[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/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: