fix (JAVA OKHTTP-GSON): don't add oneOf interface as CustomTypeAdapterFactory

Author: Nicklas2751

modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/JSON.mustache

@@ -128,7 +128,9 @@ public class JSON {
         {{#model}}
         {{^isEnum}}
         {{^hasChildren}}
+        {{^vendorExtensions.x-is-one-of-interface}}
         gsonBuilder.registerTypeAdapterFactory(new {{modelPackage}}.{{{classname}}}.CustomTypeAdapterFactory());
+        {{/vendorExtensions.x-is-one-of-interface}}
         {{/hasChildren}}
         {{/isEnum}}
         {{/model}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java

@@ -3621,4 +3621,32 @@ public void testClientWithUseOneOfInterfaceShouldntGenerateOneOfExample_issue_17
 
     }
 
-}
+    @Test
+    public void testOkHttpGsonClientWithUseOneOfInterfaceShouldntRegisterInterfaceAsTypeAdapter_issue_17419() {
+        // given
+        final Path output = newTempFolder();
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setLibrary(OKHTTP_GSON)
+                .setAdditionalProperties(Map.of("useOneOfInterfaces", "true"))
+                .setInputSpec("src/test/resources/3_0/typescript-fetch/oneOf.yaml")
+                .setOutputDir(output.toString().replace("\\", "/"));
+
+        final ClientOptInput input = configurator.toClientOptInput();
+
+        // when
+        List<File> files = new DefaultGenerator().opts(input).generate();
+
+        // then
+        validateJavaSourceFiles(files);
+        TestUtils.assertFileNotContains(
+                output.resolve("src/main/java/org/openapitools/client/model/TestResponse.java"),
+                "CustomTypeAdapterFactory"
+        );
+        TestUtils.assertFileNotContains(
+                output.resolve("src/main/java/org/openapitools/client/JSON.java"),
+                "new org.openapitools.client.model.TestResponse.CustomTypeAdapterFactory()"
+        );
+    }
+
+}