Creating a nested Map using Streams and Collectors

Question

class QuizAnswers {
  List multipleChoiceAnswers;
  List freeResponseAnswers; //not relevant to this question
}

class MultipleChoiceAnswer {
  int questionId;
  // The index of the selected multiple choice question
  int answer_selection;
}

The input to my function is a List.

I want to create an output of Map> that maps . In other words, I want to create a nested map that maps each multiple choice quiz question to a map representing the total number of selections on each answer choice of that quiz question.

Suppose the input List quizAnswersList as:

[ {questionId: 1, answer_selection: 2},    
  {questionId: 1, answer_selection:2},  
  {questionId: 1, answer_selection:3},   
  {questionId: 2, answer_selection:1} ]

Then I would want the output to be:

{1 : {2:2, 3:1}, 2: {1, 1}}

Because the question with Id = 1 received two selections on answer choice 2 and 1 selection on answer choice 3 while the question with Id=2 had 1 selection on answer choice 1.

I have tried

quizAnswersList.stream()
            .map(
                quizAnswers ->
                    quizAnswers.getMultipleChoiceAnswers().stream()
                        .collect(
                            Collectors.groupingBy(
                                MultipleChoiceAnswer::getQuestionId,
                                Collectors.groupingBy(
                                    MultipleChoiceAnswer::getAnswerSelection,
                                    Collectors.counting()))));

Which is giving me an error. I am not very familiar with streams and collectors in general, so I'd love to learn how to do this correctly.

user_17949945 · Answer

input to my function is a List. I want to create an output of Map>

I have tried quizAnswersList.stream().map(quizAnswers -> ... ) which is giving me an error.

Method map() is an intermediate operation, i.e. it yields a stream. Therefore, if you try to assign the stream-statement you've listed to a variable of type Map you'll get a compilation error.

A stream pipeline needs to end with a terminal operation like collect in order to be executed and produce a result.

And before collect() you have to apply flatMap(), which expects a stream as an argument, to transform the stream of QuizAnswers in to a stream of MultipleChoiceAnswer.

Your usage of collectors is correct and doesn't require any changes.

public static void main(String[] args) {
    List quizAnswersList =
        List.of(new QuizAnswers(List.of(new MultipleChoiceAnswer(1, 2),
                                        new MultipleChoiceAnswer(1, 2))),
                new QuizAnswers(List.of(new MultipleChoiceAnswer(1, 3),
                                        new MultipleChoiceAnswer(2, 1))));

    Map> totalCountOfAnswerSelectionByQuestion =
        quizAnswersList.stream()
            .flatMap(quizAnswers -> quizAnswers.getMultipleChoiceAnswers().stream())
            .collect(Collectors.groupingBy(MultipleChoiceAnswer::getQuestionId,
                        Collectors.groupingBy(MultipleChoiceAnswer::getAnswerSelection,
                            Collectors.counting())));

    System.out.println(totalCountOfAnswerSelectionByQuestion);
}

Output

{1={2=2, 3=1}, 2={1=1}}

user_1552534 · Answer

I want to create an output of Map> that maps , total count of answer_selection>.

You were close. You just didn't flatMap the MultipleChoiceAnswers onto the stream so you had a nested stream and that was causing the problem.

Based on your edited question, here is what I came up with.

List mca =
        List.of(new MultipleChoiceAnswer(1, 2),
                new MultipleChoiceAnswer(1, 2),
                new MultipleChoiceAnswer(1, 3),
                new MultipleChoiceAnswer(2, 1));

// more could be added to the List.  You only provided one.
List list = List.of(new QuizAnswers(mca));

flatMap all the MultipleChoice lists
group them by the questionId
then subgroup them according to AnswerSelection and get a count
then you get the Map output you requested.

Map> map = list.stream()
        .flatMap(s -> s.getMultipleChoiceAnswers().stream())
        .collect(Collectors.groupingBy(
                MultipleChoiceAnswer::getQuestionId,
                Collectors.groupingBy(
                        MultipleChoiceAnswer::getAnswerSelection,
                        Collectors.counting())));

map.entrySet().forEach(System.out::println);

prints

1={2=2, 3=1}
2={1=1}

Questions

how do you want to handle multiple QuizAnswer instances?
how do you want to handle multiple MulitpleChoiceAnswer lists. You only provided one of each.

They could all be flatmapped together and processed as above. But I think there could be some differences in the Answers (perhaps for different tests) which you don't want grouped and counted as the same.

Example

If I add the following to the List

List mca2 =
List.of(new MultipleChoiceAnswer(1, 2),
        new MultipleChoiceAnswer(1, 2),
        new MultipleChoiceAnswer(5, 2),
        new MultipleChoiceAnswer(5, 2));

And process using the above solution, the output would be

1={2=4, 3=1}
2={1=1}
5={2=2}

Creating a nested Map using Streams and Collectors

2 Answers