet extends GrandparentAnimal {
            public ParentPet read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ParentPet instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Pet.java

@@ -40,6 +40,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -278,6 +279,42 @@ public class Pet {
     this.status = status;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Pet putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -293,12 +330,13 @@ public class Pet {
         Objects.equals(this.name, pet.name) &&
         Objects.equals(this.photoUrls, pet.photoUrls) &&
         Objects.equals(this.tags, pet.tags) &&
-        Objects.equals(this.status, pet.status);
+        Objects.equals(this.status, pet.status)&&
+        Objects.equals(this.additionalProperties, pet.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(id, category, name, photoUrls, tags, status);
+    return Objects.hash(id, category, name, photoUrls, tags, status, additionalProperties);
   }
 
   @Override
@@ -311,6 +349,7 @@ public class Pet {
     sb.append("    photoUrls: ").append(toIndentedString(photoUrls)).append("\n");
     sb.append("    tags: ").append(toIndentedString(tags)).append("\n");
     sb.append("    status: ").append(toIndentedString(status)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -360,13 +399,6 @@ public class Pet {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Pet is not found in the empty JSON string", Pet.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Pet.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Pet` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : Pet.openapiRequiredFields) {
@@ -402,6 +434,23 @@ public class Pet {
            @Override
            public void write(JsonWriter out, Pet value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -409,7 +458,25 @@ public class Pet {
            public Pet read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Pet instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/PetWithRequiredTags.java

@@ -40,6 +40,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -275,6 +276,42 @@ public class PetWithRequiredTags {
     this.status = status;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public PetWithRequiredTags putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -290,12 +327,13 @@ public class PetWithRequiredTags {
         Objects.equals(this.name, petWithRequiredTags.name) &&
         Objects.equals(this.photoUrls, petWithRequiredTags.photoUrls) &&
         Objects.equals(this.tags, petWithRequiredTags.tags) &&
-        Objects.equals(this.status, petWithRequiredTags.status);
+        Objects.equals(this.status, petWithRequiredTags.status)&&
+        Objects.equals(this.additionalProperties, petWithRequiredTags.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(id, category, name, photoUrls, tags, status);
+    return Objects.hash(id, category, name, photoUrls, tags, status, additionalProperties);
   }
 
   @Override
@@ -308,6 +346,7 @@ public class PetWithRequiredTags {
     sb.append("    photoUrls: ").append(toIndentedString(photoUrls)).append("\n");
     sb.append("    tags: ").append(toIndentedString(tags)).append("\n");
     sb.append("    status: ").append(toIndentedString(status)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -358,13 +397,6 @@ public class PetWithRequiredTags {
           throw new IllegalArgumentException(String.format("The required field(s) %s in PetWithRequiredTags is not found in the empty JSON string", PetWithRequiredTags.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!PetWithRequiredTags.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `PetWithRequiredTags` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : PetWithRequiredTags.openapiRequiredFields) {
@@ -400,6 +432,23 @@ public class PetWithRequiredTags {
            @Override
            public void write(JsonWriter out, PetWithRequiredTags value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -407,7 +456,25 @@ public class PetWithRequiredTags {
            public PetWithRequiredTags read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             PetWithRequiredTags instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/QuadrilateralInterface.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class QuadrilateralInterface {
     this.quadrilateralType = quadrilateralType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public QuadrilateralInterface putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class QuadrilateralInterface {
       return false;
     }
     QuadrilateralInterface quadrilateralInterface = (QuadrilateralInterface) o;
-    return Objects.equals(this.quadrilateralType, quadrilateralInterface.quadrilateralType);
+    return Objects.equals(this.quadrilateralType, quadrilateralInterface.quadrilateralType)&&
+        Objects.equals(this.additionalProperties, quadrilateralInterface.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(quadrilateralType);
+    return Objects.hash(quadrilateralType, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class QuadrilateralInterface {
     StringBuilder sb = new StringBuilder();
     sb.append("class QuadrilateralInterface {\n");
     sb.append("    quadrilateralType: ").append(toIndentedString(quadrilateralType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class QuadrilateralInterface {
           throw new IllegalArgumentException(String.format("The required field(s) %s in QuadrilateralInterface is not found in the empty JSON string", QuadrilateralInterface.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!QuadrilateralInterface.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `QuadrilateralInterface` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : QuadrilateralInterface.openapiRequiredFields) {
@@ -174,6 +206,23 @@ public class QuadrilateralInterface {
            @Override
            public void write(JsonWriter out, QuadrilateralInterface value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -181,7 +230,25 @@ public class QuadrilateralInterface {
            public QuadrilateralInterface read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             QuadrilateralInterface instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ReadOnlyFirst.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -103,6 +104,42 @@ public class ReadOnlyFirst {
     this.baz = baz;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ReadOnlyFirst putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -114,12 +151,13 @@ public class ReadOnlyFirst {
     }
     ReadOnlyFirst readOnlyFirst = (ReadOnlyFirst) o;
     return Objects.equals(this.bar, readOnlyFirst.bar) &&
-        Objects.equals(this.baz, readOnlyFirst.baz);
+        Objects.equals(this.baz, readOnlyFirst.baz)&&
+        Objects.equals(this.additionalProperties, readOnlyFirst.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(bar, baz);
+    return Objects.hash(bar, baz, additionalProperties);
   }
 
