diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache
index 73a5749fba7c..9b1812a955bc 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache
@@ -1528,11 +1528,12 @@ public class ApiClient {
                 for (Object item: list) {
                     if (item instanceof File) {
                         addPartToMultiPartBuilder(mpBuilder, param.getKey(), (File) item);
+                    } else {
+                        addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
                     }
                 }
             } else {
-                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
-                mpBuilder.addPart(partHeaders, RequestBody.create(parameterToString(param.getValue()), null));
+                addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
             }
         }
         return mpBuilder.build();
@@ -1566,6 +1567,31 @@ public class ApiClient {
         mpBuilder.addPart(partHeaders, RequestBody.create(file, mediaType));
     }
 
+    /**
+     * Add a Content-Disposition Header for the given key and complex object to the MultipartBody Builder.
+     *
+     * @param mpBuilder MultipartBody.Builder
+     * @param key The key of the Header element
+     * @param obj The complex object to add to the Header
+     */
+    private void addPartToMultiPartBuilder(MultipartBody.Builder mpBuilder, String key, Object obj) {
+        RequestBody requestBody;
+        if (obj instanceof String) {
+            requestBody = RequestBody.create((String) obj, MediaType.parse("text/plain"));
+        } else {
+            String content;
+            if (obj != null) {
+                content = json.serialize(obj);
+            } else {
+                content = null;
+            }
+            requestBody = RequestBody.create(content, MediaType.parse("application/json"));
+        }
+
+        Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + key + "\"");
+        mpBuilder.addPart(partHeaders, requestBody);
+    }
+
     /**
      * Get network interceptor to add it to the httpClient to track download progress for
      * async requests.
@@ -1737,7 +1763,7 @@ public class ApiClient {
     /**
      * Convert the HTTP request body to a string.
      *
-     * @param request The HTTP request object
+     * @param requestBody The HTTP request object
      * @return The string representation of the HTTP request body
      * @throws {{invokerPackage}}.ApiException If fail to serialize the request body object into a string
      */
diff --git a/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java
index 20c24352d3d8..89471e3b8bf4 100644
--- a/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java
@@ -1323,11 +1323,12 @@ public class ApiClient {
                 for (Object item: list) {
                     if (item instanceof File) {
                         addPartToMultiPartBuilder(mpBuilder, param.getKey(), (File) item);
+                    } else {
+                        addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
                     }
                 }
             } else {
-                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
-                mpBuilder.addPart(partHeaders, RequestBody.create(parameterToString(param.getValue()), null));
+                addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
             }
         }
         return mpBuilder.build();
@@ -1361,6 +1362,31 @@ public class ApiClient {
         mpBuilder.addPart(partHeaders, RequestBody.create(file, mediaType));
     }
 
+    /**
+     * Add a Content-Disposition Header for the given key and complex object to the MultipartBody Builder.
+     *
+     * @param mpBuilder MultipartBody.Builder
+     * @param key The key of the Header element
+     * @param obj The complex object to add to the Header
+     */
+    private void addPartToMultiPartBuilder(MultipartBody.Builder mpBuilder, String key, Object obj) {
+        RequestBody requestBody;
+        if (obj instanceof String) {
+            requestBody = RequestBody.create((String) obj, MediaType.parse("text/plain"));
+        } else {
+            String content;
+            if (obj != null) {
+                content = json.serialize(obj);
+            } else {
+                content = null;
+            }
+            requestBody = RequestBody.create(content, MediaType.parse("application/json"));
+        }
+
+        Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + key + "\"");
+        mpBuilder.addPart(partHeaders, requestBody);
+    }
+
     /**
      * Get network interceptor to add it to the httpClient to track download progress for
      * async requests.
@@ -1461,7 +1487,7 @@ public class ApiClient {
     /**
      * Convert the HTTP request body to a string.
      *
-     * @param request The HTTP request object
+     * @param requestBody The HTTP request object
      * @return The string representation of the HTTP request body
      * @throws org.openapitools.client.ApiException If fail to serialize the request body object into a string
      */
diff --git a/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java
index c065d7d109fe..e7332008c008 100644
--- a/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java
@@ -1401,11 +1401,12 @@ public class ApiClient {
                 for (Object item: list) {
                     if (item instanceof File) {
                         addPartToMultiPartBuilder(mpBuilder, param.getKey(), (File) item);
+                    } else {
+                        addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
                     }
                 }
             } else {
-                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
-                mpBuilder.addPart(partHeaders, RequestBody.create(parameterToString(param.getValue()), null));
+                addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
             }
         }
         return mpBuilder.build();
@@ -1439,6 +1440,31 @@ public class ApiClient {
         mpBuilder.addPart(partHeaders, RequestBody.create(file, mediaType));
     }
 
