diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache
index 4a34934dce77..c2cceb5b1fe4 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache
@@ -48,15 +48,21 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
             {{#composedSchemas}}
             {{#oneOf}}
             {{^isArray}}
+            {{^isMap}}
             {{^vendorExtensions.x-duplicated-data-type}}
             final TypeAdapter<{{{dataType}}}> adapter{{{dataType}}} = gson.getDelegateAdapter(this, TypeToken.get({{{dataType}}}.class));
             {{/vendorExtensions.x-duplicated-data-type}}
+            {{/isMap}}
             {{/isArray}}
             {{#isArray}}
 
             final Type typeInstance = new TypeToken<{{{dataType}}}>(){}.getType();
             final TypeAdapter<{{{dataType}}}> adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}} = (TypeAdapter<{{{dataType}}}>) gson.getDelegateAdapter(this, TypeToken.get(typeInstance));
             {{/isArray}}
+            {{#isMap}}
+            final Type typeInstance = new TypeToken<{{{dataType}}}>(){}.getType();
+            final TypeAdapter<{{{dataType}}}> adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}} = (TypeAdapter<{{{dataType}}}>) gson.getDelegateAdapter(this, TypeToken.get(typeInstance));
+            {{/isMap}}
             {{/oneOf}}
             {{/composedSchemas}}
 
@@ -72,11 +78,18 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                     {{#oneOf}}
                     {{^vendorExtensions.x-duplicated-data-type}}
                     // check if the actual instance is of the type `{{{dataType}}}`
-                    if (value.getActualInstance() instanceof {{#isArray}}List<?>{{/isArray}}{{^isArray}}{{{dataType}}}{{/isArray}}) {
+                    if (value.getActualInstance() instanceof {{#isArray}}List<?>{{/isArray}}{{#isMap}}Map<?, ?>{{/isMap}}{{^isMap}}{{^isArray}}{{{dataType}}}{{/isArray}}{{/isMap}}) {
                     {{#isPrimitiveType}}
+                    {{^isMap}}
                         JsonPrimitive primitive = adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}}.toJsonTree(({{{dataType}}})value.getActualInstance()).getAsJsonPrimitive();
                         elementAdapter.write(out, primitive);
                         return;
+                    {{/isMap}}
+                    {{#isMap}}
+                        JsonObject object = adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}}.toJsonTree(({{{dataType}}})value.getActualInstance()).getAsJsonObject();
+                        elementAdapter.write(out, object);
+                        return;
+                    {{/isMap}}
                     {{/isPrimitiveType}}
                     {{^isPrimitiveType}}
                     {{#isArray}}
@@ -88,6 +101,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                         }
                     {{/isArray}}
                     {{/isPrimitiveType}}
+                    {{^isMap}}
                     {{^isArray}}
                     {{^isPrimitiveType}}
                         JsonElement element = adapter{{{dataType}}}.toJsonTree(({{{dataType}}})value.getActualInstance());
@@ -95,6 +109,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                         return;
                     {{/isPrimitiveType}}
                     {{/isArray}}
+                    {{/isMap}}
                     }
                     {{/vendorExtensions.x-duplicated-data-type}}
                     {{/oneOf}}
@@ -143,6 +158,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                     try {
                         // validate the JSON object to see if any exception is thrown
                     {{^isArray}}
+                    {{^isMap}}
                     {{#isNumber}}
                         if (!jsonElement.getAsJsonPrimitive().isNumber()) {
                             throw new IllegalArgumentException(String.format("Expected json element to be of type Number in the JSON string but got `%s`", jsonElement.toString()));
@@ -163,6 +179,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                         actualAdapter = adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}};
                     {{/isPrimitiveType}}
                     {{/isNumber}}
+                    {{/isMap}}
                     {{/isArray}}
                     {{#isArray}}
                         if (!jsonElement.isJsonArray()) {
@@ -194,6 +211,38 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                         }
                         actualAdapter = adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}};
                     {{/isArray}}
+                    {{#isMap}}
+                        if (!jsonElement.isJsonObject()) {
+                            throw new IllegalArgumentException(String.format("Expected json element to be a object type in the JSON string but got `%s`", jsonElement.toString()));
+                        }
+
+                        {{^isFreeFormObject}}
+                        Map<String, JsonElement> map = jsonElement.getAsJsonObject().asMap();
+                        // validate map items
+                        for(JsonElement element : map.values()) {
+                        {{#items}}
+                        {{#isNumber}}
+                            if (!jsonElement.getAsJsonPrimitive().isNumber()) {
+                                throw new IllegalArgumentException(String.format("Expected json element to be of type Number in the JSON string but got `%s`", jsonElement.toString()));
+                            }
+                        {{/isNumber}}
+                        {{^isNumber}}
+                        {{#isPrimitiveType}}
+                            if (!element.getAsJsonPrimitive().is{{#isBoolean}}Boolean{{/isBoolean}}{{#isString}}String{{/isString}}{{^isString}}{{^isBoolean}}Number{{/isBoolean}}{{/isString}}()) {
