rxJavaObserver.java

private List<Movies> movies = new ArrayList<>();

movieService.getMovies()
       .flatMap(new Function<List<Movie>, ObservableSource<Movie>>() {
             @Override
             public ObservableSource<Movie> apply(List<Movie> movies) 
                           throws Exception {
                  return Observable.fromIterable(movies);
             }
        })
        .flatMap(new Function<Movie, ObservableSource<Pair<Movie, List<Cast>>>() {
             @Override
             public ObservableSource<Pair<Movie, List<Cast>>> apply(Movie movie) 
                            throws Exception {
                return Observable.zip(movieService.getMovieCasts(movie.getId()),
                    Observable.just(movie),
                    new BiFunction<List<Cast>, Movie, Pair<Movie, List<Cast>>>() {
                        @Override
                        public Pair<Movie, List<Cast>> apply(List<Cast> casts, Movie movie) 
                            throws Exception {
                               return new Pair<>(movie,casts);
                           }
                        });
                   }
            })
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new Observer<Pair<Movie, List<Cast>>>() {
                @Override
                public void onSubscribe(Disposable d) {
                            
                }

                @Override
                public void onNext(Pair<Movie, List<Cast>> value) {
                    Movie movie =  value.first;
                    movie.setCasts(value.second);
                    movies.add(movie);
                }

                @Override
                public void onError(Throwable e) {

                }

                @Override
                public void onComplete() {
                    updateUI(movies)
                }
            });