   @Override
@@ -128,6 +166,7 @@ public class ReadOnlyFirst {
     sb.append("class ReadOnlyFirst {\n");
     sb.append("    bar: ").append(toIndentedString(bar)).append("\n");
     sb.append("    baz: ").append(toIndentedString(baz)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -171,13 +210,6 @@ public class ReadOnlyFirst {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ReadOnlyFirst is not found in the empty JSON string", ReadOnlyFirst.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ReadOnlyFirst.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ReadOnlyFirst` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -195,6 +227,23 @@ public class ReadOnlyFirst {
            @Override
            public void write(JsonWriter out, ReadOnlyFirst value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -202,7 +251,25 @@ public class ReadOnlyFirst {
            public ReadOnlyFirst read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ReadOnlyFirst instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ScaleneTriangle.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -106,6 +107,42 @@ public class ScaleneTriangle {
     this.triangleType = triangleType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ScaleneTriangle putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -117,12 +154,13 @@ public class ScaleneTriangle {
     }
     ScaleneTriangle scaleneTriangle = (ScaleneTriangle) o;
     return Objects.equals(this.shapeType, scaleneTriangle.shapeType) &&
-        Objects.equals(this.triangleType, scaleneTriangle.triangleType);
+        Objects.equals(this.triangleType, scaleneTriangle.triangleType)&&
+        Objects.equals(this.additionalProperties, scaleneTriangle.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(shapeType, triangleType);
+    return Objects.hash(shapeType, triangleType, additionalProperties);
   }
 
   @Override
@@ -131,6 +169,7 @@ public class ScaleneTriangle {
     sb.append("class ScaleneTriangle {\n");
     sb.append("    shapeType: ").append(toIndentedString(shapeType)).append("\n");
     sb.append("    triangleType: ").append(toIndentedString(triangleType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -176,13 +215,6 @@ public class ScaleneTriangle {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ScaleneTriangle is not found in the empty JSON string", ScaleneTriangle.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ScaleneTriangle.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ScaleneTriangle` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : ScaleneTriangle.openapiRequiredFields) {
@@ -207,6 +239,23 @@ public class ScaleneTriangle {
            @Override
            public void write(JsonWriter out, ScaleneTriangle value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -214,7 +263,25 @@ public class ScaleneTriangle {
            public ScaleneTriangle read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ScaleneTriangle instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ShapeInterface.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class ShapeInterface {
     this.shapeType = shapeType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public ShapeInterface putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class ShapeInterface {
       return false;
     }
     ShapeInterface shapeInterface = (ShapeInterface) o;
-    return Objects.equals(this.shapeType, shapeInterface.shapeType);
+    return Objects.equals(this.shapeType, shapeInterface.shapeType)&&
+        Objects.equals(this.additionalProperties, shapeInterface.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(shapeType);
+    return Objects.hash(shapeType, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class ShapeInterface {
     StringBuilder sb = new StringBuilder();
     sb.append("class ShapeInterface {\n");
     sb.append("    shapeType: ").append(toIndentedString(shapeType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class ShapeInterface {
           throw new IllegalArgumentException(String.format("The required field(s) %s in ShapeInterface is not found in the empty JSON string", ShapeInterface.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!ShapeInterface.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `ShapeInterface` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : ShapeInterface.openapiRequiredFields) {
@@ -174,6 +206,23 @@ public class ShapeInterface {
            @Override
            public void write(JsonWriter out, ShapeInterface value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -181,7 +230,25 @@ public class ShapeInterface {
            public ShapeInterface read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             ShapeInterface instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/SimpleQuadrilateral.java