+                                throw new IllegalArgumentException(String.format("Expected array items to be of type {{#isBoolean}}Boolean{{/isBoolean}}{{#isString}}String{{/isString}}{{^isString}}{{^isBoolean}}Number{{/isBoolean}}{{/isString}} in the JSON string but got `%s`", jsonElement.toString()));
+                            }
+                        {{/isPrimitiveType}}
+                        {{/isNumber}}
+                        {{^isNumber}}
+                        {{^isPrimitiveType}}
+                            {{{dataType}}}.validateJsonElement(element);
+                        {{/isPrimitiveType}}
+                        {{/isNumber}}
+                        {{/items}}
+                        }
+                        {{/isFreeFormObject}}
+                        actualAdapter = adapter{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}};
+                    {{/isMap}}
                         match++;
                         log.log(Level.FINER, "Input data matches schema '{{{dataType}}}'");
                     } catch (Exception e) {
@@ -280,7 +329,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
         {{#composedSchemas}}
         {{#oneOf}}
         {{^vendorExtensions.x-duplicated-data-type}}
-        if (instance instanceof {{#isArray}}List<?>{{/isArray}}{{^isArray}}{{{dataType}}}{{/isArray}}) {
+        if (instance instanceof {{#isArray}}List<?>{{/isArray}}{{#isMap}}Map<?, ?>{{/isMap}}{{^isMap}}{{^isArray}}{{{dataType}}}{{/isArray}}{{/isMap}}) {
         {{#isArray}}
             List<?> list = (List<?>) instance;
             if (list.get(0) instanceof {{{items.dataType}}}) {
@@ -322,7 +371,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
      * @return The actual instance of `{{{dataType}}}`
      * @throws ClassCastException if the instance is not `{{{dataType}}}`
      */
-    public {{{dataType}}} get{{#isArray}}{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}}{{/isArray}}{{^isArray}}{{{dataType}}}{{/isArray}}() throws ClassCastException {
+    public {{{dataType}}} get{{#sanitizeGeneric}}{{{dataType}}}{{/sanitizeGeneric}}() throws ClassCastException {
         return ({{{dataType}}})super.getActualInstance();
     }
     {{/vendorExtensions.x-duplicated-data-type}}
@@ -345,6 +394,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
         // validate the json string with {{{dataType}}}
         try {
         {{^hasVars}}
+        {{^isMap}}
         {{^isArray}}
         {{#isNumber}}
             if (!jsonElement.getAsJsonPrimitive().isNumber()) {
@@ -364,6 +414,7 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
         {{/isPrimitiveType}}
         {{/isNumber}}
         {{/isArray}}
+        {{/isMap}}
         {{#isArray}}
             if (!jsonElement.isJsonArray()) {
                 throw new IllegalArgumentException(String.format("Expected json element to be a array type in the JSON string but got `%s`", jsonElement.toString()));
@@ -392,6 +443,37 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
             {{/items}}
             }
         {{/isArray}}
+        {{#isMap}}
+            if (!jsonElement.isJsonObject()) {
+                throw new IllegalArgumentException(String.format("Expected json element to be a object type in the JSON string but got `%s`", jsonElement.toString()));
+            }
+
+            {{^isFreeFormObject}}
+            Map<String, JsonElement> map = jsonElement.getAsJsonObject().asMap();
+            // validate map items
+            for(JsonElement element : map.values()) {
+            {{#items}}
+            {{#isNumber}}
+                if (!jsonElement.getAsJsonPrimitive().isNumber()) {
+                    throw new IllegalArgumentException(String.format("Expected json element to be of type Number in the JSON string but got `%s`", jsonElement.toString()));
+                }
+            {{/isNumber}}
+            {{^isNumber}}
+            {{#isPrimitiveType}}
+                if (!element.getAsJsonPrimitive().is{{#isBoolean}}Boolean{{/isBoolean}}{{#isString}}String{{/isString}}{{^isString}}{{^isBoolean}}Number{{/isBoolean}}{{/isString}}()) {
+                    throw new IllegalArgumentException(String.format("Expected array items to be of type {{#isBoolean}}Boolean{{/isBoolean}}{{#isString}}String{{/isString}}{{^isString}}{{^isBoolean}}Number{{/isBoolean}}{{/isString}} in the JSON string but got `%s`", jsonElement.toString()));
+                }
+            {{/isPrimitiveType}}
+            {{/isNumber}}
+            {{^isNumber}}
+            {{^isPrimitiveType}}
+                {{{dataType}}}.validateJsonElement(element);
+            {{/isPrimitiveType}}
+            {{/isNumber}}
+            {{/items}}
+            }
+            {{/isFreeFormObject}}
+        {{/isMap}}
         {{/hasVars}}
         {{#hasVars}}
             {{{.}}}.validateJsonElement(jsonElement);
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 0aff5a7c0957..5e763f1989ab 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
@@ -1048,6 +1048,18 @@ paths:
             schema:
               $ref: '#/components/schemas/FileSchemaTestClass'
         required: true
+  /fake/get-free-form-object:
+    get:
+      tags:
+        - fake
+      description: Get a free form object or Json string
+      responses:
+        '200':
+          description: Success
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/FreeFormObjectTestClass'
   /fake/test-query-parameters:
     put:
       tags:
@@ -2035,6 +2047,16 @@ components:
           type: array
           items:
             $ref: '#/components/schemas/File'
+    FreeFormObjectTestClass:
+      type: object
+      properties:
+        name:
+          type: string
+        properties:
+          oneOf:
+            - type: string
+            - type: object
+              additionalProperties: true
     File:
       type: object
       description: Must be named `File` for test.
