diff --git a/bin/utils/test_file_list.yaml b/bin/utils/test_file_list.yaml
index 3412fcdca713..5e0ea8a9e346 100644
--- a/bin/utils/test_file_list.yaml
+++ b/bin/utils/test_file_list.yaml
@@ -10,7 +10,7 @@
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ClientTest.java"
   sha256: 325fdd5d7e2c97790c0fb44f712ab7b2ba022d7e1a5b0056f47b07f342682b6d
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java"
-  sha256: 67941355a0a27ed9ff9318b1caa103e78b81b9aff61b594b18be5cd2bb9f6591
+  sha256: b1b1d31e0df17f0b68cf2747a4a53879f12acb1bf2860e45385c679c1efe9894
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/api/PetApiTest.java"
   sha256: 8b1b8f2a2ad00ccb090873a94a5f73e328b98317d2ec715f53bd7a1accb2a023
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/PetTest.java"
diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/pojo.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/pojo.mustache
index 881f2acf72d2..cb19eb603679 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/pojo.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/pojo.mustache
@@ -385,16 +385,16 @@ public class {{classname}} {{#parent}}extends {{{.}}} {{/parent}}{{#vendorExtens
       {{#isArray}}
       {{#items.isModel}}
       {{#required}}
-      // ensure the json data is an array
-      if (!jsonObj.get("{{{baseName}}}").isJsonArray()) {
-        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `{{{baseName}}}` to be an array in the JSON string but got `%s`", jsonObj.get("{{{baseName}}}").toString()));
+      if (jsonObj.get("{{{baseName}}}") != null{{#isNullable}} && !jsonObj.get("{{{baseName}}}").isJsonNull(){{/isNullable}}) {
+        if (!jsonObj.get("{{{baseName}}}").isJsonArray()) {
+          throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `{{{baseName}}}` to be an array in the JSON string but got `%s`", jsonObj.get("{{{baseName}}}").toString()));
+        }
+        JsonArray jsonArray{{name}} = jsonObj.getAsJsonArray("{{{baseName}}}");
+        // validate the required field `{{{baseName}}}` (array)
+        for (int i = 0; i < jsonArray{{name}}.size(); i++) {
+          {{{items.dataType}}}.validateJsonElement(jsonArray{{name}}.get(i));
+        }
       }
-
-      JsonArray jsonArray{{name}} = jsonObj.getAsJsonArray("{{{baseName}}}");
-      // validate the required field `{{{baseName}}}` (array)
-      for (int i = 0; i < jsonArray{{name}}.size(); i++) {
-        {{{items.dataType}}}.validateJsonElement(jsonArray{{name}}.get(i));
-      };
       {{/required}}
       {{^required}}
       if (jsonObj.get("{{{baseName}}}") != null && !jsonObj.get("{{{baseName}}}").isJsonNull()) {
@@ -424,7 +424,7 @@ public class {{classname}} {{#parent}}extends {{{.}}} {{/parent}}{{#vendorExtens
       // ensure the required json array is present
       if (jsonObj.get("{{{baseName}}}") == null) {
         throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
-      } else if (!jsonObj.get("{{{baseName}}}").isJsonArray()) {
+      } else if (!jsonObj.get("{{{baseName}}}").isJsonArray(){{#isNullable}} && !jsonObj.get("{{baseName}}").isJsonNull(){{/isNullable}}) {
         throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `{{{baseName}}}` to be an array in the JSON string but got `%s`", jsonObj.get("{{{baseName}}}").toString()));
       }
       {{/required}}
@@ -438,8 +438,14 @@ public class {{classname}} {{#parent}}extends {{{.}}} {{/parent}}{{#vendorExtens
       {{/isString}}
       {{#isModel}}
       {{#required}}
+      {{#isNullable}}
+      if (jsonObj.get("{{{baseName}}}") != null && !jsonObj.get("{{{baseName}}}").isJsonNull()) {
+      {{/isNullable}}
       // validate the required field `{{{baseName}}}`
       {{{dataType}}}.validateJsonElement(jsonObj.get("{{{baseName}}}"));
+      {{#isNullable}}
+      }
+      {{/isNullable}}
       {{/required}}
       {{^required}}
       // validate the optional field `{{{baseName}}}`
@@ -450,8 +456,14 @@ public class {{classname}} {{#parent}}extends {{{.}}} {{/parent}}{{#vendorExtens
       {{/isModel}}
       {{#isEnum}}
       {{#required}}
+      {{#isNullable}}
+      if (jsonObj.get("{{{baseName}}}") != null && !jsonObj.get("{{{baseName}}}").isJsonNull()) {
+      {{/isNullable}}
       // validate the required field `{{{baseName}}}`
       {{{datatypeWithEnum}}}.validateJsonElement(jsonObj.get("{{{baseName}}}"));
+      {{#isNullable}}
+      }
+      {{/isNullable}}
       {{/required}}
       {{^required}}
       // validate the optional field `{{{baseName}}}`
@@ -462,8 +474,14 @@ public class {{classname}} {{#parent}}extends {{{.}}} {{/parent}}{{#vendorExtens
       {{/isEnum}}
       {{#isEnumRef}}
       {{#required}}
+      {{#isNullable}}
+      if (jsonObj.get("{{{baseName}}}") != null && !jsonObj.get("{{{baseName}}}").isJsonNull()) {
+      {{/isNullable}}
       // validate the required field `{{{baseName}}}`
       {{{dataType}}}.validateJsonElement(jsonObj.get("{{{baseName}}}"));
+      {{#isNullable}}
+      }
+      {{/isNullable}}
       {{/required}}
       {{^required}}
       // validate the optional field `{{{baseName}}}`
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
index bb92138888da..94a0a6f4535e 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
@@ -3769,6 +3769,35 @@ public class JavaClientCodegenTest {
         );
     }
 
+    @Test
+    public void testRequiredAndNullableAreBothTrue() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setLibrary(JavaClientCodegen.OKHTTP_GSON)
+                .setInputSpec("src/test/resources/bugs/issue_18516.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        validateJavaSourceFiles(files);
+
+        Path modelFile = Paths.get(output + "/src/main/java/org/openapitools/client/model/SomeObject.java");
+        TestUtils.assertFileContains(
+                modelFile,
+                "} else if (!jsonObj.get(\"ids\").isJsonArray()  && !jsonObj.get(\"ids\").isJsonNull()) {",
+                "if (jsonObj.get(\"users\") != null && !jsonObj.get(\"users\").isJsonNull()) {",
+                "if (!jsonObj.get(\"users\").isJsonArray()) {",
+                "if (jsonObj.get(\"user\") != null && !jsonObj.get(\"user\").isJsonNull()) {",
+                "if (jsonObj.get(\"role\") != null && !jsonObj.get(\"role\").isJsonNull()) {",
+                "if (jsonObj.get(\"custom\") != null && !jsonObj.get(\"custom\").isJsonNull()) {");
+    }
+
     @Test(description = "Issue #21051")
     public void givenComplexObjectHasDefaultValueWhenGenerateThenDefaultAssignmentsAreValid() throws Exception {
         File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
diff --git a/modules/openapi-generator/src/test/resources/3_0/java/petstore-with-fake-endpoints-models-for-testing-okhttp-gson.yaml b/modules/openapi-generator/src/test/resources/3_0/java/petstore-with-fake-endpoints-models-for-testing-okhttp-gson.yaml
index 9cf24ac86351..bde436fcabcb 100644
--- a/modules/openapi-generator/src/test/resources/3_0/java/petstore-with-fake-endpoints-models-for-testing-okhttp-gson.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/java/petstore-with-fake-endpoints-models-for-testing-okhttp-gson.yaml
@@ -1369,6 +1369,19 @@ paths:
                 $ref: '#/components/schemas/Variable'
         '400':
           description: Invalid Value
+  /fake/required-nullable-body:
+    get:
+      tags:
+        - fake
+      summary: fields in the response body, required and nullable are both true
+      description: ''
+      responses:
+        '200':
+          description: success
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RequiredNullableBody'
 servers:
   - url: 'http://{server}.swagger.io:{port}/v2'
     description: petstore server
@@ -2768,6 +2781,38 @@ components:
           $ref: '#/components/schemas/ArrayOneOf'
         anyof_prop:
           $ref: '#/components/schemas/ArrayAnyOf'
+    NullableEnum:
+      type: string
+      nullable: true
