RxExample.java

void getNowPlayingMovies() {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(BuildConfig.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();

        TmdbInterface tmdbInterface = retrofit.create(TmdbInterface.class);

        Observable<HashMap<Integer, String>> genresListResponseObservable =
                tmdbInterface.getGenresList(BuildConfig.API_KEY)
                        .map(new Function<GenresListResponse, HashMap<Integer, String>>() {
                            @Override
                            public HashMap<Integer, String> apply(GenresListResponse genresListResponse) throws Exception {
                                HashMap<Integer, String> genresMap = new HashMap<>();

                                for (Genre genre : genresListResponse.getGenres()) {
                                    genresMap.put(genre.getId(), genre.getName());
                                }

                                return genresMap;
                            }
                        })
                        .subscribeOn(Schedulers.newThread())
                        .observeOn(Schedulers.newThread());

        Observable<NowPlayingMoviesListResponse> nowPlayingMoviesListResponseObservable =
                tmdbInterface.getNowPlayingMovies(BuildConfig.API_KEY)
                        .subscribeOn(Schedulers.newThread())
                        .observeOn(Schedulers.newThread());

        Observable.zip(genresListResponseObservable, nowPlayingMoviesListResponseObservable,
                new BiFunction<HashMap<Integer, String>, NowPlayingMoviesListResponse, NowPlayingMoviesListResponse>() {
                    @Override
                    public NowPlayingMoviesListResponse apply(HashMap<Integer, String> genresMap, NowPlayingMoviesListResponse nowPlayingMoviesListResponse) throws Exception {

                        for (Result nowPlayingMovie : nowPlayingMoviesListResponse.getResults()) {
                            StringBuilder genre = new StringBuilder();

                            for (int i = 0; i < nowPlayingMovie.getGenreIds().size(); i++) {
                                Integer id = nowPlayingMovie.getGenreIds().get(i);
                                if (i == 0) {
                                    genre.append(genresMap.get(id));
                                } else {
                                    genre.append(", ").append(genresMap.get(id).toLowerCase());
                                }
                            }

                            nowPlayingMovie.setGenre(genre.toString());
                        }

                        return nowPlayingMoviesListResponse;
                    }
                }).observeOn(AndroidSchedulers.mainThread())
                .subscribeOn(Schedulers.newThread())
                .subscribe(new Observer<NowPlayingMoviesListResponse>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onNext(NowPlayingMoviesListResponse nowPlayingMoviesListResponse) {
                        mNowPlayingMoviesListView
                                .addNowPlayingMoviesToList(nowPlayingMoviesListResponse.getResults());
                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onComplete() {

                    }
                });
    }