diff --git a/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES b/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES
index 9dd3140d3e43..d42445a12dac 100644
--- a/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES
+++ b/samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES
@@ -48,6 +48,8 @@ docs/FileSchemaTestClass.md
 docs/Foo.md
 docs/FooGetDefaultResponse.md
 docs/FormatTest.md
+docs/FreeFormObjectTestClass.md
+docs/FreeFormObjectTestClassProperties.md
 docs/Fruit.md
 docs/FruitReq.md
 docs/GmFruit.md
@@ -187,6 +189,8 @@ src/main/java/org/openapitools/client/model/FileSchemaTestClass.java
 src/main/java/org/openapitools/client/model/Foo.java
 src/main/java/org/openapitools/client/model/FooGetDefaultResponse.java
 src/main/java/org/openapitools/client/model/FormatTest.java
+src/main/java/org/openapitools/client/model/FreeFormObjectTestClass.java
+src/main/java/org/openapitools/client/model/FreeFormObjectTestClassProperties.java
 src/main/java/org/openapitools/client/model/Fruit.java
 src/main/java/org/openapitools/client/model/FruitReq.java
 src/main/java/org/openapitools/client/model/GmFruit.java
diff --git a/samples/client/petstore/java/okhttp-gson/README.md b/samples/client/petstore/java/okhttp-gson/README.md
index d892ad8cf0e5..abd680c94f49 100644
--- a/samples/client/petstore/java/okhttp-gson/README.md
+++ b/samples/client/petstore/java/okhttp-gson/README.md
@@ -119,6 +119,7 @@ Class | Method | HTTP request | Description
 *AnotherFakeApi* | [**getParameterStringNumber**](docs/AnotherFakeApi.md#getParameterStringNumber) | **GET** /fake/parameter-string-number | parameter string number
 *AnotherFakeApi* | [**nullRequestBody**](docs/AnotherFakeApi.md#nullRequestBody) | **GET** /fake/null-request-body | null request body
 *DefaultApi* | [**fooGet**](docs/DefaultApi.md#fooGet) | **GET** /foo | 
+*FakeApi* | [**fakeGetFreeFormObjectGet**](docs/FakeApi.md#fakeGetFreeFormObjectGet) | **GET** /fake/get-free-form-object | 
 *FakeApi* | [**fakeOuterBooleanSerialize**](docs/FakeApi.md#fakeOuterBooleanSerialize) | **POST** /fake/outer/boolean | 
 *FakeApi* | [**fakeOuterCompositeSerialize**](docs/FakeApi.md#fakeOuterCompositeSerialize) | **POST** /fake/outer/composite | 
 *FakeApi* | [**fakeOuterNumberSerialize**](docs/FakeApi.md#fakeOuterNumberSerialize) | **POST** /fake/outer/number | 
@@ -205,6 +206,8 @@ Class | Method | HTTP request | Description
  - [Foo](docs/Foo.md)
  - [FooGetDefaultResponse](docs/FooGetDefaultResponse.md)
  - [FormatTest](docs/FormatTest.md)
+ - [FreeFormObjectTestClass](docs/FreeFormObjectTestClass.md)
+ - [FreeFormObjectTestClassProperties](docs/FreeFormObjectTestClassProperties.md)
  - [Fruit](docs/Fruit.md)
  - [FruitReq](docs/FruitReq.md)
  - [GmFruit](docs/GmFruit.md)
diff --git a/samples/client/petstore/java/okhttp-gson/api/openapi.yaml b/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
index 131f50a91650..7170d2f6afb9 100644
--- a/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
+++ b/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
@@ -1117,6 +1117,20 @@ paths:
       x-content-type: application/json
       x-accepts:
       - application/json
+  /fake/get-free-form-object:
+    get:
+      description: Get a free form object or Json string
+      responses:
+        "200":
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/FreeFormObjectTestClass'
+          description: Success
+      tags:
+      - fake
+      x-accepts:
+      - application/json
   /fake/test-query-parameters:
     put:
       description: To test the collection format in query parameters
@@ -2134,6 +2148,16 @@ components:
             $ref: '#/components/schemas/File'
           type: array
       type: object
+    FreeFormObjectTestClass:
+      example:
+        name: name
+        properties: FreeFormObjectTestClass_properties
+      properties:
+        name:
+          type: string
+        properties:
+          $ref: '#/components/schemas/FreeFormObjectTestClass_properties'
+      type: object
     File:
       description: Must be named `File` for test.