+      enum:
+        - custom
+    RequiredNullableBody:
+      allOf:
+        - $ref: '#/components/schemas/NullableClass'
+        - type: object
+          required:
+            - custom_ref_enum
+            - custom_enum
+            - integer_prop
+            - number_prop
+            - boolean_prop
+            - string_prop
+            - date_prop
+            - datetime_prop
+            - array_nullable_prop
+            - array_and_items_nullable_prop
+            - array_items_nullable
+            - object_nullable_prop
+            - object_and_items_nullable_prop
+            - object_items_nullable
+          properties:
+            custom_ref_enum:
+              $ref: "#/components/schemas/NullableEnum"
+            custom_enum:
+              type: string
+              nullable: true
+              enum:
+                - custom
     NestedArrayWithDefaultValues:
       type: object
       properties:
diff --git a/modules/openapi-generator/src/test/resources/bugs/issue_18516.yaml b/modules/openapi-generator/src/test/resources/bugs/issue_18516.yaml
new file mode 100644
index 000000000000..fcefeb6b521f
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/bugs/issue_18516.yaml
@@ -0,0 +1,61 @@
+openapi: 3.0.3
+info:
+  title: test
+  description: Test API
+  version: 1.0.1
+
+paths:
+  /test:
+    get:
+      responses:
+        200:
+          description: Valid response
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/SomeObject"
+
+components:
+  schemas:
+    SomeObject:
+      type: object
+      required: 
+        - ids
+        - users
+        - user
+        - role
+        - custom
+      properties:
+        ids:
+          type: array
+          nullable: true
+          items:
+            type: integer
+        users:
+          type: array
+          nullable: true
+          items:
+            type: object
+            properties:
+              id:
+                type: string
+        user:
+          type: object
+          nullable: true
+          properties:
+            id:
+              type: string
+        role:
+          type: string
+          nullable: true
+          enum:
+            - admin
+            - tenant
+        custom:
+          $ref: "#/components/schemas/customEnum"
+    customEnum:
+      type: string
+      nullable: true
+      enum:
+        - custom
+        
\ No newline at end of file
diff --git a/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java b/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java
index 6e11c8a5366b..387783611b19 100644
--- a/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java
+++ b/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java
@@ -424,7 +424,7 @@ public class PetWithRequiredNullableCases1 {
       // ensure the required json array is present
       if (jsonObj.get("photoUrls") == null) {
         throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
-      } else if (!jsonObj.get("photoUrls").isJsonArray()) {
+      } else if (!jsonObj.get("photoUrls").isJsonArray() && !jsonObj.get("photoUrls").isJsonNull()) {
         throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `photoUrls` to be an array in the JSON string but got `%s`", jsonObj.get("photoUrls").toString()));
       }
       if (jsonObj.get("tags") != null && !jsonObj.get("tags").isJsonNull()) {
diff --git a/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES b/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES
index 9de4894ad1e8..c64d152e6706 100644
--- a/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES
+++ b/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES
@@ -79,6 +79,7 @@ docs/NewPet.md
 docs/NewPetCategoryInlineAllof.md
 docs/NewPetCategoryInlineAllofAllOfCategoryTag.md
 docs/NullableClass.md
+docs/NullableEnum.md
 docs/NullableShape.md
 docs/NumberOnly.md
 docs/ObjectWithDeprecatedFields.md
@@ -100,6 +101,7 @@ docs/PropertyNameCollision.md
 docs/Quadrilateral.md
 docs/QuadrilateralInterface.md
 docs/ReadOnlyFirst.md
+docs/RequiredNullableBody.md
 docs/Scalar.md
 docs/ScalarAnyOf.md
 docs/ScaleneTriangle.md
@@ -228,6 +230,7 @@ src/main/java/org/openapitools/client/model/NewPet.java
 src/main/java/org/openapitools/client/model/NewPetCategoryInlineAllof.java
 src/main/java/org/openapitools/client/model/NewPetCategoryInlineAllofAllOfCategoryTag.java
 src/main/java/org/openapitools/client/model/NullableClass.java
+src/main/java/org/openapitools/client/model/NullableEnum.java
 src/main/java/org/openapitools/client/model/NullableShape.java
 src/main/java/org/openapitools/client/model/NumberOnly.java
 src/main/java/org/openapitools/client/model/ObjectWithDeprecatedFields.java
@@ -248,6 +251,7 @@ src/main/java/org/openapitools/client/model/PropertyNameCollision.java
 src/main/java/org/openapitools/client/model/Quadrilateral.java
 src/main/java/org/openapitools/client/model/QuadrilateralInterface.java
 src/main/java/org/openapitools/client/model/ReadOnlyFirst.java
+src/main/java/org/openapitools/client/model/RequiredNullableBody.java
 src/main/java/org/openapitools/client/model/Scalar.java
 src/main/java/org/openapitools/client/model/ScalarAnyOf.java
 src/main/java/org/openapitools/client/model/ScaleneTriangle.java
diff --git a/samples/client/petstore/java/okhttp-gson/README.md b/samples/client/petstore/java/okhttp-gson/README.md
index 588113b62f87..b39145bd144d 100644
--- a/samples/client/petstore/java/okhttp-gson/README.md
+++ b/samples/client/petstore/java/okhttp-gson/README.md
@@ -127,6 +127,7 @@ Class | Method | HTTP request | Description
 *FakeApi* | [**fakeOuterNumberSerialize**](docs/FakeApi.md#fakeOuterNumberSerialize) | **POST** /fake/outer/number | 
 *FakeApi* | [**fakeOuterStringSerialize**](docs/FakeApi.md#fakeOuterStringSerialize) | **POST** /fake/outer/string | 
 *FakeApi* | [**fakeRefParameter**](docs/FakeApi.md#fakeRefParameter) | **POST** /fake/pet/{petId}/reference/parameter | fake reference parameter
+*FakeApi* | [**fakeRequiredNullableBodyGet**](docs/FakeApi.md#fakeRequiredNullableBodyGet) | **GET** /fake/required-nullable-body | fields in the response body, required and nullable are both true
 *FakeApi* | [**fakeUploadRefRequestBodies**](docs/FakeApi.md#fakeUploadRefRequestBodies) | **POST** /fake/pet/{petId}/uploadImage | fake reference parameter
 *FakeApi* | [**getFakeArrayofenums**](docs/FakeApi.md#getFakeArrayofenums) | **GET** /fake/array-of-enums | Array of Enums
 *FakeApi* | [**getFakeHealth**](docs/FakeApi.md#getFakeHealth) | **GET** /fake/health | Health check endpoint
@@ -240,6 +241,7 @@ Class | Method | HTTP request | Description
  - [NewPetCategoryInlineAllof](docs/NewPetCategoryInlineAllof.md)
  - [NewPetCategoryInlineAllofAllOfCategoryTag](docs/NewPetCategoryInlineAllofAllOfCategoryTag.md)
  - [NullableClass](docs/NullableClass.md)
+ - [NullableEnum](docs/NullableEnum.md)
  - [NullableShape](docs/NullableShape.md)
  - [NumberOnly](docs/NumberOnly.md)
  - [ObjectWithDeprecatedFields](docs/ObjectWithDeprecatedFields.md)
@@ -260,6 +262,7 @@ Class | Method | HTTP request | Description
  - [Quadrilateral](docs/Quadrilateral.md)
  - [QuadrilateralInterface](docs/QuadrilateralInterface.md)
  - [ReadOnlyFirst](docs/ReadOnlyFirst.md)
+ - [RequiredNullableBody](docs/RequiredNullableBody.md)
  - [Scalar](docs/Scalar.md)
  - [ScalarAnyOf](docs/ScalarAnyOf.md)
  - [ScaleneTriangle](docs/ScaleneTriangle.md)
diff --git a/samples/client/petstore/java/okhttp-gson/api/openapi.yaml b/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
index 5d897fbe1465..ef5aa7ad125b 100644
--- a/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
+++ b/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
@@ -1494,6 +1494,21 @@ paths:
       x-internal: true
       x-accepts:
       - application/json
+  /fake/required-nullable-body:
+    get:
+      description: ""
+      responses:
+        "200":
