{"id":"e0d7bbbb67cd63af","repo":"square/retrofit","slug":"name-return-type-must-be-parameterized-as-nam","errorCode":null,"errorMessage":"${name} return type must be parameterized as ${name}<Foo> or ${name}<? extends Foo>","messagePattern":"(.+?) return type must be parameterized as (.+?)<Foo> or (.+?)<\\? extends Foo>","errorType":"exception","errorClass":"IllegalStateException","httpStatus":null,"severity":"error","filePath":"retrofit-adapters/rxjava/src/main/java/retrofit2/adapter/rxjava/RxJavaCallAdapterFactory.java","lineNumber":114,"sourceCode":"  public @Nullable CallAdapter<?, ?> get(\n      Type returnType, Annotation[] annotations, Retrofit retrofit) {\n    Class<?> rawType = getRawType(returnType);\n    boolean isSingle = rawType == Single.class;\n    boolean isCompletable = rawType == Completable.class;\n    if (rawType != Observable.class && !isSingle && !isCompletable) {\n      return null;\n    }\n\n    if (isCompletable) {\n      return new RxJavaCallAdapter(Void.class, scheduler, isAsync, false, true, false, true);\n    }\n\n    boolean isResult = false;\n    boolean isBody = false;\n    Type responseType;\n    if (!(returnType instanceof ParameterizedType)) {\n      String name = isSingle ? \"Single\" : \"Observable\";\n      throw new IllegalStateException(\n          name\n              + \" return type must be parameterized\"\n              + \" as \"\n              + name\n              + \"<Foo> or \"\n              + name\n              + \"<? extends Foo>\");\n    }\n\n    Type observableType = getParameterUpperBound(0, (ParameterizedType) returnType);\n    Class<?> rawObservableType = getRawType(observableType);\n    if (rawObservableType == Response.class) {\n      if (!(observableType instanceof ParameterizedType)) {\n        throw new IllegalStateException(\n            \"Response must be parameterized\" + \" as Response<Foo> or Response<? extends Foo>\");\n      }\n      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);\n    } else if (rawObservableType == Result.class) {","sourceCodeStart":96,"sourceCodeEnd":132,"githubUrl":"https://github.com/square/retrofit/blob/d0b112dad073b7fe49c953ebc46ff1b424cb1e51/retrofit-adapters/rxjava/src/main/java/retrofit2/adapter/rxjava/RxJavaCallAdapterFactory.java#L96-L132","documentation":"Thrown by RxJavaCallAdapterFactory while validating a service method's return type. The placeholder `${name}` resolves to `Single` or `Observable` depending on the raw type detected (set at RxJavaCallAdapterFactory.java:113). A raw reactive type gives the factory no body type, so it cannot pick a converter; the check at line 112 (`returnType instanceof ParameterizedType`) rejects it. (Completable is handled separately and does not need a type parameter.)","triggerScenarios":"A service method returns a raw `Observable` or raw `Single` (RxJava 1.x), e.g. `@GET(\"/\") Observable getUser();`. name becomes 'Observable' or 'Single' in the message.","commonSituations":"Migrating to RxJava call adapters and forgetting the generic; suppressing raw-types warnings; or copying a method and dropping `<User>`. Surfaces at first invocation of the method because Retrofit validates lazily.","solutions":["Parameterize the reactive type: `Observable<User> getUser();` or `Single<User> getUser();`.","For HTTP metadata, wrap the body: `Observable<Response<User>>` or `Observable<Result<User>>`.","Use `Completable` for fire-and-forget endpoints with no body (no parameterization needed)."],"exampleFix":"// before\n@GET(\"users/{id}\")\nObservable getUser(@Path(\"id\") String id);\n\n// after\n@GET(\"users/{id}\")\nObservable<User> getUser(@Path(\"id\") String id);","handlingStrategy":"validation","validationCode":"// Compile-time: javac -Xlint:rawtypes -Werror.\n// Reflection guard covering Observable and Single:\nfor (Method m : MyService.class.getDeclaredMethods()) {\n    Class<?> raw = m.getReturnType();\n    if (raw == rx.Observable.class || raw == rx.Single.class) {\n        throw new IllegalStateException(m + \" returns raw \" + raw.getSimpleName() + \"; parameterize it.\");\n    }\n}","typeGuard":null,"tryCatchPattern":"// Surface it loudly in a wiring test:\ntry {\n    retrofit.create(MyService.class).getUser(\"1\");\n} catch (IllegalStateException e) {\n    if (e.getMessage().contains(\"return type must be parameterized\")) {\n        throw new IllegalStateException(\"Raw Observable/Single in MyService; parameterize it.\", e);\n    }\n    throw e;\n}","preventionTips":["Treat raw-types warnings as errors.","Remember Completable is the only RxJava1 type that needs no parameter.","Add a smoke test that invokes one method per service to trigger adapter validation."],"tags":["retrofit","rxjava","java","reflection","type-erasure","call-adapter"],"analyzedSha":"d0b112dad073b7fe49c953ebc46ff1b424cb1e51","analyzedAt":"2026-08-04T19:12:59.096Z","schemaVersion":2}