       example:
@@ -2881,6 +2905,11 @@ components:
       required:
       - requiredFile
       type: object
+    FreeFormObjectTestClass_properties:
+      oneOf:
+      - type: string
+      - additionalProperties: true
+        type: object
     ArrayOfInlineAllOf_array_allof_dog_property_inner:
       allOf:
       - properties:
diff --git a/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md b/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md
index 97e59270c5a8..18f99688c925 100644
--- a/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md
+++ b/samples/client/petstore/java/okhttp-gson/docs/FakeApi.md
@@ -4,6 +4,7 @@ All URIs are relative to *http://petstore.swagger.io:80/v2*
 
 | Method | HTTP request | Description |
 |------------- | ------------- | -------------|
+| [**fakeGetFreeFormObjectGet**](FakeApi.md#fakeGetFreeFormObjectGet) | **GET** /fake/get-free-form-object |  |
 | [**fakeOuterBooleanSerialize**](FakeApi.md#fakeOuterBooleanSerialize) | **POST** /fake/outer/boolean |  |
 | [**fakeOuterCompositeSerialize**](FakeApi.md#fakeOuterCompositeSerialize) | **POST** /fake/outer/composite |  |
 | [**fakeOuterNumberSerialize**](FakeApi.md#fakeOuterNumberSerialize) | **POST** /fake/outer/number |  |
@@ -26,6 +27,64 @@ All URIs are relative to *http://petstore.swagger.io:80/v2*
 | [**testStringMapReference**](FakeApi.md#testStringMapReference) | **POST** /fake/stringMap-reference | test referenced string map |
 
 
+<a id="fakeGetFreeFormObjectGet"></a>
+# **fakeGetFreeFormObjectGet**
+> FreeFormObjectTestClass fakeGetFreeFormObjectGet()
+
+
+
+Get a free form object or Json string
+
+### 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 {
+      FreeFormObjectTestClass result = apiInstance.fakeGetFreeFormObjectGet();
+      System.out.println(result);
+    } catch (ApiException e) {
+      System.err.println("Exception when calling FakeApi#fakeGetFreeFormObjectGet");
+      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
+
+[**FreeFormObjectTestClass**](FreeFormObjectTestClass.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="fakeOuterBooleanSerialize"></a>
 # **fakeOuterBooleanSerialize**
 > Boolean fakeOuterBooleanSerialize(body)
diff --git a/samples/client/petstore/java/okhttp-gson/docs/FreeFormObjectTestClass.md b/samples/client/petstore/java/okhttp-gson/docs/FreeFormObjectTestClass.md
new file mode 100644
index 000000000000..3bb6144bc90b
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/docs/FreeFormObjectTestClass.md
@@ -0,0 +1,14 @@
+
+
+# FreeFormObjectTestClass
+
+
+## Properties
+
+| Name | Type | Description | Notes |
+|------------ | ------------- | ------------- | -------------|
+|**name** | **String** |  |  [optional] |
+|**properties** | [**FreeFormObjectTestClassProperties**](FreeFormObjectTestClassProperties.md) |  |  [optional] |
+
+
+
diff --git a/samples/client/petstore/java/okhttp-gson/docs/FreeFormObjectTestClassProperties.md b/samples/client/petstore/java/okhttp-gson/docs/FreeFormObjectTestClassProperties.md
new file mode 100644
index 000000000000..8a7c9d2838a3
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/docs/FreeFormObjectTestClassProperties.md
@@ -0,0 +1,12 @@
+
+
+# FreeFormObjectTestClassProperties
+
+
+## Properties
+
+| Name | Type | Description | Notes |
+|------------ | ------------- | ------------- | -------------|
+
+
+
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 60c3ef11d94b..69d7ec7bf943 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
@@ -273,6 +273,8 @@ private static Class getClassByDiscriminator(Map classByDiscriminatorValue, Stri
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Foo.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.FooGetDefaultResponse.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.FormatTest.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.FreeFormObjectTestClass.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.FreeFormObjectTestClassProperties.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Fruit.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.FruitReq.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.GmFruit.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 c2e2176d5f0e..664a08c41d6f 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
@@ -31,6 +31,7 @@
 import org.openapitools.client.model.Client;
 import java.io.File;
 import org.openapitools.client.model.FileSchemaTestClass;
+import org.openapitools.client.model.FreeFormObjectTestClass;
 import org.openapitools.client.model.HealthCheckResult;
 import java.time.LocalDate;
 import org.openapitools.client.model.ModelApiResponse;
@@ -83,6 +84,119 @@ public void setCustomBaseUrl(String customBaseUrl) {
         this.localCustomBaseUrl = customBaseUrl;
     }
 
+    /**
+     * Build call for fakeGetFreeFormObjectGet
+     * @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 summary="Response Details" border="1">
+        <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 fakeGetFreeFormObjectGetCall(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/get-free-form-object";
+
+        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 fakeGetFreeFormObjectGetValidateBeforeCall(final ApiCallback _callback) throws ApiException {
