How to throw custom exception with Retrofit2 and RxJava2

Question

I have a problem with throwing custom exceptions when using Retrofit2 and RxJava2.

Retrofit Api definition

public interface BackendInterface {

    @Headers({"......"})
    @POST("international-services/")
    Observable<BackendResponse> post(@Body BackendRequest request, @Header("sessionId") String sessionId);
}

Usage of Retrofit Api

@Override
public Observable<SomeBusinessObject> getSomething(@NonNull String sessionId, @NonNull SomeOtherBusinessObject something) {
    BackendRequest br = createBackendRequest(something);
    return backendInterface.post(br, sessionId).map(backendResponse -> {

        if (backendResponse.getResult().getCode() != 0) {
            return Observable.error(new ServiceException("my custom exception"));
        }

        SomeBusinessObject result = mapBusinessObject(br);
        return result;
    });
}

The problem is the following line

return Observerable.error(new ServiceException("my custom exception"));

Android Studio marks it as error and says "no instance(s) of type variable(y) T exist so that Observable conforms to SomeBusinessObject inference variable R has incompatible bounds: equality contraints: SomeBusinessObject lower bounds: Observable

The IDE's quick fix suggests to make my method "getSomething" return Observable.

I have no idea how to forward custom exceptions to the subscriber of my observable.

Answer 1

Your .map is trying to return both SomeBusinessObject as well as Observable<"SomeType"> . Since they don't have a common parent object it cannot resolve what Observable.error() should return.

The solution it to use .flatmap instead and replacing the return with return Observable.just(result);