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/RequiredNullableBody"
+          description: success
+      summary: "fields in the response body, required and nullable are both true"
+      tags:
+      - fake
+      x-accepts:
+      - application/json
 components:
   parameters:
     pet_id:
@@ -2870,6 +2885,62 @@ components:
         anyof_prop:
           $ref: "#/components/schemas/ArrayAnyOf"
       type: object
+    NullableEnum:
+      enum:
+      - custom
+      nullable: true
+      type: string
+    RequiredNullableBody:
+      allOf:
+      - $ref: "#/components/schemas/NullableClass"
+      - properties:
+          custom_ref_enum:
+            $ref: "#/components/schemas/NullableEnum"
+          custom_enum:
+            enum:
+            - custom
+            nullable: true
+            type: string
+        required:
+        - array_and_items_nullable_prop
+        - array_items_nullable
+        - array_nullable_prop
+        - boolean_prop
+        - custom_enum
+        - custom_ref_enum
+        - date_prop
+        - datetime_prop
+        - integer_prop
+        - number_prop
+        - object_and_items_nullable_prop
+        - object_items_nullable
+        - object_nullable_prop
+        - string_prop
+        type: object
+      example:
+        number_prop: 6.027456183070403
+        datetime_prop: 2000-01-23T04:56:07.000+00:00
+        custom_ref_enum: custom
+        boolean_prop: true
+        string_prop: string_prop
+        array_nullable_prop:
+        - "{}"
+        - "{}"
+        custom_enum: custom
+        integer_prop: 0
+        array_and_items_nullable_prop:
+        - "{}"
+        - "{}"
+        object_items_nullable:
+          key: "{}"
+        object_nullable_prop:
+          key: "{}"
+        object_and_items_nullable_prop:
+          key: "{}"
+        date_prop: 2000-01-23
+        array_items_nullable:
+        - "{}"
+        - "{}"
     NestedArrayWithDefaultValues:
       properties:
         nestedArray:
diff --git a/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md b/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md
index eb2f76491a83..4a9b877cd205 100644
--- a/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md
+++ b/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md
@@ -10,6 +10,7 @@ All URIs are relative to *http://petstore.swagger.io:80/v2*
 | [**fakeOuterNumberSerialize**](FakeApi.md#fakeOuterNumberSerialize) | **POST** /fake/outer/number |  |
 | [**fakeOuterStringSerialize**](FakeApi.md#fakeOuterStringSerialize) | **POST** /fake/outer/string |  |
 | [**fakeRefParameter**](FakeApi.md#fakeRefParameter) | **POST** /fake/pet/{petId}/reference/parameter | fake reference parameter |
+| [**fakeRequiredNullableBodyGet**](FakeApi.md#fakeRequiredNullableBodyGet) | **GET** /fake/required-nullable-body | fields in the response body, required and nullable are both true |
 | [**fakeUploadRefRequestBodies**](FakeApi.md#fakeUploadRefRequestBodies) | **POST** /fake/pet/{petId}/uploadImage | fake reference parameter |
 | [**getFakeArrayofenums**](FakeApi.md#getFakeArrayofenums) | **GET** /fake/array-of-enums | Array of Enums |
 | [**getFakeHealth**](FakeApi.md#getFakeHealth) | **GET** /fake/health | Health check endpoint |
@@ -395,6 +396,64 @@ No authorization required
 |-------------|-------------|------------------|
 | **200** | successful operation |  -  |
 
+<a id="fakeRequiredNullableBodyGet"></a>
+# **fakeRequiredNullableBodyGet**
+> RequiredNullableBody fakeRequiredNullableBodyGet()
+
+fields in the response body, required and nullable are both true
+
+
+
+### Example
+```java
+// Import classes:
+import org.openapitools.client.ApiClient;
+import org.openapitools.client.ApiException;
+import org.openapitools.client.Configuration;
+import org.openapitools.client.models.*;
+import org.openapitools.client.api.FakeApi;
+
+public class Example {
+  public static void main(String[] args) {
+    ApiClient defaultClient = Configuration.getDefaultApiClient();
+    defaultClient.setBasePath("http://petstore.swagger.io:80/v2");
+
+    FakeApi apiInstance = new FakeApi(defaultClient);
+    try {
+      RequiredNullableBody result = apiInstance.fakeRequiredNullableBodyGet();
+      System.out.println(result);
+    } catch (ApiException e) {
+      System.err.println("Exception when calling FakeApi#fakeRequiredNullableBodyGet");
+      System.err.println("Status code: " + e.getCode());
+      System.err.println("Reason: " + e.getResponseBody());
+      System.err.println("Response headers: " + e.getResponseHeaders());
+      e.printStackTrace();
+    }
+  }
+}
+```
+
+### Parameters
+This endpoint does not need any parameter.
+
+### Return type
+
+[**RequiredNullableBody**](RequiredNullableBody.md)
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: Not defined
+ - **Accept**: application/json
+
+### HTTP response details
+| Status code | Description | Response headers |
+|-------------|-------------|------------------|
+| **200** | success |  -  |
+
 <a id="fakeUploadRefRequestBodies"></a>
 # **fakeUploadRefRequestBodies**
 > ModelApiResponse fakeUploadRefRequestBodies(petId, additionalMetadata, _file)
diff --git a/samples/client/petstore/java/okhttp-gson/docs/NullableEnum.md b/samples/client/petstore/java/okhttp-gson/docs/NullableEnum.md
new file mode 100644
index 000000000000..774c8d6b1d70
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/docs/NullableEnum.md
@@ -0,0 +1,11 @@
+
+
+# NullableEnum
+
+## Enum
+
+
+* `CUSTOM` (value: `"custom"`)
+
+
+
diff --git a/samples/client/petstore/java/okhttp-gson/docs/RequiredNullableBody.md b/samples/client/petstore/java/okhttp-gson/docs/RequiredNullableBody.md
new file mode 100644
index 000000000000..b732ff05be76
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/docs/RequiredNullableBody.md
@@ -0,0 +1,34 @@
+
+
+# RequiredNullableBody
+
+
+## Properties
+
+| Name | Type | Description | Notes |
+|------------ | ------------- | ------------- | -------------|
+|**integerProp** | **Integer** |  |  |
+|**numberProp** | **BigDecimal** |  |  |
+|**booleanProp** | **Boolean** |  |  |
+|**stringProp** | **String** |  |  |
+|**dateProp** | **LocalDate** |  |  |
+|**datetimeProp** | **OffsetDateTime** |  |  |
+|**arrayNullableProp** | **List&lt;Object&gt;** |  |  |
+|**arrayAndItemsNullableProp** | **List&lt;Object&gt;** |  |  |
+|**arrayItemsNullable** | **List&lt;Object&gt;** |  |  |
+|**objectNullableProp** | **Map&lt;String, Object&gt;** |  |  |
+|**objectAndItemsNullableProp** | **Map&lt;String, Object&gt;** |  |  |
+|**objectItemsNullable** | **Map&lt;String, Object&gt;** |  |  |
+|**customRefEnum** | **NullableEnum** |  |  |
+|**customEnum** | [**CustomEnumEnum**](#CustomEnumEnum) |  |  |
+
+
+
+## Enum: CustomEnumEnum
+
+| Name | Value |
+|---- | -----|
+| CUSTOM | &quot;custom&quot; |
+
+
+
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/JSON.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/JSON.java
index 5aaa5ac339df..98b9c070691b 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/JSON.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/JSON.java
@@ -327,6 +327,7 @@ public class JSON {
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Quadrilateral.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.QuadrilateralInterface.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.ReadOnlyFirst.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.RequiredNullableBody.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Scalar.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.ScalarAnyOf.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.ScaleneTriangle.CustomTypeAdapterFactory());
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/api/FakeApi.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/api/FakeApi.java
index 7055253c1a0b..eea2b5f01fce 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/api/FakeApi.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/api/FakeApi.java