@@ -38,6 +38,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -106,6 +107,42 @@ public class SimpleQuadrilateral {
     this.quadrilateralType = quadrilateralType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public SimpleQuadrilateral putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -117,12 +154,13 @@ public class SimpleQuadrilateral {
     }
     SimpleQuadrilateral simpleQuadrilateral = (SimpleQuadrilateral) o;
     return Objects.equals(this.shapeType, simpleQuadrilateral.shapeType) &&
-        Objects.equals(this.quadrilateralType, simpleQuadrilateral.quadrilateralType);
+        Objects.equals(this.quadrilateralType, simpleQuadrilateral.quadrilateralType)&&
+        Objects.equals(this.additionalProperties, simpleQuadrilateral.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(shapeType, quadrilateralType);
+    return Objects.hash(shapeType, quadrilateralType, additionalProperties);
   }
 
   @Override
@@ -131,6 +169,7 @@ public class SimpleQuadrilateral {
     sb.append("class SimpleQuadrilateral {\n");
     sb.append("    shapeType: ").append(toIndentedString(shapeType)).append("\n");
     sb.append("    quadrilateralType: ").append(toIndentedString(quadrilateralType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -176,13 +215,6 @@ public class SimpleQuadrilateral {
           throw new IllegalArgumentException(String.format("The required field(s) %s in SimpleQuadrilateral is not found in the empty JSON string", SimpleQuadrilateral.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!SimpleQuadrilateral.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `SimpleQuadrilateral` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : SimpleQuadrilateral.openapiRequiredFields) {
@@ -207,6 +239,23 @@ public class SimpleQuadrilateral {
            @Override
            public void write(JsonWriter out, SimpleQuadrilateral value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -214,7 +263,25 @@ public class SimpleQuadrilateral {
            public SimpleQuadrilateral read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             SimpleQuadrilateral instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/SpecialModelName.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -104,6 +105,42 @@ public class SpecialModelName {
     this.specialModelName = specialModelName;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public SpecialModelName putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -115,12 +152,13 @@ public class SpecialModelName {
     }
     SpecialModelName specialModelName = (SpecialModelName) o;
     return Objects.equals(this.$specialPropertyName, specialModelName.$specialPropertyName) &&
-        Objects.equals(this.specialModelName, specialModelName.specialModelName);
+        Objects.equals(this.specialModelName, specialModelName.specialModelName)&&
+        Objects.equals(this.additionalProperties, specialModelName.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash($specialPropertyName, specialModelName);
+    return Objects.hash($specialPropertyName, specialModelName, additionalProperties);
   }
 
   @Override
@@ -129,6 +167,7 @@ public class SpecialModelName {
     sb.append("class SpecialModelName {\n");
     sb.append("    $specialPropertyName: ").append(toIndentedString($specialPropertyName)).append("\n");
     sb.append("    specialModelName: ").append(toIndentedString(specialModelName)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -172,13 +211,6 @@ public class SpecialModelName {
           throw new IllegalArgumentException(String.format("The required field(s) %s in SpecialModelName is not found in the empty JSON string", SpecialModelName.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!SpecialModelName.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `SpecialModelName` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -196,6 +228,23 @@ public class SpecialModelName {
            @Override
            public void write(JsonWriter out, SpecialModelName value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -203,7 +252,25 @@ public class SpecialModelName {
            public SpecialModelName read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             SpecialModelName instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Tag.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -104,6 +105,42 @@ public class Tag {
     this.name = name;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Tag putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -115,12 +152,13 @@ public class Tag {
     }
     Tag tag = (Tag) o;
     return Objects.equals(this.id, tag.id) &&
-        Objects.equals(this.name, tag.name);
+        Objects.equals(this.name, tag.name)&&
+        Objects.equals(this.additionalProperties, tag.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(id, name);
+    return Objects.hash(id, name, additionalProperties);
   }
 
