Collecting lists from an object list using Java 8 Stream API

Question

I have a class like this

public class Example {
    private List ids;

    public getIds() {
        return this.ids; 
    }
}

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());