@@ -39,6 +39,7 @@ import org.openapitools.client.model.ModelApiResponse;
 import java.time.OffsetDateTime;
 import org.openapitools.client.model.OuterComposite;
 import org.openapitools.client.model.OuterEnum;
+import org.openapitools.client.model.RequiredNullableBody;
 import org.openapitools.client.model.TestInlineFreeformAdditionalPropertiesRequest;
 import org.openapitools.client.model.User;
 
@@ -812,6 +813,123 @@ public class FakeApi {
         localVarApiClient.executeAsync(localVarCall, _callback);
         return localVarCall;
     }
+    /**
+     * Build call for fakeRequiredNullableBodyGet
+     * @param _callback Callback for upload/download progress
+     * @return Call to execute
+     * @throws ApiException If fail to serialize the request body object
+     * @http.response.details
+     <table border="1">
+       <caption>Response Details</caption>
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> success </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call fakeRequiredNullableBodyGetCall(final ApiCallback _callback) throws ApiException {
+        String basePath = null;
+        // Operation Servers
+        String[] localBasePaths = new String[] {  };
+
+        // Determine Base Path to Use
+        if (localCustomBaseUrl != null){
+            basePath = localCustomBaseUrl;
+        } else if ( localBasePaths.length > 0 ) {
+            basePath = localBasePaths[localHostIndex];
+        } else {
+            basePath = null;
+        }
+
+        Object localVarPostBody = null;
+
+        // create path and map variables
+        String localVarPath = "/fake/required-nullable-body";
+
+        List<Pair> localVarQueryParams = new ArrayList<Pair>();
+        List<Pair> localVarCollectionQueryParams = new ArrayList<Pair>();
+        Map<String, String> localVarHeaderParams = new HashMap<String, String>();
+        Map<String, String> localVarCookieParams = new HashMap<String, String>();
+        Map<String, Object> localVarFormParams = new HashMap<String, Object>();
+
+        final String[] localVarAccepts = {
+            "application/json"
+        };
+        final String localVarAccept = localVarApiClient.selectHeaderAccept(localVarAccepts);
+        if (localVarAccept != null) {
+            localVarHeaderParams.put("Accept", localVarAccept);
+        }
+
+        final String[] localVarContentTypes = {
+        };
+        final String localVarContentType = localVarApiClient.selectHeaderContentType(localVarContentTypes);
+        if (localVarContentType != null) {
+            localVarHeaderParams.put("Content-Type", localVarContentType);
+        }
+
+        String[] localVarAuthNames = new String[] {  };
+        return localVarApiClient.buildCall(basePath, localVarPath, "GET", localVarQueryParams, localVarCollectionQueryParams, localVarPostBody, localVarHeaderParams, localVarCookieParams, localVarFormParams, localVarAuthNames, _callback);
+    }
+
+    @SuppressWarnings("rawtypes")
+    private okhttp3.Call fakeRequiredNullableBodyGetValidateBeforeCall(final ApiCallback _callback) throws ApiException {
+        return fakeRequiredNullableBodyGetCall(_callback);
+
+    }
+
+    /**
+     * fields in the response body, required and nullable are both true
+     * 
+     * @return RequiredNullableBody
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table border="1">
+       <caption>Response Details</caption>
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> success </td><td>  -  </td></tr>
+     </table>
+     */
+    public RequiredNullableBody fakeRequiredNullableBodyGet() throws ApiException {
+        ApiResponse<RequiredNullableBody> localVarResp = fakeRequiredNullableBodyGetWithHttpInfo();
+        return localVarResp.getData();
+    }
+
+    /**
+     * fields in the response body, required and nullable are both true
+     * 
+     * @return ApiResponse&lt;RequiredNullableBody&gt;
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table border="1">
+       <caption>Response Details</caption>
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> success </td><td>  -  </td></tr>
+     </table>
+     */
+    public ApiResponse<RequiredNullableBody> fakeRequiredNullableBodyGetWithHttpInfo() throws ApiException {
+        okhttp3.Call localVarCall = fakeRequiredNullableBodyGetValidateBeforeCall(null);
+        Type localVarReturnType = new TypeToken<RequiredNullableBody>(){}.getType();
+        return localVarApiClient.execute(localVarCall, localVarReturnType);
+    }
+
+    /**
+     * fields in the response body, required and nullable are both true (asynchronously)
+     * 
+     * @param _callback The callback to be executed when the API call finishes
+     * @return The request call
+     * @throws ApiException If fail to process the API call, e.g. serializing the request body object
+     * @http.response.details
+     <table border="1">
+       <caption>Response Details</caption>
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> success </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call fakeRequiredNullableBodyGetAsync(final ApiCallback<RequiredNullableBody> _callback) throws ApiException {
+
+        okhttp3.Call localVarCall = fakeRequiredNullableBodyGetValidateBeforeCall(_callback);
+        Type localVarReturnType = new TypeToken<RequiredNullableBody>(){}.getType();
+        localVarApiClient.executeAsync(localVarCall, localVarReturnType, _callback);
+        return localVarCall;
+    }
     /**
      * Build call for fakeUploadRefRequestBodies
      * @param petId ID of pet to update (required)
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AllOfModelArrayAnyOfAllOfLinkListColumn1.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AllOfModelArrayAnyOfAllOfLinkListColumn1.java
index 0039137bb4f3..5fa05e48a089 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AllOfModelArrayAnyOfAllOfLinkListColumn1.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AllOfModelArrayAnyOfAllOfLinkListColumn1.java
@@ -204,16 +204,16 @@ public class AllOfModelArrayAnyOfAllOfLinkListColumn1 {
         }
       }
         JsonObject jsonObj = jsonElement.getAsJsonObject();
-      // ensure the json data is an array
-      if (!jsonObj.get("value").isJsonArray()) {
-        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `value` to be an array in the JSON string but got `%s`", jsonObj.get("value").toString()));
+      if (jsonObj.get("value") != null) {
+        if (!jsonObj.get("value").isJsonArray()) {
+          throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `value` to be an array in the JSON string but got `%s`", jsonObj.get("value").toString()));
+        }
+        JsonArray jsonArrayvalue = jsonObj.getAsJsonArray("value");
+        // validate the required field `value` (array)
+        for (int i = 0; i < jsonArrayvalue.size(); i++) {
+          AllOfModelArrayAnyOfAllOfLinkListColumn1Value.validateJsonElement(jsonArrayvalue.get(i));
+        }
       }
-
-      JsonArray jsonArrayvalue = jsonObj.getAsJsonArray("value");
-      // validate the required field `value` (array)
-      for (int i = 0; i < jsonArrayvalue.size(); i++) {
-        AllOfModelArrayAnyOfAllOfLinkListColumn1Value.validateJsonElement(jsonArrayvalue.get(i));
-      };
   }
 
   public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/NullableEnum.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/NullableEnum.java
new file mode 100644
index 000000000000..d28af0a9d1ea
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/NullableEnum.java
@@ -0,0 +1,76 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import com.google.gson.annotations.SerializedName;
+
+import java.io.IOException;
+import com.google.gson.TypeAdapter;
+import com.google.gson.JsonElement;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+
+/**
+ * Gets or Sets NullableEnum
