How to collect multiple lists to one list with java-streams?

Question

How can I collect multiple List values into one list, using java-streams?

List<MyListService> services;

services.stream().XXX.collect(Collectors.toList());


interface MyListService {
   List<MyObject> getObjects();
}

As I have full control over the interface: or should I change the method to return an Array instead of a List?

That depends on what `MyListService` is and how you obtain List[s] from instances of it. — Eran, Jun 06 '16 at 07:55
What exactly is `MyListService`? Can you share its public methods please? How should the result look? — Mureinik, Jun 06 '16 at 07:58
The service may contain *any* method that returns a list of objects — membersound, Jun 06 '16 at 08:02

score 56 · Accepted Answer · edited Jun 06 '16 at 08:03

56

You can collect the Lists contained in the MyListService instances with flatMap :

List<MyObject> list = services.stream()
                              .flatMap(s -> s.getObjects().stream())
                              .collect(Collectors.toList());

edited Jun 06 '16 at 08:03

membersound

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answered Jun 06 '16 at 08:02

Eran

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Great that works, if it's the preferred way. – membersound Jun 06 '16 at 08:03
6

@membersound That's what `flatMap` exists for. – Eran Jun 06 '16 at 08:06
Omg thanks this saved me from creating extra lists and moving them all 1 by 1 with a For Each – wheelerswebservices Sep 08 '17 at 21:19

How to collect multiple lists to one list with java-streams?

1 Answers1