+        return fakeGetFreeFormObjectGetCall(_callback);
+
+    }
+
+    /**
+     * 
+     * Get a free form object or Json string
+     * @return FreeFormObjectTestClass
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <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 FreeFormObjectTestClass fakeGetFreeFormObjectGet() throws ApiException {
+        ApiResponse<FreeFormObjectTestClass> localVarResp = fakeGetFreeFormObjectGetWithHttpInfo();
+        return localVarResp.getData();
+    }
+
+    /**
+     * 
+     * Get a free form object or Json string
+     * @return ApiResponse&lt;FreeFormObjectTestClass&gt;
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <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<FreeFormObjectTestClass> fakeGetFreeFormObjectGetWithHttpInfo() throws ApiException {
+        okhttp3.Call localVarCall = fakeGetFreeFormObjectGetValidateBeforeCall(null);
+        Type localVarReturnType = new TypeToken<FreeFormObjectTestClass>(){}.getType();
+        return localVarApiClient.execute(localVarCall, localVarReturnType);
+    }
+
+    /**
+     *  (asynchronously)
+     * Get a free form object or Json string
+     * @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 summary="Response Details" border="1">
+        <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 fakeGetFreeFormObjectGetAsync(final ApiCallback<FreeFormObjectTestClass> _callback) throws ApiException {
+
+        okhttp3.Call localVarCall = fakeGetFreeFormObjectGetValidateBeforeCall(_callback);
+        Type localVarReturnType = new TypeToken<FreeFormObjectTestClass>(){}.getType();
+        localVarApiClient.executeAsync(localVarCall, localVarReturnType, _callback);
+        return localVarCall;
+    }
     /**
      * Build call for fakeOuterBooleanSerialize
      * @param body Input boolean as post body (optional)
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FreeFormObjectTestClass.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FreeFormObjectTestClass.java
new file mode 100644
index 000000000000..335b40973cf7
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FreeFormObjectTestClass.java
@@ -0,0 +1,318 @@
+/*
+ * 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.util.Arrays;
+import org.openapitools.client.model.FreeFormObjectTestClassProperties;
+
+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.lang.reflect.Type;
+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;
+
+/**
+ * FreeFormObjectTestClass
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.6.0-SNAPSHOT")
+public class FreeFormObjectTestClass {
+  public static final String SERIALIZED_NAME_NAME = "name";
+  @SerializedName(SERIALIZED_NAME_NAME)
+  private String name;
+
+  public static final String SERIALIZED_NAME_PROPERTIES = "properties";
+  @SerializedName(SERIALIZED_NAME_PROPERTIES)
+  private FreeFormObjectTestClassProperties properties;
+
+  public FreeFormObjectTestClass() {
+  }
+
+  public FreeFormObjectTestClass name(String name) {
+    this.name = name;
+    return this;
+  }
+
+   /**
+   * Get name
+   * @return name
+  **/
+  @javax.annotation.Nullable
+  public String getName() {
+    return name;
+  }
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+
+  public FreeFormObjectTestClass properties(FreeFormObjectTestClassProperties properties) {
+    this.properties = properties;
+    return this;
+  }
+
+   /**
+   * Get properties
+   * @return properties
+  **/
+  @javax.annotation.Nullable
+  public FreeFormObjectTestClassProperties getProperties() {
+    return properties;
+  }
+
+  public void setProperties(FreeFormObjectTestClassProperties properties) {
+    this.properties = properties;
+  }
+
+  /**
+   * 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 FreeFormObjectTestClass instance itself
+   */
+  public FreeFormObjectTestClass 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;
+    }
+    FreeFormObjectTestClass freeFormObjectTestClass = (FreeFormObjectTestClass) o;
+    return Objects.equals(this.name, freeFormObjectTestClass.name) &&
+        Objects.equals(this.properties, freeFormObjectTestClass.properties)&&
+        Objects.equals(this.additionalProperties, freeFormObjectTestClass.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(name, properties, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class FreeFormObjectTestClass {\n");
+    sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    properties: ").append(toIndentedString(properties)).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>();
+    openapiFields.add("name");
+    openapiFields.add("properties");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+  }
+
+ /**
+  * 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 FreeFormObjectTestClass
+  */
+  public static void validateJsonElement(JsonElement jsonElement) throws IOException {
+      if (jsonElement == null) {
+        if (!FreeFormObjectTestClass.openapiRequiredFields.isEmpty()) { // has required fields but JSON element is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in FreeFormObjectTestClass is not found in the empty JSON string", FreeFormObjectTestClass.openapiRequiredFields.toString()));
+        }
+      }
+        JsonObject jsonObj = jsonElement.getAsJsonObject();
