如何使用分组依据获取实体中新列的列表

Question

我有一个Answers实体，其值是这样的

question answer 
1           x
2           y
3           z
4           p
1           x
2           q
3           r

我需要按问题分组回答

Map<Integer, List<String>>
<1, [x,x]
 2, [y,p]
 3, [z,r]
 4, [p]>

我可以这样得到List<Answer>

Map<Integer, List<Answer>> collect = answers
    .stream()
    .collect(Collectors.groupingBy(Answer::getQuestion));

但是我找不到一种将其作为List<String>而非List<Answer>的方式吗？

Answer 1

 Map<Integer, List<String>> collect = answers
         .stream()
          .collect(Collectors.groupingBy(
                Answer::getQuestion,
                Collectors.mapping(Answer::getAnswerAsString, Collectors.toList())       
));

只需将getAnswerAsString替换为可以从Answer获得该String的任何方法即可。

Answer 2

您可以这样做

Map<Integer, List<String>> collect = answers.stream()
    .collect(Collectors.groupingBy(Answer::getQuestion, 
        Collectors.mapping(Answer::getAnswer, Collectors.toList())));