How to convert List of Lists to single List using Java8 streams [duplicate]

Question

How to convert following code using stream without using for each loop.

getAllSubjects() returns all List and each Subject has List<Topic>. all List should be combined as List<Topic>.
Needs to get Map<id,topicName> from List<Topic>

Object Model:

Subject
  id,....
  List<Topic>
Topic
  id,name

public Map<String, String> getSubjectIdAndName(final String subjectId) {

    List<Subject> list = getAllSubjects(); // api method returns all subjects
    //NEEDS TO IMPROVE CODE USING STREAMS
    list = list.stream().filter(e -> e.getId().equals(subjectId)).collect(Collectors.toList());
    List<Topic> topicList = new ArrayList<>();
    for (Subject s : list) {
        List<Topic> tlist = s.getTopics();
        topicList.addAll(tlist);
    }
    return topicList.stream().collect(Collectors.toMap(Topic::getId, Topic::getName));

}

Just remove these intermediate collections into Lists, then, study flatMap and you’re done. In other words, return getAllSubjects() .stream() .filter(e -> e.getId().equals(subjectId)) .flatMap(s -> s.getTopics().stream()) .collect(Collectors.toMap(Topic::getId, Topic::getName)); — Holger
– Holger, Commented Feb 28, 2018 at 10:21

Eugene · Accepted Answer · 2018-02-28 10:28:00Z

9

Use flatMap here, to not stream again. Just notice that this toMap assumes that there will be no duplicate keys (or nulls)

list.stream()
    .filter(e -> subjectId.equals(e.getId()))
    .flatMap(subject -> subject.getTopics().stream())
    .collect(Collectors.toMap(Topic::getId, Topic::getName));

answered Feb 28, 2018 at 10:28

Eugene

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