{"record":{"id":"308617c5c2c47222","repo":"lysine-dev/retrofit","slug":"response-must-be-parameterized-as-response-foo-or-308617","errorCode":null,"errorMessage":"Response must be parameterized as Response<Foo> or Response<? extends Foo>","messagePattern":"Response must be parameterized as Response<Foo> or Response<\\? extends Foo>","errorType":"exception","errorClass":"IllegalStateException","httpStatus":null,"severity":"error","filePath":"retrofit-adapters/java8/src/main/java/retrofit2/adapter/java8/Java8CallAdapterFactory.java","lineNumber":80,"sourceCode":"      Type returnType, Annotation[] annotations, Retrofit retrofit) {\n    if (getRawType(returnType) != CompletableFuture.class) {\n      return null;\n    }\n    if (!(returnType instanceof ParameterizedType)) {\n      throw new IllegalStateException(\n          \"CompletableFuture return type must be parameterized\"\n              + \" as CompletableFuture<Foo> or CompletableFuture<? extends Foo>\");\n    }\n    Type innerType = getParameterUpperBound(0, (ParameterizedType) returnType);\n\n    if (getRawType(innerType) != Response.class) {\n      // Generic type is not Response<T>. Use it for body-only adapter.\n      return new BodyCallAdapter<>(innerType);\n    }\n\n    // Generic type is Response<T>. Extract T and create the Response version of the adapter.\n    if (!(innerType instanceof ParameterizedType)) {\n      throw new IllegalStateException(\n          \"Response must be parameterized\" + \" as Response<Foo> or Response<? extends Foo>\");\n    }\n    Type responseType = getParameterUpperBound(0, (ParameterizedType) innerType);\n    return new ResponseCallAdapter<>(responseType);\n  }\n\n  private static final class BodyCallAdapter<R> implements CallAdapter<R, CompletableFuture<R>> {\n    private final Type responseType;\n\n    BodyCallAdapter(Type responseType) {\n      this.responseType = responseType;\n    }\n\n    @Override\n    public Type responseType() {\n      return responseType;\n    }\n","sourceCodeStart":62,"sourceCodeEnd":98,"githubUrl":"https://github.com/lysine-dev/retrofit/blob/d0b112dad073b7fe49c953ebc46ff1b424cb1e51/retrofit-adapters/java8/src/main/java/retrofit2/adapter/java8/Java8CallAdapterFactory.java#L62-L98","documentation":"For Java8-adapter methods declared as CompletableFuture<Response<...>>, the nested Response must be generic so the adapter can extract the body type. A raw Response (CompletableFuture<Response>) yields no deserialization target, so Java8CallAdapterFactory throws IllegalStateException when the inner type is not a ParameterizedType.","triggerScenarios":"A method like `CompletableFuture<Response> getUser();` — outer CompletableFuture is parameterized but the inner Response is raw. Thrown in get() after confirming the inner raw type is Response.class.","commonSituations":"Adding the Response wrapper for status/headers and forgetting the body generic; partial refactor.","solutions":["Parameterize Response: `CompletableFuture<Response<User>> getUser(@Path(\"id\") long id);`","Drop Response if you only need the body: `CompletableFuture<User> getUser(@Path(\"id\") long id);`"],"exampleFix":"// before\nCompletableFuture<Response> getUser(@Path(\"id\") long id);\n// after\nCompletableFuture<Response<User>> getUser(@Path(\"id\") long id);","handlingStrategy":"validation","validationCode":"import java.lang.reflect.*;\nstatic void assertNestedResponseParameterized(Class<?> iface) {\n  for (Method m : iface.getDeclaredMethods()) {\n    if (!(m.getGenericReturnType() instanceof ParameterizedType)) continue;\n    Type inner = ((ParameterizedType) m.getGenericReturnType()).getActualTypeArguments()[0];\n    if (inner.getTypeName().startsWith(\"retrofit2.Response\") && !(inner instanceof ParameterizedType)) {\n      throw new AssertionError(\"Raw nested Response on \" + m);\n    }\n  }\n}","typeGuard":null,"tryCatchPattern":"try {\n  CompletableFuture<Response<User>> f = service.getUser(id);\n} catch (IllegalStateException e) {\n  throw new IllegalStateException(\"Nested Response return type is raw: \" + e.getMessage(), e);\n}","preventionTips":["Fail builds on raw types with -Xlint:rawtypes.","Unit-test that nested Response types are parameterized.","Use the body-only form unless status/headers are needed."],"tags":["retrofit","java8","generics","java","type-system","response"],"backgroundTag":null,"analyzedSha":"d0b112dad073b7fe49c953ebc46ff1b424cb1e51","analyzedAt":"2026-08-13T23:49:47.955Z","schemaVersion":2},"datasetVersion":"2026-08-14T00:17:13.853Z"}