Java Streams and List of List of List

Question

For some reason I can't wrap my head around how to turn this deeply nested list into a new list using streams.

   every A in List<A> contains -> 
   List<B> where every B contains -> 
   List<C> where every C contains -> List<String>

I have tried many different iterations like:

  List<String> newlist =  listA.getB()
                   .stream()
                   .filter(b -> b.getC()
                   .stream()
                   .filter(c -> c.getPeople)
                   .collect(Collectors.toList())

I'm full of confusion... I could easily do this with for loops but I've heard that streams are simple and easy and I would like to start using them more.

Possibly related: https://stackoverflow.com/questions/25147094/how-can-i-turn-a-list-of-lists-into-a-list-in-java-8 — Lino, Feb 05 '19 at 11:46
You could also use: https://stackoverflow.com/a/40399455/6419007 — Eric Duminil, Feb 05 '19 at 12:53

score 14 · Accepted Answer · answered Feb 05 '19 at 11:41

14

You should use flatMap:

List<String> newList =
    listA.stream() // Stream<A>
         .flatMap(a->a.getB().stream()) // Stream<B>
         .flatMap(b->b.getC().stream()) // Stream<C>
         .flatMap(c->c.gtPeople().stream()) // Stream<String>
         .collect(Collectors.toList()); // List<String>

answered Feb 05 '19 at 11:41

Eran

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whelp, sure. Thanks! :) – Clomez Feb 05 '19 at 11:47

Java Streams and List of List of List

1 Answers1