[Java][okhttp-gson] Fix UUID inner enum compilation

modules/openapi-generator/pom.xml

@@ -280,6 +280,12 @@
         </plugins>
     </reporting>
     <dependencies>
+        <dependency>
+            <groupId>com.google.code.gson</groupId>
+            <artifactId>gson</artifactId>
+            <version>2.10.1</version>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>${swagger-parser-groupid.version}</groupId>
             <artifactId>swagger-parser</artifactId>

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

@@ -49,18 +49,18 @@
     public static class Adapter extends TypeAdapter<{{{datatypeWithEnum}}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}> {
       @Override
       public void write(final JsonWriter jsonWriter, final {{{datatypeWithEnum}}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}} enumeration) throws IOException {
-        jsonWriter.value(enumeration.getValue(){{#isUri}}.toASCIIString(){{/isUri}});
+        jsonWriter.value(enumeration.getValue(){{#isUri}}.toASCIIString(){{/isUri}}{{#isUuid}}.toString(){{/isUuid}});
       }
 
       @Override
       public {{{datatypeWithEnum}}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}} read(final JsonReader jsonReader) throws IOException {
-        {{^isNumber}}{{{dataType}}}{{/isNumber}}{{#isNumber}}String{{/isNumber}} value = {{#isFloat}}(float){{/isFloat}} {{#isUri}}URI.create({{/isUri}}jsonReader.{{#isNumber}}nextString(){{/isNumber}}{{#isInteger}}nextInt(){{/isInteger}}{{#isUri}}nextString()){{/isUri}}{{^isNumber}}{{^isInteger}}{{^isUri}}{{#isFloat}}nextDouble{{/isFloat}}{{^isFloat}}next{{{dataType}}}{{/isFloat}}(){{/isUri}}{{/isInteger}}{{/isNumber}};
+        {{^isNumber}}{{{dataType}}}{{/isNumber}}{{#isNumber}}String{{/isNumber}} value = {{#isFloat}}(float){{/isFloat}} {{#isUuid}}UUID.fromString({{/isUuid}}{{#isUri}}URI.create({{/isUri}}jsonReader.{{#isNumber}}nextString(){{/isNumber}}{{#isInteger}}nextInt(){{/isInteger}}{{#isUri}}nextString()){{/isUri}}{{#isUuid}}nextString()){{/isUuid}}{{^isNumber}}{{^isInteger}}{{^isUri}}{{^isUuid}}{{#isFloat}}nextDouble{{/isFloat}}{{^isFloat}}next{{{dataType}}}{{/isFloat}}(){{/isUuid}}{{/isUri}}{{/isInteger}}{{/isNumber}};
         return {{{datatypeWithEnum}}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}.fromValue({{#isNumber}}new BigDecimal({{/isNumber}}value{{#isNumber}}){{/isNumber}});
       }
     }
 
     public static void validateJsonElement(JsonElement jsonElement) throws IOException {
-      {{^isNumber}}{{{dataType}}}{{/isNumber}}{{#isNumber}}String{{/isNumber}} value = {{#isUri}}URI.create({{/isUri}}jsonElement.{{#isNumber}}getAsString(){{/isNumber}}{{#isInteger}}getAsInt(){{/isInteger}}{{#isUri}}getAsString()){{/isUri}}{{^isNumber}}{{^isInteger}}{{^isUri}}getAs{{{dataType}}}(){{/isUri}}{{/isInteger}}{{/isNumber}};
+      {{^isNumber}}{{{dataType}}}{{/isNumber}}{{#isNumber}}String{{/isNumber}} value = {{#isUuid}}UUID.fromString({{/isUuid}}{{#isUri}}URI.create({{/isUri}}jsonElement.{{#isNumber}}getAsString(){{/isNumber}}{{#isInteger}}getAsInt(){{/isInteger}}{{#isUri}}getAsString()){{/isUri}}{{#isUuid}}getAsString()){{/isUuid}}{{^isNumber}}{{^isInteger}}{{^isUri}}{{^isUuid}}getAs{{{dataType}}}(){{/isUuid}}{{/isUri}}{{/isInteger}}{{/isNumber}};
       {{{datatypeWithEnum}}}{{^datatypeWithEnum}}{{{classname}}}{{/datatypeWithEnum}}.fromValue({{#isNumber}}new BigDecimal({{/isNumber}}value{{#isNumber}}){{/isNumber}});
     }
   }

