Get response from CompletableFuture

Question

I'm trying to add Async to my fetch operation from the db. However, I'm stuck at trying to get the response from CompletableFuture.allOf(). Join return a 'void. Instead how can I return a List<QuestionResponseData> as mentioned in the code. Below is my code

@RequestMapping(method = RequestMethod.POST, path = "/questionsByAssessmentIdAndQuestionNumber")
    public List<QuestionResponseData> getQuestionsByIdAndLimit(@RequestBody QuestionOptionRequest questionOptionRequest) {
        String assessmentId = questionOptionRequest.getAssessmentId();
        QuestionOptionRequest request = QuestionOptionRequest.builder().assessmentId(questionOptionRequest.getAssessmentId()).startingQuestion(1).build();
        CompletableFuture<List<QuestionResponseData>> questions = questionService.getQuestions(request);
        QuestionOptionRequest request1 = QuestionOptionRequest.builder().assessmentId(assessmentId).startingQuestion(26).build();
        CompletableFuture<List<QuestionResponseData>> questions1 = questionService.getQuestions(request1);
        QuestionOptionRequest request2 = QuestionOptionRequest.builder().assessmentId(assessmentId).startingQuestion(51).build();
        CompletableFuture<List<QuestionResponseData>> questions2 = questionService.getQuestions(request2);
        QuestionOptionRequest request3 = QuestionOptionRequest.builder().assessmentId(assessmentId).startingQuestion(76).build();
        CompletableFuture<List<QuestionResponseData>> questions3 = questionService.getQuestions(request3);

        CompletableFuture.allOf(questions, questions1, questions2, questions3).join();

        //return questionService.getQuestions(questionOptionRequest);
    }
    

Answer 1

 CompletableFuture<List<QuestionResponseData>> futures = new CompletableFuture[]{request, request1, request2, request3};

 List<QuestionResponseData> result =
    CompletableFuture.allOf(futures)
            .thenApply(_void -> Arrays.stream(futures).map(CompletableFuture::join).flatMap(List::stream).collect(Collectors.toList()))
            .join();

Notice that calling CompletableFuture::join does not actually block, the blocking has already happened in CompletableFuture::allOf