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

java java-stream

25,852

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());

25,852

membersound

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Updated on January 25, 2020

Comments

membersound over 4 years
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?
- Eran almost 8 years
  
  That depends on what MyListService is and how you obtain List[s] from instances of it.
- Mureinik almost 8 years
  
  What exactly is MyListService? Can you share its public methods please? How should the result look?
- membersound almost 8 years
  
  The service may contain any method that returns a list of objects
membersound almost 8 years

Great that works, if it's the preferred way.
Eran almost 8 years

@membersound That's what flatMap exists for.
wheelerswebservices over 6 years

Omg thanks this saved me from creating extra lists and moving them all 1 by 1 with a For Each