Completable Future Suggestion needed

Question

i am new to CompletableFutures and need some to help better understand this topic. Trying to implement a personal project where i write a service to read some data from the mysql sakila db. Here is the db schema : enter link description here

Here is what i am trying to achieve :

1.) CompletableFuture to Load a Film object by id. 2.) If it doesnt exist then dont proceed. 3.) If it exists then load : Categories , Languages in parallel and update Category Names and Language Name in the Film object returned at 1.)

While i can do it using thenApply , i would ideally like to load : the filmCategories ,categories and languages in parallel and then merge the data into my dto.

Any help would be appreciated

    CompletableFuture<Map<Integer,String>> categories = CompletableFuture.supplyAsync(
            () -> categoryService.getCategories().stream()
                    .collect(Collectors.toMap(CategoryDto::getCategoryId,CategoryDto::getCategoryName))

    ) ;

    CompletableFuture<Map<Integer,String>> languages = CompletableFuture.supplyAsync(
            () -> languageService.getLanguages().stream()
                    .collect(Collectors.toMap(LanguageDto::getLanguageId,LanguageDto::getLanguageName))
    ) ;

    CompletableFuture<Map<Integer,List<FilmCategoryDto>>> filmCategories = CompletableFuture.supplyAsync(
            () -> filmCategoryRepository.findFilmCategoryEntitiesByFilmId(filmId)
                    .stream().map(
                            e -> {
                                FilmCategoryDto filmCategoryDto = mapper.filmCategoryToFilmCategoryDto(e);
                                filmCategoryDto.setCategoryName(categories.join().get(filmCategoryDto.getCategoryId()));
                                return filmCategoryDto;
                            }

                    )
                    .collect(Collectors.groupingBy(FilmCategoryDto::getFilmId))
    ) ;

    CompletableFuture.allOf(filmCategories,languages).join();
    return filmRepository.findById(filmId)
            .map(e -> {
                FilmDto f = mapper.filmToFilmDto(e);
                f.setLanguageName(languages.join().get(f.getLanguageId()));
                f.setFilmCategories(filmCategories.join().get(f.getFilmId()));
                return f;
            })
            .orElse(null);