+ */
+@JsonAdapter(NullableEnum.Adapter.class)
+public enum NullableEnum {
+  
+  CUSTOM("custom");
+
+  private String value;
+
+  NullableEnum(String value) {
+    this.value = value;
+  }
+
+  public String getValue() {
+    return value;
+  }
+
+  @Override
+  public String toString() {
+    return String.valueOf(value);
+  }
+
+  public static NullableEnum fromValue(String value) {
+    for (NullableEnum b : NullableEnum.values()) {
+      if (b.value.equals(value)) {
+        return b;
+      }
+    }
+    return null;
+  }
+
+  public static class Adapter extends TypeAdapter<NullableEnum> {
+    @Override
+    public void write(final JsonWriter jsonWriter, final NullableEnum enumeration) throws IOException {
+      jsonWriter.value(enumeration.getValue());
+    }
+
+    @Override
+    public NullableEnum read(final JsonReader jsonReader) throws IOException {
+      String value = jsonReader.nextString();
+      return NullableEnum.fromValue(value);
+    }
+  }
+
+  public static void validateJsonElement(JsonElement jsonElement) throws IOException {
+    String value = jsonElement.getAsString();
+    NullableEnum.fromValue(value);
+  }
+}
+
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/PetWithRequiredTags.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/PetWithRequiredTags.java
index 041a975c7de0..78182b25bd62 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/PetWithRequiredTags.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/PetWithRequiredTags.java
@@ -410,16 +410,16 @@ public class PetWithRequiredTags {
       } else if (!jsonObj.get("photoUrls").isJsonArray()) {
         throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `photoUrls` to be an array in the JSON string but got `%s`", jsonObj.get("photoUrls").toString()));
       }
-      // ensure the json data is an array
-      if (!jsonObj.get("tags").isJsonArray()) {
-        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `tags` to be an array in the JSON string but got `%s`", jsonObj.get("tags").toString()));
+      if (jsonObj.get("tags") != null) {
+        if (!jsonObj.get("tags").isJsonArray()) {
+          throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `tags` to be an array in the JSON string but got `%s`", jsonObj.get("tags").toString()));
+        }
+        JsonArray jsonArraytags = jsonObj.getAsJsonArray("tags");
+        // validate the required field `tags` (array)
+        for (int i = 0; i < jsonArraytags.size(); i++) {
+          Tag.validateJsonElement(jsonArraytags.get(i));
+        }
       }
-
-      JsonArray jsonArraytags = jsonObj.getAsJsonArray("tags");
-      // validate the required field `tags` (array)
-      for (int i = 0; i < jsonArraytags.size(); i++) {
-        Tag.validateJsonElement(jsonArraytags.get(i));
-      };
       if ((jsonObj.get("status") != null && !jsonObj.get("status").isJsonNull()) && !jsonObj.get("status").isJsonPrimitive()) {
         throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `status` to be a primitive type in the JSON string but got `%s`", jsonObj.get("status").toString()));
       }
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/RequiredNullableBody.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/RequiredNullableBody.java
new file mode 100644
index 000000000000..0cd332f2fc81
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/RequiredNullableBody.java
@@ -0,0 +1,766 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.math.BigDecimal;
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import org.openapitools.client.model.NullableEnum;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * RequiredNullableBody
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.20.0-SNAPSHOT")
+public class RequiredNullableBody {
+  public static final String SERIALIZED_NAME_INTEGER_PROP = "integer_prop";
+  @SerializedName(SERIALIZED_NAME_INTEGER_PROP)
+  @javax.annotation.Nullable
+  private Integer integerProp;
+
+  public static final String SERIALIZED_NAME_NUMBER_PROP = "number_prop";
+  @SerializedName(SERIALIZED_NAME_NUMBER_PROP)
+  @javax.annotation.Nullable
+  private BigDecimal numberProp;
+
+  public static final String SERIALIZED_NAME_BOOLEAN_PROP = "boolean_prop";
+  @SerializedName(SERIALIZED_NAME_BOOLEAN_PROP)
+  @javax.annotation.Nullable
+  private Boolean booleanProp;
+
+  public static final String SERIALIZED_NAME_STRING_PROP = "string_prop";
+  @SerializedName(SERIALIZED_NAME_STRING_PROP)
+  @javax.annotation.Nullable
+  private String stringProp;
+
+  public static final String SERIALIZED_NAME_DATE_PROP = "date_prop";
+  @SerializedName(SERIALIZED_NAME_DATE_PROP)
+  @javax.annotation.Nullable
+  private LocalDate dateProp;
+
+  public static final String SERIALIZED_NAME_DATETIME_PROP = "datetime_prop";
+  @SerializedName(SERIALIZED_NAME_DATETIME_PROP)
+  @javax.annotation.Nullable
+  private OffsetDateTime datetimeProp;
+
+  public static final String SERIALIZED_NAME_ARRAY_NULLABLE_PROP = "array_nullable_prop";
+  @SerializedName(SERIALIZED_NAME_ARRAY_NULLABLE_PROP)
+  @javax.annotation.Nullable
+  private List<Object> arrayNullableProp;
+
+  public static final String SERIALIZED_NAME_ARRAY_AND_ITEMS_NULLABLE_PROP = "array_and_items_nullable_prop";
+  @SerializedName(SERIALIZED_NAME_ARRAY_AND_ITEMS_NULLABLE_PROP)
+  @javax.annotation.Nullable
+  private List<Object> arrayAndItemsNullableProp;
+
+  public static final String SERIALIZED_NAME_ARRAY_ITEMS_NULLABLE = "array_items_nullable";
+  @SerializedName(SERIALIZED_NAME_ARRAY_ITEMS_NULLABLE)
+  @javax.annotation.Nonnull
+  private List<Object> arrayItemsNullable = new ArrayList<>();
+
+  public static final String SERIALIZED_NAME_OBJECT_NULLABLE_PROP = "object_nullable_prop";
+  @SerializedName(SERIALIZED_NAME_OBJECT_NULLABLE_PROP)
+  @javax.annotation.Nullable
+  private Map<String, Object> objectNullableProp;
+
+  public static final String SERIALIZED_NAME_OBJECT_AND_ITEMS_NULLABLE_PROP = "object_and_items_nullable_prop";
+  @SerializedName(SERIALIZED_NAME_OBJECT_AND_ITEMS_NULLABLE_PROP)
+  @javax.annotation.Nullable
+  private Map<String, Object> objectAndItemsNullableProp;
+
+  public static final String SERIALIZED_NAME_OBJECT_ITEMS_NULLABLE = "object_items_nullable";
+  @SerializedName(SERIALIZED_NAME_OBJECT_ITEMS_NULLABLE)
+  @javax.annotation.Nonnull
+  private Map<String, Object> objectItemsNullable = new HashMap<>();
+
+  public static final String SERIALIZED_NAME_CUSTOM_REF_ENUM = "custom_ref_enum";
+  @SerializedName(SERIALIZED_NAME_CUSTOM_REF_ENUM)
+  @javax.annotation.Nullable
+  private NullableEnum customRefEnum;
+
+  /**
+   * Gets or Sets customEnum
+   */
+  @JsonAdapter(CustomEnumEnum.Adapter.class)
+  public enum CustomEnumEnum {
+    CUSTOM("custom");
+
+    private String value;
+
+    CustomEnumEnum(String value) {
+      this.value = value;
+    }
+
+    public String getValue() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return String.valueOf(value);
+    }
+
+    public static CustomEnumEnum fromValue(String value) {
+      for (CustomEnumEnum b : CustomEnumEnum.values()) {
+        if (b.value.equals(value)) {
+          return b;
+        }
+      }
+      return null;
+    }
+
+    public static class Adapter extends TypeAdapter<CustomEnumEnum> {
+      @Override
+      public void write(final JsonWriter jsonWriter, final CustomEnumEnum enumeration) throws IOException {
+        jsonWriter.value(enumeration.getValue());
+      }
+
+      @Override
+      public CustomEnumEnum read(final JsonReader jsonReader) throws IOException {
+        String value =  jsonReader.nextString();
+        return CustomEnumEnum.fromValue(value);
+      }
+    }
+
+    public static void validateJsonElement(JsonElement jsonElement) throws IOException {
+      String value = jsonElement.getAsString();
+      CustomEnumEnum.fromValue(value);
+    }
+  }
+
+  public static final String SERIALIZED_NAME_CUSTOM_ENUM = "custom_enum";
