square/retrofit · error · IllegalStateException

Result must be parameterized as Result<Foo> or Result<? exte

Error message

Result must be parameterized as Result<Foo> or Result<? extends Foo>

What it means

Thrown by RxJava2CallAdapterFactory when the outer reactive type is parameterized with raw `Result`. The factory needs `Result<T>` to extract the body type T; raw Result fails the check at RxJava2CallAdapterFactory.java:137. Result captures both successful responses and errors as onNext emissions so neither path invokes onError.

Source

Thrown at retrofit-adapters/rxjava2/src/main/java/retrofit2/adapter/rxjava2/RxJava2CallAdapterFactory.java:138

              + " return type must be parameterized"
              + " as "
              + name
              + "<Foo> or "
              + name
              + "<? extends Foo>");
    }

    Type observableType = getParameterUpperBound(0, (ParameterizedType) returnType);
    Class<?> rawObservableType = getRawType(observableType);
    if (rawObservableType == Response.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException(
            "Response must be parameterized" + " as Response<Foo> or Response<? extends Foo>");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
    } else if (rawObservableType == Result.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException(
            "Result must be parameterized" + " as Result<Foo> or Result<? extends Foo>");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
      isResult = true;
    } else {
      responseType = observableType;
      isBody = true;
    }

    return new RxJava2CallAdapter(
        responseType, scheduler, isAsync, isResult, isBody, isFlowable, isSingle, isMaybe, false);
  }
}

View on GitHub (pinned to d0b112dad0)

Solutions

1. Parameterize Result: `Observable<Result<User>> getUser();`.
2. If you want standard Rx error semantics, use `Observable<User>` (HttpException on non-2xx) or `Observable<Response<User>>`.

Example fix

// before
@GET("users/{id}")
Observable<Result> getUser(@Path("id") String id);

// after
@GET("users/{id}")
Observable<Result<User>> getUser(@Path("id") String id);

for (Method m : MyService.class.getDeclaredMethods()) {
    Type rt = m.getGenericReturnType();
    if (rt instanceof ParameterizedType
            && ((ParameterizedType) rt).getActualTypeArguments()[0] == retrofit2.adapter.rxjava2.Result.class) {
        throw new IllegalStateException(m + " uses raw Result inside an RxJava2 type.");
    }
}

try {
    retrofit.create(MyService.class).getUser("1");
} catch (IllegalStateException e) {
    if (e.getMessage().contains("Result must be parameterized")) {
        throw new IllegalStateException("Fix raw Result<> inside RxJava2 return in MyService.", e);
    }
    throw e;
}

When it happens

Trigger: A service method returns `Observable<Result>` (or Flowable/Single/Maybe) with raw Result instead of `Observable<Result<User>>`.

Common situations: Choosing the Result strategy and forgetting the inner generic; autocompleting Result without its argument.

Related errors

• Result must be parameterized as Result<Foo> or Result<? exte
• ${name} return type must be parameterized as ${name}<Foo> or
• Response must be parameterized as Response<Foo> or Response<
• ListenableFuture return type must be parameterized as Listen
• Response must be parameterized as Response<Foo> or Response<

AI-assisted analysis of square/retrofit@d0b112dad0 (2026-08-04). Data as JSON: /data/errors/f730c765bc5f5f77.json. Report an issue: GitHub.