+      if ((jsonObj.get("name") != null && !jsonObj.get("name").isJsonNull()) && !jsonObj.get("name").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `name` to be a primitive type in the JSON string but got `%s`", jsonObj.get("name").toString()));
+      }
+      // validate the optional field `properties`
+      if (jsonObj.get("properties") != null && !jsonObj.get("properties").isJsonNull()) {
+        FreeFormObjectTestClassProperties.validateJsonElement(jsonObj.get("properties"));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!FreeFormObjectTestClass.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'FreeFormObjectTestClass' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<FreeFormObjectTestClass> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(FreeFormObjectTestClass.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<FreeFormObjectTestClass>() {
+           @Override
+           public void write(JsonWriter out, FreeFormObjectTestClass 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 FreeFormObjectTestClass read(JsonReader in) throws IOException {
+             JsonElement jsonElement = elementAdapter.read(in);
+             validateJsonElement(jsonElement);
+             JsonObject jsonObj = jsonElement.getAsJsonObject();
+             // store additional fields in the deserialized instance
+             FreeFormObjectTestClass instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else 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 FreeFormObjectTestClass given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of FreeFormObjectTestClass
+  * @throws IOException if the JSON string is invalid with respect to FreeFormObjectTestClass
+  */
+  public static FreeFormObjectTestClass fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, FreeFormObjectTestClass.class);
+  }
+
+ /**
+  * Convert an instance of FreeFormObjectTestClass 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/main/java/org/openapitools/client/model/FreeFormObjectTestClassProperties.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FreeFormObjectTestClassProperties.java
new file mode 100644
index 000000000000..9ac1b95f8186
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FreeFormObjectTestClassProperties.java
@@ -0,0 +1,282 @@
+/*
+ * 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 java.util.Map;
+
+
+
+import java.io.IOException;
+import java.lang.reflect.Type;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.JsonPrimitive;
+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 com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonSerializationContext;
+import com.google.gson.JsonSerializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonParseException;
+
+import org.openapitools.client.JSON;
+
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.6.0-SNAPSHOT")
+public class FreeFormObjectTestClassProperties extends AbstractOpenApiSchema {
+    private static final Logger log = Logger.getLogger(FreeFormObjectTestClassProperties.class.getName());
+
+    public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+        @SuppressWarnings("unchecked")
+        @Override
+        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+            if (!FreeFormObjectTestClassProperties.class.isAssignableFrom(type.getRawType())) {
+                return null; // this class only serializes 'FreeFormObjectTestClassProperties' and its subtypes
+            }
+            final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+            final TypeAdapter<String> adapterString = gson.getDelegateAdapter(this, TypeToken.get(String.class));
+            final Type typeInstance = new TypeToken<Map<String, Object>>(){}.getType();
+            final TypeAdapter<Map<String, Object>> adapterMapStringObject = (TypeAdapter<Map<String, Object>>) gson.getDelegateAdapter(this, TypeToken.get(typeInstance));
+
+            return (TypeAdapter<T>) new TypeAdapter<FreeFormObjectTestClassProperties>() {
+                @Override
+                public void write(JsonWriter out, FreeFormObjectTestClassProperties value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
+                    // check if the actual instance is of the type `String`
+                    if (value.getActualInstance() instanceof String) {
+                        JsonPrimitive primitive = adapterString.toJsonTree((String)value.getActualInstance()).getAsJsonPrimitive();
+                        elementAdapter.write(out, primitive);
+                        return;
+                    }
+                    // check if the actual instance is of the type `Map<String, Object>`
+                    if (value.getActualInstance() instanceof Map<?, ?>) {
+                        JsonObject object = adapterMapStringObject.toJsonTree((Map<String, Object>)value.getActualInstance()).getAsJsonObject();
+                        elementAdapter.write(out, object);
+                        return;
+                    }
+                    throw new IOException("Failed to serialize as the type doesn't match oneOf schemas: Map<String, Object>, String");
+                }
+
+                @Override
+                public FreeFormObjectTestClassProperties read(JsonReader in) throws IOException {