+  @SerializedName(SERIALIZED_NAME_CUSTOM_ENUM)
+  @javax.annotation.Nullable
+  private CustomEnumEnum customEnum;
+
+  public RequiredNullableBody() {
+  }
+
+  public RequiredNullableBody integerProp(@javax.annotation.Nullable Integer integerProp) {
+    this.integerProp = integerProp;
+    return this;
+  }
+
+  /**
+   * Get integerProp
+   * @return integerProp
+   */
+  @javax.annotation.Nullable
+  public Integer getIntegerProp() {
+    return integerProp;
+  }
+
+  public void setIntegerProp(@javax.annotation.Nullable Integer integerProp) {
+    this.integerProp = integerProp;
+  }
+
+
+  public RequiredNullableBody numberProp(@javax.annotation.Nullable BigDecimal numberProp) {
+    this.numberProp = numberProp;
+    return this;
+  }
+
+  /**
+   * Get numberProp
+   * @return numberProp
+   */
+  @javax.annotation.Nullable
+  public BigDecimal getNumberProp() {
+    return numberProp;
+  }
+
+  public void setNumberProp(@javax.annotation.Nullable BigDecimal numberProp) {
+    this.numberProp = numberProp;
+  }
+
+
+  public RequiredNullableBody booleanProp(@javax.annotation.Nullable Boolean booleanProp) {
+    this.booleanProp = booleanProp;
+    return this;
+  }
+
+  /**
+   * Get booleanProp
+   * @return booleanProp
+   */
+  @javax.annotation.Nullable
+  public Boolean getBooleanProp() {
+    return booleanProp;
+  }
+
+  public void setBooleanProp(@javax.annotation.Nullable Boolean booleanProp) {
+    this.booleanProp = booleanProp;
+  }
+
+
+  public RequiredNullableBody stringProp(@javax.annotation.Nullable String stringProp) {
+    this.stringProp = stringProp;
+    return this;
+  }
+
+  /**
+   * Get stringProp
+   * @return stringProp
+   */
+  @javax.annotation.Nullable
+  public String getStringProp() {
+    return stringProp;
+  }
+
+  public void setStringProp(@javax.annotation.Nullable String stringProp) {
+    this.stringProp = stringProp;
+  }
+
+
+  public RequiredNullableBody dateProp(@javax.annotation.Nullable LocalDate dateProp) {
+    this.dateProp = dateProp;
+    return this;
+  }
+
+  /**
+   * Get dateProp
+   * @return dateProp
+   */
+  @javax.annotation.Nullable
+  public LocalDate getDateProp() {
+    return dateProp;
+  }
+
+  public void setDateProp(@javax.annotation.Nullable LocalDate dateProp) {
+    this.dateProp = dateProp;
+  }
+
+
+  public RequiredNullableBody datetimeProp(@javax.annotation.Nullable OffsetDateTime datetimeProp) {
+    this.datetimeProp = datetimeProp;
+    return this;
+  }
+
+  /**
+   * Get datetimeProp
+   * @return datetimeProp
+   */
+  @javax.annotation.Nullable
+  public OffsetDateTime getDatetimeProp() {
+    return datetimeProp;
+  }
+
+  public void setDatetimeProp(@javax.annotation.Nullable OffsetDateTime datetimeProp) {
+    this.datetimeProp = datetimeProp;
+  }
+
+
+  public RequiredNullableBody arrayNullableProp(@javax.annotation.Nullable List<Object> arrayNullableProp) {
+    this.arrayNullableProp = arrayNullableProp;
+    return this;
+  }
+
+  public RequiredNullableBody addArrayNullablePropItem(Object arrayNullablePropItem) {
+    if (this.arrayNullableProp == null) {
+      this.arrayNullableProp = new ArrayList<>();
+    }
+    this.arrayNullableProp.add(arrayNullablePropItem);
+    return this;
+  }
+
+  /**
+   * Get arrayNullableProp
+   * @return arrayNullableProp
+   */
+  @javax.annotation.Nullable
+  public List<Object> getArrayNullableProp() {
+    return arrayNullableProp;
+  }
+
+  public void setArrayNullableProp(@javax.annotation.Nullable List<Object> arrayNullableProp) {
+    this.arrayNullableProp = arrayNullableProp;
+  }
+
+
+  public RequiredNullableBody arrayAndItemsNullableProp(@javax.annotation.Nullable List<Object> arrayAndItemsNullableProp) {
+    this.arrayAndItemsNullableProp = arrayAndItemsNullableProp;
+    return this;
+  }
+
+  public RequiredNullableBody addArrayAndItemsNullablePropItem(Object arrayAndItemsNullablePropItem) {
+    if (this.arrayAndItemsNullableProp == null) {
+      this.arrayAndItemsNullableProp = new ArrayList<>();
+    }
+    this.arrayAndItemsNullableProp.add(arrayAndItemsNullablePropItem);
+    return this;
+  }
+
+  /**
+   * Get arrayAndItemsNullableProp
+   * @return arrayAndItemsNullableProp
+   */
+  @javax.annotation.Nullable
+  public List<Object> getArrayAndItemsNullableProp() {
+    return arrayAndItemsNullableProp;
+  }
+
+  public void setArrayAndItemsNullableProp(@javax.annotation.Nullable List<Object> arrayAndItemsNullableProp) {
+    this.arrayAndItemsNullableProp = arrayAndItemsNullableProp;
+  }
+
+
+  public RequiredNullableBody arrayItemsNullable(@javax.annotation.Nonnull List<Object> arrayItemsNullable) {
+    this.arrayItemsNullable = arrayItemsNullable;
+    return this;
+  }
+
+  public RequiredNullableBody addArrayItemsNullableItem(Object arrayItemsNullableItem) {
+    if (this.arrayItemsNullable == null) {
+      this.arrayItemsNullable = new ArrayList<>();
+    }
+    this.arrayItemsNullable.add(arrayItemsNullableItem);
+    return this;
+  }
+
+  /**
+   * Get arrayItemsNullable
+   * @return arrayItemsNullable
+   */
+  @javax.annotation.Nonnull
+  public List<Object> getArrayItemsNullable() {
+    return arrayItemsNullable;
+  }
+
+  public void setArrayItemsNullable(@javax.annotation.Nonnull List<Object> arrayItemsNullable) {
+    this.arrayItemsNullable = arrayItemsNullable;
+  }
+
+
+  public RequiredNullableBody objectNullableProp(@javax.annotation.Nullable Map<String, Object> objectNullableProp) {
+    this.objectNullableProp = objectNullableProp;
+    return this;
+  }
+
+  public RequiredNullableBody putObjectNullablePropItem(String key, Object objectNullablePropItem) {
+    if (this.objectNullableProp == null) {
+      this.objectNullableProp = new HashMap<>();
+    }
+    this.objectNullableProp.put(key, objectNullablePropItem);
+    return this;
+  }
+
+  /**
+   * Get objectNullableProp
+   * @return objectNullableProp
+   */
+  @javax.annotation.Nullable
+  public Map<String, Object> getObjectNullableProp() {
+    return objectNullableProp;
+  }
+
+  public void setObjectNullableProp(@javax.annotation.Nullable Map<String, Object> objectNullableProp) {
+    this.objectNullableProp = objectNullableProp;
+  }
+
+
+  public RequiredNullableBody objectAndItemsNullableProp(@javax.annotation.Nullable Map<String, Object> objectAndItemsNullableProp) {
+    this.objectAndItemsNullableProp = objectAndItemsNullableProp;
+    return this;
+  }
+
+  public RequiredNullableBody putObjectAndItemsNullablePropItem(String key, Object objectAndItemsNullablePropItem) {
+    if (this.objectAndItemsNullableProp == null) {
+      this.objectAndItemsNullableProp = new HashMap<>();
+    }
+    this.objectAndItemsNullableProp.put(key, objectAndItemsNullablePropItem);
+    return this;
+  }
+
+  /**
+   * Get objectAndItemsNullableProp
+   * @return objectAndItemsNullableProp
+   */
+  @javax.annotation.Nullable
+  public Map<String, Object> getObjectAndItemsNullableProp() {
+    return objectAndItemsNullableProp;
+  }
+
+  public void setObjectAndItemsNullableProp(@javax.annotation.Nullable Map<String, Object> objectAndItemsNullableProp) {
+    this.objectAndItemsNullableProp = objectAndItemsNullableProp;
+  }
+
+
+  public RequiredNullableBody objectItemsNullable(@javax.annotation.Nonnull Map<String, Object> objectItemsNullable) {
+    this.objectItemsNullable = objectItemsNullable;
+    return this;
+  }
+
+  public RequiredNullableBody putObjectItemsNullableItem(String key, Object objectItemsNullableItem) {
+    if (this.objectItemsNullable == null) {
+      this.objectItemsNullable = new HashMap<>();
+    }
+    this.objectItemsNullable.put(key, objectItemsNullableItem);
+    return this;
+  }
+
+  /**
+   * Get objectItemsNullable
+   * @return objectItemsNullable
+   */
+  @javax.annotation.Nonnull
+  public Map<String, Object> getObjectItemsNullable() {
