Collecting lists from an object list using Java 8 Stream API

Question

I have a class like this

public class Example {     private List<Integer> ids;      public getIds() {         return this.ids;      } } 

If I have a list of objects of this class like this

List<Example> examples; 

How would I be able to map the id lists of all examples into one list? I tried like this:

List<Integer> concat = examples.stream().map(Example::getIds).collect(Collectors.toList()); 

but getting an error with Collectors.toList()

What would be the correct way to achive this with Java 8 stream api?

Answer 1

Use flatMap:

List<Integer> concat = examples.stream()     .flatMap(e -> e.getIds().stream())     .collect(Collectors.toList()); 

Answer 2

Another solution by using method reference expression instead of lambda expression:

List<Integer> concat = examples.stream()                                .map(Example::getIds)                                .flatMap(List::stream)                                .collect(Collectors.toList());