Your Accumulator

MoviesSelectors.java

public static BiFunction<ArrayList<GenreMovies>, Movie, ArrayList<GenreMovies>> selectAndSeparate(String... genres) {
    return (findeds, movie) -> {
        for(String genre : genres) {
            GenreMovies genreMovieToAdd = getGenreMoviesInListByGenre(genre, findeds);
            if(genreMovieToAdd == null) {
                genreMovieToAdd = new GenreMovies(genre);
                findeds.add(genreMovieToAdd);
            }
            if(Arrays.asList(movie.getGenresAsArray()).contains(genre)) {
                genreMovieToAdd.getMovies().add(movie);
            }
        }
        return findeds;
    };
}