+                    Object deserialized = null;
+                    JsonElement jsonElement = elementAdapter.read(in);
+
+                    int match = 0;
+                    ArrayList<String> errorMessages = new ArrayList<>();
+                    TypeAdapter actualAdapter = elementAdapter;
+
+                    // deserialize String
+                    try {
+                        // validate the JSON object to see if any exception is thrown
+                        if (!jsonElement.getAsJsonPrimitive().isString()) {
+                            throw new IllegalArgumentException(String.format("Expected json element to be of type String in the JSON string but got `%s`", jsonElement.toString()));
+                        }
+                        actualAdapter = adapterString;
+                        match++;
+                        log.log(Level.FINER, "Input data matches schema 'String'");
+                    } catch (Exception e) {
+                        // deserialization failed, continue
+                        errorMessages.add(String.format("Deserialization for String failed with `%s`.", e.getMessage()));
+                        log.log(Level.FINER, "Input data does not match schema 'String'", e);
+                    }
+                    // deserialize Map<String, Object>
+                    try {
+                        // validate the JSON object to see if any exception is thrown
+                        if (!jsonElement.isJsonObject()) {
+                            throw new IllegalArgumentException(String.format("Expected json element to be a object type in the JSON string but got `%s`", jsonElement.toString()));
+                        }
+
+                        actualAdapter = adapterMapStringObject;
+                        match++;
+                        log.log(Level.FINER, "Input data matches schema 'Map<String, Object>'");
+                    } catch (Exception e) {
+                        // deserialization failed, continue
+                        errorMessages.add(String.format("Deserialization for Map<String, Object> failed with `%s`.", e.getMessage()));
+                        log.log(Level.FINER, "Input data does not match schema 'Map<String, Object>'", e);
+                    }
+
+                    if (match == 1) {
+                        FreeFormObjectTestClassProperties ret = new FreeFormObjectTestClassProperties();
+                        ret.setActualInstance(actualAdapter.fromJsonTree(jsonElement));
+                        return ret;
+                    }
+
+                    throw new IOException(String.format("Failed deserialization for FreeFormObjectTestClassProperties: %d classes match result, expected 1. Detailed failure message for oneOf schemas: %s. JSON: %s", match, errorMessages, jsonElement.toString()));
+                }
+            }.nullSafe();
+        }
+    }
+
+    // store a list of schema names defined in oneOf
+    public static final Map<String, Class<?>> schemas = new HashMap<String, Class<?>>();
+
+    public FreeFormObjectTestClassProperties() {
+        super("oneOf", Boolean.FALSE);
+    }
+
+    public FreeFormObjectTestClassProperties(Map<String, Object> o) {
+        super("oneOf", Boolean.FALSE);
+        setActualInstance(o);
+    }
+
+    public FreeFormObjectTestClassProperties(String o) {
+        super("oneOf", Boolean.FALSE);
+        setActualInstance(o);
+    }
+
+    static {
+        schemas.put("String", String.class);
+        schemas.put("Map<String, Object>", Map.class);
+    }
+
+    @Override
+    public Map<String, Class<?>> getSchemas() {
+        return FreeFormObjectTestClassProperties.schemas;
+    }
+
+    /**
+     * Set the instance that matches the oneOf child schema, check
+     * the instance parameter is valid against the oneOf child schemas:
+     * Map<String, Object>, String
+     *
+     * It could be an instance of the 'oneOf' schemas.
+     */
+    @Override
+    public void setActualInstance(Object instance) {
+        if (instance instanceof String) {
+            super.setActualInstance(instance);
+            return;
+        }
+
+        if (instance instanceof Map<?, ?>) {
+            super.setActualInstance(instance);
+            return;
+        }
+
+        throw new RuntimeException("Invalid instance type. Must be Map<String, Object>, String");
+    }
+
+    /**
+     * Get the actual instance, which can be the following:
+     * Map<String, Object>, String
+     *
+     * @return The actual instance (Map<String, Object>, String)
+     */
+    @SuppressWarnings("unchecked")
+    @Override
+    public Object getActualInstance() {
+        return super.getActualInstance();
+    }
+
+    /**
+     * Get the actual instance of `String`. If the actual instance is not `String`,
+     * the ClassCastException will be thrown.
+     *
+     * @return The actual instance of `String`
+     * @throws ClassCastException if the instance is not `String`
+     */
+    public String getString() throws ClassCastException {
+        return (String)super.getActualInstance();
+    }
+    /**
+     * Get the actual instance of `Map<String, Object>`. If the actual instance is not `Map<String, Object>`,
+     * the ClassCastException will be thrown.