+    return objectItemsNullable;
+  }
+
+  public void setObjectItemsNullable(@javax.annotation.Nonnull Map<String, Object> objectItemsNullable) {
+    this.objectItemsNullable = objectItemsNullable;
+  }
+
+
+  public RequiredNullableBody customRefEnum(@javax.annotation.Nullable NullableEnum customRefEnum) {
+    this.customRefEnum = customRefEnum;
+    return this;
+  }
+
+  /**
+   * Get customRefEnum
+   * @return customRefEnum
+   */
+  @javax.annotation.Nullable
+  public NullableEnum getCustomRefEnum() {
+    return customRefEnum;
+  }
+
+  public void setCustomRefEnum(@javax.annotation.Nullable NullableEnum customRefEnum) {
+    this.customRefEnum = customRefEnum;
+  }
+
+
+  public RequiredNullableBody customEnum(@javax.annotation.Nullable CustomEnumEnum customEnum) {
+    this.customEnum = customEnum;
+    return this;
+  }
+
+  /**
+   * Get customEnum
+   * @return customEnum
+   */
+  @javax.annotation.Nullable
+  public CustomEnumEnum getCustomEnum() {
+    return customEnum;
+  }
+
+  public void setCustomEnum(@javax.annotation.Nullable CustomEnumEnum customEnum) {
+    this.customEnum = customEnum;
+  }
+
+  /**
+   * 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.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the RequiredNullableBody instance itself
+   */
+  public RequiredNullableBody 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.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    RequiredNullableBody requiredNullableBody = (RequiredNullableBody) o;
+    return Objects.equals(this.integerProp, requiredNullableBody.integerProp) &&
+        Objects.equals(this.numberProp, requiredNullableBody.numberProp) &&
+        Objects.equals(this.booleanProp, requiredNullableBody.booleanProp) &&
+        Objects.equals(this.stringProp, requiredNullableBody.stringProp) &&
+        Objects.equals(this.dateProp, requiredNullableBody.dateProp) &&
+        Objects.equals(this.datetimeProp, requiredNullableBody.datetimeProp) &&
+        Objects.equals(this.arrayNullableProp, requiredNullableBody.arrayNullableProp) &&
+        Objects.equals(this.arrayAndItemsNullableProp, requiredNullableBody.arrayAndItemsNullableProp) &&
+        Objects.equals(this.arrayItemsNullable, requiredNullableBody.arrayItemsNullable) &&
+        Objects.equals(this.objectNullableProp, requiredNullableBody.objectNullableProp) &&
+        Objects.equals(this.objectAndItemsNullableProp, requiredNullableBody.objectAndItemsNullableProp) &&
+        Objects.equals(this.objectItemsNullable, requiredNullableBody.objectItemsNullable) &&
+        Objects.equals(this.customRefEnum, requiredNullableBody.customRefEnum) &&
+        Objects.equals(this.customEnum, requiredNullableBody.customEnum)&&
+        Objects.equals(this.additionalProperties, requiredNullableBody.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(integerProp, numberProp, booleanProp, stringProp, dateProp, datetimeProp, arrayNullableProp, arrayAndItemsNullableProp, arrayItemsNullable, objectNullableProp, objectAndItemsNullableProp, objectItemsNullable, customRefEnum, customEnum, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class RequiredNullableBody {\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");
+    sb.append("    stringProp: ").append(toIndentedString(stringProp)).append("\n");
+    sb.append("    dateProp: ").append(toIndentedString(dateProp)).append("\n");
+    sb.append("    datetimeProp: ").append(toIndentedString(datetimeProp)).append("\n");
+    sb.append("    arrayNullableProp: ").append(toIndentedString(arrayNullableProp)).append("\n");
+    sb.append("    arrayAndItemsNullableProp: ").append(toIndentedString(arrayAndItemsNullableProp)).append("\n");
+    sb.append("    arrayItemsNullable: ").append(toIndentedString(arrayItemsNullable)).append("\n");
+    sb.append("    objectNullableProp: ").append(toIndentedString(objectNullableProp)).append("\n");
+    sb.append("    objectAndItemsNullableProp: ").append(toIndentedString(objectAndItemsNullableProp)).append("\n");
+    sb.append("    objectItemsNullable: ").append(toIndentedString(objectItemsNullable)).append("\n");
+    sb.append("    customRefEnum: ").append(toIndentedString(customRefEnum)).append("\n");
+    sb.append("    customEnum: ").append(toIndentedString(customEnum)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>(Arrays.asList("integer_prop", "number_prop", "boolean_prop", "string_prop", "date_prop", "datetime_prop", "array_nullable_prop", "array_and_items_nullable_prop", "array_items_nullable", "object_nullable_prop", "object_and_items_nullable_prop", "object_items_nullable", "custom_ref_enum", "custom_enum"));
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>(Arrays.asList("integer_prop", "number_prop", "boolean_prop", "string_prop", "date_prop", "datetime_prop", "array_nullable_prop", "array_and_items_nullable_prop", "array_items_nullable", "object_nullable_prop", "object_and_items_nullable_prop", "object_items_nullable", "custom_ref_enum", "custom_enum"));
+  }
+
+  /**
+   * Validates the JSON Element and throws an exception if issues found
+   *
+   * @param jsonElement JSON Element
+   * @throws IOException if the JSON Element is invalid with respect to RequiredNullableBody
+   */
+  public static void validateJsonElement(JsonElement jsonElement) throws IOException {
+      if (jsonElement == null) {
+        if (!RequiredNullableBody.openapiRequiredFields.isEmpty()) { // has required fields but JSON element is null
+          throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "The required field(s) %s in RequiredNullableBody is not found in the empty JSON string", RequiredNullableBody.openapiRequiredFields.toString()));
+        }
+      }
+
+      // check to make sure all required properties/fields are present in the JSON string
+      for (String requiredField : RequiredNullableBody.openapiRequiredFields) {
+        if (jsonElement.getAsJsonObject().get(requiredField) == null) {
+          throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "The required field `%s` is not found in the JSON string: %s", requiredField, jsonElement.toString()));
+        }
+      }
+        JsonObject jsonObj = jsonElement.getAsJsonObject();
+      if ((jsonObj.get("string_prop") != null && !jsonObj.get("string_prop").isJsonNull()) && !jsonObj.get("string_prop").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `string_prop` to be a primitive type in the JSON string but got `%s`", jsonObj.get("string_prop").toString()));
+      }
+      // ensure the required json array is present
+      if (jsonObj.get("array_nullable_prop") == null) {
+        throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
+      } else if (!jsonObj.get("array_nullable_prop").isJsonArray() && !jsonObj.get("array_nullable_prop").isJsonNull()) {
+        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `array_nullable_prop` to be an array in the JSON string but got `%s`", jsonObj.get("array_nullable_prop").toString()));
+      }
+      // ensure the required json array is present
+      if (jsonObj.get("array_and_items_nullable_prop") == null) {
+        throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
+      } else if (!jsonObj.get("array_and_items_nullable_prop").isJsonArray() && !jsonObj.get("array_and_items_nullable_prop").isJsonNull()) {
+        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `array_and_items_nullable_prop` to be an array in the JSON string but got `%s`", jsonObj.get("array_and_items_nullable_prop").toString()));
+      }
+      // ensure the required json array is present
+      if (jsonObj.get("array_items_nullable") == null) {
+        throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
+      } else if (!jsonObj.get("array_items_nullable").isJsonArray()) {
+        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `array_items_nullable` to be an array in the JSON string but got `%s`", jsonObj.get("array_items_nullable").toString()));