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

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2026 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openapitools.codegen.java;
+
+import com.github.javaparser.StaticJavaParser;
+import com.github.javaparser.ast.body.EnumDeclaration;
+import com.google.gson.Gson;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonPrimitive;
+import org.openapitools.codegen.DefaultGenerator;
+import org.openapitools.codegen.config.CodegenConfigurator;
+import org.testng.annotations.DataProvider;
+import org.testng.annotations.Test;
+
+import javax.tools.ToolProvider;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import static org.openapitools.codegen.TestUtils.newTempFolder;
+import static org.testng.Assert.*;
+
+public class JavaOkHttpGsonUuidEnumTest {
+
+    @DataProvider
+    public Object[][] caseInsensitiveOptions() {
+        return new Object[][]{{false}, {true}};
+    }
+
+    @Test(dataProvider = "caseInsensitiveOptions")
+    public void testUuidEnumGsonRoundTrip(boolean caseInsensitive) throws Exception {
+        Path output = newTempFolder();
+        CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setLibrary("okhttp-gson")
+                .setInputSpec("src/test/resources/3_0/issue_20952_inner_enum_comparison.yaml")
+                .setOutputDir(output.toString())
+                .addGlobalProperty("models", "")
+                .addGlobalProperty("modelDocs", "false")
+                .addGlobalProperty("modelTests", "false")
+                .addAdditionalProperty("useEnumCaseInsensitive", caseInsensitive);
+        File model = new DefaultGenerator().opts(configurator.toClientOptInput()).generate().stream()
+                .filter(file -> file.getName().equals("Example.java"))
+                .findFirst().orElseThrow(() -> new AssertionError("Example.java was not generated"));
+        EnumDeclaration uuidEnum = StaticJavaParser.parse(model).findAll(EnumDeclaration.class).stream()
+                .filter(declaration -> declaration.getNameAsString().equals("UuidEnum"))
+                .findFirst().orElseThrow(() -> new AssertionError("UuidEnum was not generated"));
+
+        // Compile the complete generated enum, including its Gson adapter and validation method.
+        // Only the enclosing model and its unrelated client dependencies are omitted.
+        Path runtime = Files.createDirectory(output.resolve("runtime"));
+        Path fixture = runtime.resolve("EnumFixture.java");
+        Files.writeString(fixture, "import com.google.gson.*;\n"
+                + "import com.google.gson.annotations.JsonAdapter;\n"
+                + "import com.google.gson.stream.*;\n"
+                + "import java.io.IOException;\n"
+                + "import java.util.UUID;\n"
+                + "public class EnumFixture {\n" + uuidEnum + "\n}\n");
+        String gsonClasspath = Paths.get(Gson.class.getProtectionDomain().getCodeSource().getLocation().toURI()).toString();
+        ByteArrayOutputStream diagnostics = new ByteArrayOutputStream();
+        assertNotNull(ToolProvider.getSystemJavaCompiler(), "Runtime regression tests require a JDK");
+        int result = ToolProvider.getSystemJavaCompiler().run(null, diagnostics, diagnostics,
+                "-classpath", gsonClasspath, "-d", runtime.toString(), fixture.toString());
+        assertEquals(result, 0, diagnostics.toString(StandardCharsets.UTF_8));
+
+        try (URLClassLoader loader = new URLClassLoader(new URL[]{runtime.toUri().toURL()}, getClass().getClassLoader())) {
+            Class<?> enumClass = loader.loadClass("EnumFixture$UuidEnum");
+            Method validate = enumClass.getMethod("validateJsonElement", JsonElement.class);
+            Gson gson = new Gson();
+            String[] values = {
+                    "123e4567-e89b-12d3-a456-426614174000",
+                    "123e4567-e89b-12d3-a456-426614174001"
+            };
+            Object[] constants = enumClass.getEnumConstants();
+            assertEquals(constants.length, values.length);
+            for (int i = 0; i < values.length; i++) {
+                String json = "\"" + values[i] + "\"";
+                assertEquals(gson.toJson(constants[i]), json);
+                assertSame(gson.fromJson(json, enumClass), constants[i]);
+                validate.invoke(null, new JsonPrimitive(values[i]));
+            }
+            for (String invalid : new String[]{"not-a-uuid", "123e4567-e89b-12d3-a456-426614174002"}) {
+                expectThrows(IllegalArgumentException.class,
+                        () -> gson.fromJson(new JsonPrimitive(invalid), enumClass));
+                InvocationTargetException exception = expectThrows(InvocationTargetException.class,
+                        () -> validate.invoke(null, new JsonPrimitive(invalid)));
+                assertTrue(exception.getCause() instanceof IllegalArgumentException,
+                        "Validation should reject malformed or unknown UUIDs: " + exception.getCause());
+            }
+        }
+    }
+}