+    /**
+     * Add a Content-Disposition Header for the given key and complex object to the MultipartBody Builder.
+     *
+     * @param mpBuilder MultipartBody.Builder
+     * @param key The key of the Header element
+     * @param obj The complex object to add to the Header
+     */
+    private void addPartToMultiPartBuilder(MultipartBody.Builder mpBuilder, String key, Object obj) {
+        RequestBody requestBody;
+        if (obj instanceof String) {
+            requestBody = RequestBody.create((String) obj, MediaType.parse("text/plain"));
+        } else {
+            String content;
+            if (obj != null) {
+                content = json.serialize(obj);
+            } else {
+                content = null;
+            }
+            requestBody = RequestBody.create(content, MediaType.parse("application/json"));
+        }
+
+        Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + key + "\"");
+        mpBuilder.addPart(partHeaders, requestBody);
+    }
+
     /**
      * Get network interceptor to add it to the httpClient to track download progress for
      * async requests.
@@ -1608,7 +1634,7 @@ public class ApiClient {
     /**
      * Convert the HTTP request body to a string.
      *
-     * @param request The HTTP request object
+     * @param requestBody The HTTP request object
      * @return The string representation of the HTTP request body
      * @throws org.openapitools.client.ApiException If fail to serialize the request body object into a string
      */
diff --git a/samples/client/petstore/java/okhttp-gson-group-parameter/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/okhttp-gson-group-parameter/src/main/java/org/openapitools/client/ApiClient.java
index 8845b13e974b..24914b502808 100644
--- a/samples/client/petstore/java/okhttp-gson-group-parameter/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/okhttp-gson-group-parameter/src/main/java/org/openapitools/client/ApiClient.java
@@ -1396,11 +1396,12 @@ public class ApiClient {
                 for (Object item: list) {
                     if (item instanceof File) {
                         addPartToMultiPartBuilder(mpBuilder, param.getKey(), (File) item);
+                    } else {
+                        addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
                     }
                 }
             } else {
-                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
-                mpBuilder.addPart(partHeaders, RequestBody.create(parameterToString(param.getValue()), null));
+                addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
             }
         }
         return mpBuilder.build();
@@ -1434,6 +1435,31 @@ public class ApiClient {
         mpBuilder.addPart(partHeaders, RequestBody.create(file, mediaType));
     }
 
+    /**
+     * Add a Content-Disposition Header for the given key and complex object to the MultipartBody Builder.
+     *
+     * @param mpBuilder MultipartBody.Builder
+     * @param key The key of the Header element
+     * @param obj The complex object to add to the Header
+     */
+    private void addPartToMultiPartBuilder(MultipartBody.Builder mpBuilder, String key, Object obj) {
+        RequestBody requestBody;
+        if (obj instanceof String) {
+            requestBody = RequestBody.create((String) obj, MediaType.parse("text/plain"));
+        } else {
+            String content;
+            if (obj != null) {
+                content = json.serialize(obj);
+            } else {
+                content = null;
+            }
+            requestBody = RequestBody.create(content, MediaType.parse("application/json"));
+        }
+
+        Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + key + "\"");
+        mpBuilder.addPart(partHeaders, requestBody);
+    }
+
     /**
      * Get network interceptor to add it to the httpClient to track download progress for
      * async requests.
@@ -1534,7 +1560,7 @@ public class ApiClient {
     /**
      * Convert the HTTP request body to a string.
      *
-     * @param request The HTTP request object
+     * @param requestBody The HTTP request object
      * @return The string representation of the HTTP request body
      * @throws org.openapitools.client.ApiException If fail to serialize the request body object into a string
      */
diff --git a/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java
index 8ae5d16c3baf..07792101ffa8 100644
--- a/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java
@@ -1402,11 +1402,12 @@ public class ApiClient {
                 for (Object item: list) {
                     if (item instanceof File) {
                         addPartToMultiPartBuilder(mpBuilder, param.getKey(), (File) item);
+                    } else {
+                        addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
                     }
                 }
             } else {
-                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
-                mpBuilder.addPart(partHeaders, RequestBody.create(parameterToString(param.getValue()), null));
+                addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
             }
         }
         return mpBuilder.build();
@@ -1440,6 +1441,31 @@ public class ApiClient {
         mpBuilder.addPart(partHeaders, RequestBody.create(file, mediaType));
     }
 
