Reverse Map structure using Java 8 streams

Question

Given a class

public class MyClass {
    public int langId;
    public int sectionId;
}

If I have a map that maps a LangId to all the instances of MyClass that have that LangId:

Map<Integer, List<MyClass>> mapLangIdToListOfMyClass = new HashMap<>();

Using Java 8 streams, would there be a simple way to consume the previous Map and create a new Map that maps SectionId to all the instances of MyClass that has that SectionId:

Map<Integer, List<MyClass>> mapSectionIdToListOfMyClass = new HashMap<>();

possible duplicate of [Reverse HashMap keys and values in Java](http://stackoverflow.com/questions/20412354/reverse-hashmap-keys-and-values-in-java) — SobiborTreblinka, May 04 '15 at 01:46
Flagging as duplicate http://stackoverflow.com/questions/20412354/reverse-hashmap-keys-and-values-in-java — SobiborTreblinka, May 04 '15 at 01:47
@SobiborTreblinka The OP is not reversing keys and values, but grouping based on a different field. — Paul Boddington, May 04 '15 at 01:50
@pbabcdefp, good call, unfortunately, I don't have the ability to retract the flag — SobiborTreblinka, May 04 '15 at 02:05

score 6 · Accepted Answer · answered May 04 '15 at 01:29

I think you can do

Map<Integer, List<MyClass>> mapSectionIdToListOfMyClass = mapLangIdToListOfMyClass
                               .values()
                               .stream()
                               .flatMap(List::stream)
                               .collect(Collectors.groupingBy(j -> j.sectionId));

Reverse Map structure using Java 8 streams

1 Answers1

Linked