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?
2 Answers
Use flatMap:
List<Integer> concat = examples.stream()
.flatMap(e -> e.getIds().stream())
.collect(Collectors.toList());
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());