   @Override
@@ -129,6 +167,7 @@ public class Tag {
     sb.append("class Tag {\n");
     sb.append("    id: ").append(toIndentedString(id)).append("\n");
     sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -172,13 +211,6 @@ public class Tag {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Tag is not found in the empty JSON string", Tag.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Tag.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Tag` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -196,6 +228,23 @@ public class Tag {
            @Override
            public void write(JsonWriter out, Tag value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -203,7 +252,25 @@ public class Tag {
            public Tag read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Tag instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/TriangleInterface.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -77,6 +78,42 @@ public class TriangleInterface {
     this.triangleType = triangleType;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public TriangleInterface putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -87,12 +124,13 @@ public class TriangleInterface {
       return false;
     }
     TriangleInterface triangleInterface = (TriangleInterface) o;
-    return Objects.equals(this.triangleType, triangleInterface.triangleType);
+    return Objects.equals(this.triangleType, triangleInterface.triangleType)&&
+        Objects.equals(this.additionalProperties, triangleInterface.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(triangleType);
+    return Objects.hash(triangleType, additionalProperties);
   }
 
   @Override
@@ -100,6 +138,7 @@ public class TriangleInterface {
     StringBuilder sb = new StringBuilder();
     sb.append("class TriangleInterface {\n");
     sb.append("    triangleType: ").append(toIndentedString(triangleType)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -143,13 +182,6 @@ public class TriangleInterface {
           throw new IllegalArgumentException(String.format("The required field(s) %s in TriangleInterface is not found in the empty JSON string", TriangleInterface.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!TriangleInterface.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `TriangleInterface` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : TriangleInterface.openapiRequiredFields) {
@@ -174,6 +206,23 @@ public class TriangleInterface {
            @Override
            public void write(JsonWriter out, TriangleInterface value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -181,7 +230,25 @@ public class TriangleInterface {
            public TriangleInterface read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             TriangleInterface instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/User.java

@@ -37,6 +37,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -375,6 +376,42 @@ public class User {
     this.anyTypePropNullable = anyTypePropNullable;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public User putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -396,7 +433,8 @@ public class User {
         Objects.equals(this.objectWithNoDeclaredProps, user.objectWithNoDeclaredProps) &&
         Objects.equals(this.objectWithNoDeclaredPropsNullable, user.objectWithNoDeclaredPropsNullable) &&
         Objects.equals(this.anyTypeProp, user.anyTypeProp) &&
-        Objects.equals(this.anyTypePropNullable, user.anyTypePropNullable);
+        Objects.equals(this.anyTypePropNullable, user.anyTypePropNullable)&&
+        Objects.equals(this.additionalProperties, user.additionalProperties);
   }
 
   private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b) {
@@ -405,7 +443,7 @@ public class User {
 
   @Override
   public int hashCode() {
-    return Objects.hash(id, username, firstName, lastName, email, password, phone, userStatus, objectWithNoDeclaredProps, objectWithNoDeclaredPropsNullable, anyTypeProp, anyTypePropNullable);
+    return Objects.hash(id, username, firstName, lastName, email, password, phone, userStatus, objectWithNoDeclaredProps, objectWithNoDeclaredPropsNullable, anyTypeProp, anyTypePropNullable, additionalProperties);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
@@ -431,6 +469,7 @@ public class User {
     sb.append("    objectWithNoDeclaredPropsNullable: ").append(toIndentedString(objectWithNoDeclaredPropsNullable)).append("\n");
     sb.append("    anyTypeProp: ").append(toIndentedString(anyTypeProp)).append("\n");
     sb.append("    anyTypePropNullable: ").append(toIndentedString(anyTypePropNullable)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -484,13 +523,6 @@ public class User {
           throw new IllegalArgumentException(String.format("The required field(s) %s in User is not found in the empty JSON string", User.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!User.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `User` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
@@ -508,6 +540,23 @@ public class User {
            @Override
            public void write(JsonWriter out, User value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -515,7 +564,25 @@ public class User {
            public User read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             User instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Whale.java

@@ -36,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -131,6 +132,42 @@ public class Whale {
     this.className = className;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Whale putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -143,12 +180,13 @@ public class Whale {
     Whale whale = (Whale) o;
     return Objects.equals(this.hasBaleen, whale.hasBaleen) &&
         Objects.equals(this.hasTeeth, whale.hasTeeth) &&
-        Objects.equals(this.className, whale.className);
+        Objects.equals(this.className, whale.className)&&
+        Objects.equals(this.additionalProperties, whale.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(hasBaleen, hasTeeth, className);
+    return Objects.hash(hasBaleen, hasTeeth, className, additionalProperties);
   }
 