+      }
+      if (jsonObj.get("custom_ref_enum") != null && !jsonObj.get("custom_ref_enum").isJsonNull()) {
+      // validate the required field `custom_ref_enum`
+      NullableEnum.validateJsonElement(jsonObj.get("custom_ref_enum"));
+      }
+      if ((jsonObj.get("custom_enum") != null && !jsonObj.get("custom_enum").isJsonNull()) && !jsonObj.get("custom_enum").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format(java.util.Locale.ROOT, "Expected the field `custom_enum` to be a primitive type in the JSON string but got `%s`", jsonObj.get("custom_enum").toString()));
+      }
+      if (jsonObj.get("custom_enum") != null && !jsonObj.get("custom_enum").isJsonNull()) {
+      // validate the required field `custom_enum`
+      CustomEnumEnum.validateJsonElement(jsonObj.get("custom_enum"));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!RequiredNullableBody.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'RequiredNullableBody' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<RequiredNullableBody> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(RequiredNullableBody.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<RequiredNullableBody>() {
+           @Override
+           public void write(JsonWriter out, RequiredNullableBody value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional 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 {
+                   JsonElement jsonElement = gson.toJsonTree(entry.getValue());
+                   if (jsonElement.isJsonArray()) {
+                     obj.add(entry.getKey(), jsonElement.getAsJsonArray());
+                   } else {
+                     obj.add(entry.getKey(), jsonElement.getAsJsonObject());
+                   }
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public RequiredNullableBody read(JsonReader in) throws IOException {
+             JsonElement jsonElement = elementAdapter.read(in);
+             validateJsonElement(jsonElement);
+             JsonObject jsonObj = jsonElement.getAsJsonObject();
+             // store additional fields in the deserialized instance
+             RequiredNullableBody 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(java.util.Locale.ROOT, "The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+  /**
+   * Create an instance of RequiredNullableBody given an JSON string
+   *
+   * @param jsonString JSON string
+   * @return An instance of RequiredNullableBody
+   * @throws IOException if the JSON string is invalid with respect to RequiredNullableBody
+   */
+  public static RequiredNullableBody fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, RequiredNullableBody.class);
+  }
+
+  /**
+   * Convert an instance of RequiredNullableBody to an JSON string
+   *
+   * @return JSON string
+   */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+
diff --git a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java
index 4552c01cca74..b406db14e130 100644
--- a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java
+++ b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java
@@ -674,6 +674,15 @@ public class JSONTest {
         }
     }
 
+    @Test
+    public void testRequiredNullableBody() throws Exception {
+        final String json1 = "{\"integer_prop\":null,\"number_prop\":null,\"boolean_prop\":null,\"string_prop\":null,\"date_prop\":null,\"datetime_prop\":null,\"array_nullable_prop\":null,\"array_and_items_nullable_prop\":null,\"array_items_nullable\":[],\"object_nullable_prop\":null,\"object_and_items_nullable_prop\":null,\"object_items_nullable\":{},\"custom_ref_enum\":null,\"custom_enum\":null}";
+        final RequiredNullableBody body = new RequiredNullableBody();
+
+        assertEquals("{\"array_items_nullable\":[],\"object_items_nullable\":{}}", json.serialize(body));
+        assertEquals(json.deserialize(json1, RequiredNullableBody.class), body);
+    }
+
     @Test
     public void testDeserializeInputStream() throws Exception {
         final String str = "\"2016-09-09\"";
diff --git a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/NullableEnumTest.java b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/NullableEnumTest.java
new file mode 100644
index 000000000000..8bcd02d221e0
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/NullableEnumTest.java
@@ -0,0 +1,32 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.annotations.SerializedName;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Model tests for NullableEnum
+ */
+public class NullableEnumTest {
+    /**
+     * Model tests for NullableEnum
+     */
+    @Test
+    public void testNullableEnum() {
+        // TODO: test NullableEnum
+    }
+
+}
diff --git a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/RequiredNullableBodyTest.java b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/RequiredNullableBodyTest.java
new file mode 100644
index 000000000000..4adb311eb1ea
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/RequiredNullableBodyTest.java
@@ -0,0 +1,160 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.math.BigDecimal;
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import org.openapitools.client.model.NullableEnum;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Model tests for RequiredNullableBody
+ */
+public class RequiredNullableBodyTest {
+    private final RequiredNullableBody model = new RequiredNullableBody();
+
+    /**
+     * Model tests for RequiredNullableBody
+     */
+    @Test
+    public void testRequiredNullableBody() {
+        // TODO: test RequiredNullableBody
+    }
+
+    /**
+     * Test the property 'integerProp'
+     */
+    @Test
+    public void integerPropTest() {
+        // TODO: test integerProp
+    }
+
+    /**
+     * Test the property 'numberProp'
+     */
+    @Test
+    public void numberPropTest() {
+        // TODO: test numberProp
+    }
+
+    /**
+     * Test the property 'booleanProp'
+     */
+    @Test
+    public void booleanPropTest() {
+        // TODO: test booleanProp
+    }
+
+    /**
+     * Test the property 'stringProp'
+     */
+    @Test
+    public void stringPropTest() {
+        // TODO: test stringProp
+    }
+
+    /**
+     * Test the property 'dateProp'
+     */
+    @Test
+    public void datePropTest() {
+        // TODO: test dateProp
+    }
+
+    /**
+     * Test the property 'datetimeProp'
+     */
+    @Test
+    public void datetimePropTest() {
+        // TODO: test datetimeProp
+    }
+
+    /**
+     * Test the property 'arrayNullableProp'
+     */
+    @Test
+    public void arrayNullablePropTest() {
+        // TODO: test arrayNullableProp
+    }
+
+    /**
+     * Test the property 'arrayAndItemsNullableProp'
+     */
+    @Test
+    public void arrayAndItemsNullablePropTest() {
+        // TODO: test arrayAndItemsNullableProp
+    }
+
+    /**
+     * Test the property 'arrayItemsNullable'
+     */
+    @Test
+    public void arrayItemsNullableTest() {
+        // TODO: test arrayItemsNullable
+    }
+
+    /**
+     * Test the property 'objectNullableProp'
+     */
+    @Test
+    public void objectNullablePropTest() {
+        // TODO: test objectNullableProp
+    }
+
+    /**
+     * Test the property 'objectAndItemsNullableProp'
+     */
+    @Test
+    public void objectAndItemsNullablePropTest() {
+        // TODO: test objectAndItemsNullableProp
+    }
+
+    /**
+     * Test the property 'objectItemsNullable'
+     */
+    @Test
+    public void objectItemsNullableTest() {
+        // TODO: test objectItemsNullable
+    }
+
+    /**
+     * Test the property 'customRefEnum'
+     */
+    @Test
+    public void customRefEnumTest() {
+        // TODO: test customRefEnum
+    }
+
+    /**
+     * Test the property 'customEnum'
+     */
+    @Test
+    public void customEnumTest() {
+        // TODO: test customEnum
+    }
+
+}