Java 8 stream: list to flat map grouped by [duplicate]

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 javagists.com/java-8-streams-list-to-map-examples — awsome
– awsome, Commented Oct 31, 2017 at 8:07

senerh · Accepted Answer · 2017-10-31 08:09:23Z

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, 2017 at 8:09

senerh

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answered Oct 31, 2017 at 8:04

Eugene

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4 Comments

shmosel Over a year ago

Assuming id is unique...

Eugene Over a year ago

@shmosel agree...

shmosel Over a year ago

My interpretation was that OP wanted to group and flatten multiple bs, but I could be wrong.

Naman Over a year ago

@shmosel Indeed, the initial question context that was and the question title still fall in the line of intuition there.

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