   @Override
@@ -158,6 +196,7 @@ public class Whale {
     sb.append("    hasBaleen: ").append(toIndentedString(hasBaleen)).append("\n");
     sb.append("    hasTeeth: ").append(toIndentedString(hasTeeth)).append("\n");
     sb.append("    className: ").append(toIndentedString(className)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -203,13 +242,6 @@ public class Whale {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Whale is not found in the empty JSON string", Whale.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Whale.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Whale` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : Whale.openapiRequiredFields) {
@@ -234,6 +266,23 @@ public class Whale {
            @Override
            public void write(JsonWriter out, Whale value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -241,7 +290,25 @@ public class Whale {
            public Whale read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Whale instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/Zebra.java

@@ -23,8 +23,6 @@ import com.google.gson.stream.JsonWriter;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 import java.io.IOException;
-import java.util.HashMap;
-import java.util.Map;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
@@ -38,6 +36,7 @@ import com.google.gson.TypeAdapterFactory;
 import com.google.gson.reflect.TypeToken;
 
 import java.lang.reflect.Type;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -49,7 +48,7 @@ import org.openapitools.client.JSON;
  * Zebra
  */
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
-public class Zebra extends HashMap<String, Object> {
+public class Zebra {
   /**
    * Gets or Sets type
    */
@@ -155,6 +154,42 @@ public class Zebra extends HashMap<String, Object> {
     this.className = className;
   }
 
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   */
+  public Zebra putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
 
   @Override
   public boolean equals(Object o) {
@@ -166,22 +201,22 @@ public class Zebra extends HashMap<String, Object> {
     }
     Zebra zebra = (Zebra) o;
     return Objects.equals(this.type, zebra.type) &&
-        Objects.equals(this.className, zebra.className) &&
-        super.equals(o);
+        Objects.equals(this.className, zebra.className)&&
+        Objects.equals(this.additionalProperties, zebra.additionalProperties);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(type, className, super.hashCode());
+    return Objects.hash(type, className, additionalProperties);
   }
 
   @Override
   public String toString() {
     StringBuilder sb = new StringBuilder();
     sb.append("class Zebra {\n");
-    sb.append("    ").append(toIndentedString(super.toString())).append("\n");
     sb.append("    type: ").append(toIndentedString(type)).append("\n");
     sb.append("    className: ").append(toIndentedString(className)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
     sb.append("}");
     return sb.toString();
   }
@@ -226,13 +261,6 @@ public class Zebra extends HashMap<String, Object> {
           throw new IllegalArgumentException(String.format("The required field(s) %s in Zebra is not found in the empty JSON string", Zebra.openapiRequiredFields.toString()));
         }
       }
-      Set<Entry<String, JsonElement>> entries = jsonObj.entrySet();
-      // check to see if the JSON string contains additional fields
-      for (Entry<String, JsonElement> entry : entries) {
-        if (!Zebra.openapiFields.contains(entry.getKey())) {
-          throw new IllegalArgumentException(String.format("The field `%s` in the JSON string is not defined in the `Zebra` properties. JSON: %s", entry.getKey(), jsonObj.toString()));
-        }
-      }
 
       // check to make sure all required properties/fields are present in the JSON string
       for (String requiredField : Zebra.openapiRequiredFields) {
@@ -257,6 +285,23 @@ public class Zebra extends HashMap<String, Object> {
            @Override
            public void write(JsonWriter out, Zebra value) throws IOException {
              JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additonal properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
              elementAdapter.write(out, obj);
            }
 
@@ -264,7 +309,25 @@ public class Zebra extends HashMap<String, Object> {
            public Zebra read(JsonReader in) throws IOException {
              JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
              validateJsonObject(jsonObj);
-             return thisAdapter.fromJsonTree(jsonObj);
+             // store additional fields in the deserialized instance
+             Zebra instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else { // non-primitive type
+                   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
            }
 
        }.nullSafe();

samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java

@@ -224,13 +224,11 @@ public class JSONTest {
     }
 
     @Test
+    @Disabled("No longer need the following test as additional field(s) should be stored in `additionalProperties`")
     public void testAdditionalFieldException() {
         IllegalArgumentException thrown = Assertions.assertThrows(IllegalArgumentException.class, () -> {
             // test json string with additional field(s) to ensure exception is thrown
             Gson gson = json.getGson();
-            //Gson gson = new GsonBuilder()
-            //        .registerTypeAdapter(Tag.class, new Tag.CustomDeserializer())
-            //        .create();
             String json = "{\"id\": 5847, \"name\":\"tag test 1\", \"new-field\": true}";
             org.openapitools.client.model.Tag t = gson.fromJson(json, org.openapitools.client.model.Tag.class);
         });
@@ -268,7 +266,13 @@ public class JSONTest {
         Pet t4 = gson.fromJson(json3, Pet.class);
         assertEquals(t4.getName(), "pet test 1");
         assertEquals(t4.getId(), Long.valueOf(5847));
+    }
 
