How to convert List of Lists to single List using Java8 streams

Question

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

1. getAllSubjects() returns all List and each Subject has List<Topic>. all List should be combined as List<Topic>.
2. 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));

}

Answer 1

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));