[BUG] [Java] Invalid code generation for oneof types. (#18544)

* [BUG] [Java] Invalid code generation for oneof types. #18517

* update samples

* [BUG] [Java] Invalid code generation for oneof types. #18517

* [BUG] [Java] Invalid code generation for oneof types. #18517

* [BUG] [Java] Invalid code generation for oneof types. #18544

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)

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] |
+
+
+

samples/client/petstore/java/okhttp-gson/docs/FreeFormObjectTestClassProperties.md

@@ -0,0 +1,12 @@
+
+
+# FreeFormObjectTestClassProperties
+
+
+## Properties
+
+| Name | Type | Description | Notes |
+|------------ | ------------- | ------------- | -------------|
+
+
+