IllegalArgumentException: Could not locate call adapter for rx.Observable RxJava, Retrofit2

Question

I am getting the above error while calling the rest api. I am using both retrofit2 and RxJava.

ServiceFactory.java

public class ServiceFactory {
public static <T> T createRetrofitService(final Class<T> clazz, final String endpoint){

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(endpoint)
            //.addConverterFactory(GsonConverterFactory.create())

            .build();

    T service = retrofit.create(clazz);
    return service;
}

}

MovieService.java

public interface MovieService{
//public final String API_KEY = "<apikey>";
public final String SERVICE_END = "https://api.mymovies.org/3/";
@GET("movie/{movieId}??api_key=xyz")
Observable<Response<Movies>> getMovies(@Field("movieId") int movieId);

}

Inside MainActivity

      MovieService   tmdbService = ServiceFactory.createRetrofitService(MovieService.class, MovieService.SERVICE_END);
    Observable<Response<Movies>> responseObservable = tmdbService.getMovies(400);
    responseObservable .subscribeOn(Schedulers.newThread())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new Subscriber<Response<Movies>>() {
                @Override
                public void onCompleted() {

                }

                @Override
                public void onError(Throwable e) {

                }

                @Override
                public void onNext(Response<Movies> moviesResponse) {

                }
            });

Answer 1

Be sure to add implementation 'com.squareup.retrofit2:adapter-rxjava2:2.4.0' or whatever version you are using to your dependencies, and then configure retrofit with that converter:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl(endpoint)
    .addConverterFactory(GsonConverterFactory.create())
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
    .build();

Updated

RxJavaCallAdapterFactory was renamed to RxJava2CallAdapterFactory. Changed the snipped above.