+     *
+     * @return The actual instance of `Map<String, Object>`
+     * @throws ClassCastException if the instance is not `Map<String, Object>`
+     */
+    public Map<String, Object> getMapStringObject() throws ClassCastException {
+        return (Map<String, Object>)super.getActualInstance();
+    }
+
+    /**
+     * 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 FreeFormObjectTestClassProperties
+     */
+    public static void validateJsonElement(JsonElement jsonElement) throws IOException {
+        // validate oneOf schemas one by one
+        int validCount = 0;
+        ArrayList<String> errorMessages = new ArrayList<>();
+        // validate the json string with String
+        try {
+            if (!jsonElement.getAsJsonPrimitive().isString()) {
+                throw new IllegalArgumentException(String.format("Expected json element to be of type String in the JSON string but got `%s`", jsonElement.toString()));
+            }
+            validCount++;
+        } catch (Exception e) {
+            errorMessages.add(String.format("Deserialization for String failed with `%s`.", e.getMessage()));
+            // continue to the next one
+        }
+        // validate the json string with Map<String, Object>
+        try {
+            if (!jsonElement.isJsonObject()) {
+                throw new IllegalArgumentException(String.format("Expected json element to be a object type in the JSON string but got `%s`", jsonElement.toString()));
+            }
+
+            validCount++;
+        } catch (Exception e) {
+            errorMessages.add(String.format("Deserialization for Map<String, Object> failed with `%s`.", e.getMessage()));
+            // continue to the next one
+        }
+        if (validCount != 1) {
+            throw new IOException(String.format("The JSON string is invalid for FreeFormObjectTestClassProperties with oneOf schemas: Map<String, Object>, String. %d class(es) match the result, expected 1. Detailed failure message for oneOf schemas: %s. JSON: %s", validCount, errorMessages, jsonElement.toString()));
+        }
+    }
+
+    /**
+     * Create an instance of FreeFormObjectTestClassProperties given an JSON string
+     *
+     * @param jsonString JSON string
+     * @return An instance of FreeFormObjectTestClassProperties
+     * @throws IOException if the JSON string is invalid with respect to FreeFormObjectTestClassProperties
+     */
+    public static FreeFormObjectTestClassProperties fromJson(String jsonString) throws IOException {
+        return JSON.getGson().fromJson(jsonString, FreeFormObjectTestClassProperties.class);
+    }
+
+    /**
+     * Convert an instance of FreeFormObjectTestClassProperties 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 978195ccf6de..765e7b71c77a 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
@@ -37,6 +37,28 @@ public void setup() {
         order = new Order();
     }
 
+    @Test
+    public void testOneOfFreeFormObject() {
+        final Map<String, Object> map = new LinkedHashMap<>();
+        map.put("someString", "abc");
+        map.put("someBoolean", false);
+
+        final String json1 = "{\"someString\":\"abc\",\"someBoolean\":false}";
+        final FreeFormObjectTestClassProperties properties = new FreeFormObjectTestClassProperties();
+        properties.setActualInstance(map);
+
+        assertEquals(json1, json.serialize(properties));
+        assertEquals(json.deserialize(json1, FreeFormObjectTestClassProperties.class), properties);
+
+
+        final String json2 = "\"abc\"";
+        final FreeFormObjectTestClassProperties properties2 = new FreeFormObjectTestClassProperties();
+        properties2.setActualInstance("abc");
+
+        assertEquals(json2, json.serialize(properties2));
+        assertEquals(json.deserialize(json2, FreeFormObjectTestClassProperties.class), properties2);
+    }
+
     @Test
     public void testSqlDateTypeAdapter() {
         final String str = "\"2015-11-07\"";
diff --git a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/FreeFormObjectTestClassPropertiesTest.java b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/FreeFormObjectTestClassPropertiesTest.java
new file mode 100644
index 000000000000..32a85ee296b7
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/FreeFormObjectTestClassPropertiesTest.java
@@ -0,0 +1,34 @@
+/*
+ * 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.Map;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Model tests for FreeFormObjectTestClassProperties
+ */
+public class FreeFormObjectTestClassPropertiesTest {
+    private final FreeFormObjectTestClassProperties model = new FreeFormObjectTestClassProperties();
+
+    /**
+     * Model tests for FreeFormObjectTestClassProperties
+     */
+    @Test
+    public void testFreeFormObjectTestClassProperties() {
+        // TODO: test FreeFormObjectTestClassProperties
+    }
+
+}
diff --git a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/FreeFormObjectTestClassTest.java b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/FreeFormObjectTestClassTest.java
new file mode 100644
index 000000000000..3d2f22b98de3
--- /dev/null
+++ b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/FreeFormObjectTestClassTest.java
@@ -0,0 +1,57 @@
+/*
+ * 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.util.Arrays;
+import org.openapitools.client.model.FreeFormObjectTestClassProperties;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Model tests for FreeFormObjectTestClass
+ */
+public class FreeFormObjectTestClassTest {
+    private final FreeFormObjectTestClass model = new FreeFormObjectTestClass();
+
+    /**
+     * Model tests for FreeFormObjectTestClass
+     */
+    @Test
+    public void testFreeFormObjectTestClass() {
+        // TODO: test FreeFormObjectTestClass
+    }
+
+    /**
+     * Test the property 'name'
+     */
+    @Test
+    public void nameTest() {
+        // TODO: test name
+    }
+
+    /**
+     * Test the property 'properties'
+     */
+    @Test
+    public void propertiesTest() {
+        // TODO: test properties
+    }
+
+}