Java 8 stream: list to flat map grouped by

Question

I have following situation. (pseudocode)

class A {
  id;
  List<B> bs;
}

class B {}

I wonder how to convert List os As -> Map of Bs

List<A> as;

// the Map key is A.id (Map<A.id, List<B>>)
Map<Integer, List<B>> bs = as.stream()
                       .map(a ->a.getBs())
                       .collect(// I dont know what to add here ???);

Here i have given a lot of examples for that. You may find it useful for your question https://www.javagists.com/java-8-streams-list-to-map-examples — awsome, Oct 31 '17 at 08:07

score 3 · Accepted Answer · edited Oct 31 '17 at 08:09

3

Seems like you want sometime like this:

 Map<Integer, List<B>> bs = as.stream()
        .collect(Collectors.toMap(A::getId, A::getBs));

edited Oct 31 '17 at 08:09

senerh

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answered Oct 31 '17 at 08:04

Eugene

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Assuming `id` is unique... – shmosel Oct 31 '17 at 08:11
@shmosel agree... – Eugene Oct 31 '17 at 08:11
1

My interpretation was that OP wanted to group and flatten multiple `bs`, but I could be wrong. – shmosel Oct 31 '17 at 08:12
1

@shmosel Indeed, the initial question context that was and the question title still fall in the line of intuition there. – Naman Oct 31 '17 at 08:56

Java 8 stream: list to flat map grouped by

1 Answers1