+    @Test
+    @Disabled("Unknown fields are now correctly deserialized into `additionalProperties`")
+    public void testUnknownFields() {
+        // test unknown fields in the payload
+        Gson gson = json.getGson();
         // test Tag
         String json5 = "{\"unknown_field\": 543, \"id\":\"tag 123\"}";
         Exception exception5 = assertThrows(java.lang.IllegalArgumentException.class, () -> {
@@ -359,6 +363,7 @@ public class JSONTest {
             assertEquals(json.getGson().toJson(o), "{\"cultivar\":\"golden delicious\",\"origin\":\"japan\"}");
             assertEquals(o.toJson(), "{\"cultivar\":\"golden delicious\",\"origin\":\"japan\"}");
 
+            /* comment out the following as we've added "additionalProperties" support
             String str2 = "{ \"origin_typo\": \"japan\" }";
             // no match
             Exception exception = assertThrows(java.lang.IllegalArgumentException.class, () -> {
@@ -374,6 +379,7 @@ public class JSONTest {
             Exception exception4 = assertThrows(com.google.gson.JsonSyntaxException.class, () -> {
                 GmFruit o2 = json.getGson().fromJson(str2, GmFruit.class);
             });
+             */
         }
     }
 
@@ -407,15 +413,15 @@ public class JSONTest {
 
             // make sure deserialization works for pojo object
             Zebra z = Zebra.fromJson(str2);
-            assertEquals(z.toJson(), "{\"className\":\"zebra\",\"type\":\"plains\"}");
+            assertEquals(z.toJson(), "{\"type\":\"plains\",\"className\":\"zebra\"}");
 
             Mammal o2 = json.getGson().fromJson(str2, Mammal.class);
             assertTrue(o2.getActualInstance() instanceof Zebra);
             Zebra inst2 = (Zebra) o2.getActualInstance();
-            assertEquals(json.getGson().toJson(inst2), "{\"className\":\"zebra\",\"type\":\"plains\"}");
-            assertEquals(inst2.toJson(), "{\"className\":\"zebra\",\"type\":\"plains\"}");
-            assertEquals(json.getGson().toJson(o2), "{\"className\":\"zebra\",\"type\":\"plains\"}");
-            assertEquals(o2.toJson(), "{\"className\":\"zebra\",\"type\":\"plains\"}");
+            assertEquals(json.getGson().toJson(inst2), "{\"type\":\"plains\",\"className\":\"zebra\"}");
+            assertEquals(inst2.toJson(), "{\"type\":\"plains\",\"className\":\"zebra\"}");
+            assertEquals(json.getGson().toJson(o2), "{\"type\":\"plains\",\"className\":\"zebra\"}");
+            assertEquals(o2.toJson(), "{\"type\":\"plains\",\"className\":\"zebra\"}");
         }
         {
             // incorrect payload results in exception
@@ -519,4 +525,21 @@ public class JSONTest {
             assertTrue(exception.getMessage().contains("Failed deserialization for FruitReq: 0 classes match result, expected 1"));
         }
     }
+
+    /**
+     * Test additional properties.
+     */
+    @Test
+    public void testAdditionalProperties() throws Exception {
+        String str = "{ \"className\": \"zebra\", \"type\": \"plains\", \"from_json\": 4567, \"from_json_map\": {\"nested_string\": \"nested_value\"} }";
+        Zebra z = Zebra.fromJson(str);
+        z.putAdditionalProperty("new_key", "new_value");
+        z.putAdditionalProperty("new_number", 1.23);
+        z.putAdditionalProperty("new_boolean", true);
+        org.openapitools.client.model.Tag t = new org.openapitools.client.model.Tag();
+        t.setId(34L);
+        t.setName("just a tag");
+        z.putAdditionalProperty("new_object", t);
+        assertEquals(z.toJson(), "{\"type\":\"plains\",\"className\":\"zebra\",\"new_key\":\"new_value\",\"new_boolean\":true,\"new_object\":{\"id\":34,\"name\":\"just a tag\"},\"from_json\":4567,\"from_json_map\":{\"nested_string\":\"nested_value\"},\"new_number\":1.23}");
+    }
 }