+    /**
+     * Add a Content-Disposition Header for the given key and complex object to the MultipartBody Builder.
+     *
+     * @param mpBuilder MultipartBody.Builder
+     * @param key The key of the Header element
+     * @param obj The complex object to add to the Header
+     */
+    private void addPartToMultiPartBuilder(MultipartBody.Builder mpBuilder, String key, Object obj) {
+        RequestBody requestBody;
+        if (obj instanceof String) {
+            requestBody = RequestBody.create((String) obj, MediaType.parse("text/plain"));
+        } else {
+            String content;
+            if (obj != null) {
+                content = json.serialize(obj);
+            } else {
+                content = null;
+            }
+            requestBody = RequestBody.create(content, MediaType.parse("application/json"));
+        }
+
+        Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + key + "\"");
+        mpBuilder.addPart(partHeaders, requestBody);
+    }
+
     /**
      * Get network interceptor to add it to the httpClient to track download progress for
      * async requests.
@@ -1540,7 +1566,7 @@ public class ApiClient {
     /**
      * Convert the HTTP request body to a string.
      *
-     * @param request The HTTP request object
+     * @param requestBody The HTTP request object
      * @return The string representation of the HTTP request body
      * @throws org.openapitools.client.ApiException If fail to serialize the request body object into a string
      */
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java
index 0e626694f6f9..4319311adf73 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java
@@ -1421,11 +1421,12 @@ public class ApiClient {
                 for (Object item: list) {
                     if (item instanceof File) {
                         addPartToMultiPartBuilder(mpBuilder, param.getKey(), (File) item);
+                    } else {
+                        addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
                     }
                 }
             } else {
-                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
-                mpBuilder.addPart(partHeaders, RequestBody.create(parameterToString(param.getValue()), null));
+                addPartToMultiPartBuilder(mpBuilder, param.getKey(), param.getValue());
             }
         }
         return mpBuilder.build();
@@ -1459,6 +1460,31 @@ public class ApiClient {
         mpBuilder.addPart(partHeaders, RequestBody.create(file, mediaType));
     }
 
+    /**
+     * Add a Content-Disposition Header for the given key and complex object to the MultipartBody Builder.
+     *
+     * @param mpBuilder MultipartBody.Builder
+     * @param key The key of the Header element
+     * @param obj The complex object to add to the Header
+     */
+    private void addPartToMultiPartBuilder(MultipartBody.Builder mpBuilder, String key, Object obj) {
+        RequestBody requestBody;
+        if (obj instanceof String) {
+            requestBody = RequestBody.create((String) obj, MediaType.parse("text/plain"));
+        } else {
+            String content;
+            if (obj != null) {
+                content = json.serialize(obj);
+            } else {
+                content = null;
+            }
+            requestBody = RequestBody.create(content, MediaType.parse("application/json"));
+        }
+
+        Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + key + "\"");
+        mpBuilder.addPart(partHeaders, requestBody);
+    }
+
     /**
      * Get network interceptor to add it to the httpClient to track download progress for
      * async requests.
@@ -1559,7 +1585,7 @@ public class ApiClient {
     /**
      * Convert the HTTP request body to a string.
      *
-     * @param request The HTTP request object
+     * @param requestBody The HTTP request object
      * @return The string representation of the HTTP request body
      * @throws org.openapitools.client.ApiException If fail to serialize the request body object into a string
      */