How to flatten a list of objects containing a list and pass each element into a method?

Question

Given the following entity:

public class Parent extends {
    private Long id;    
    private List<Child> children = new ArrayList<>();
}

I am attempting to learn how to use the Java 8 Stream API to flatten a List<Parent>, each containing a List<Child> and pass the stream of Child into a void method.

For example:

List<Parent> parents = //..

schedules.stream()
        .flatMap(Parent::getChildren)
        .forEach(this::invoke);

This gives the error:

Bad return type in method reference: cannot convert java.util.List< Child > to java.util.stream.Stream < ? extends R >

You flatMap to a stream. So `.flatMap(parent -> parent.getChildren().stream())` — Michael, Jul 08 '19 at 20:48
Also I think that if the children of that first operation have any children themselves, no flat map construct will recursively invoke `getChildren`. It's one level of children only. — markspace, Jul 08 '19 at 20:50

score 2 · Accepted Answer · answered Jul 08 '19 at 20:55

Stream.flatMap() requires a Function which returns a Stream as parameter. You are trying to pass a Function which returns a List. Use this instead:

List<Parent> parents = ...;
parents.stream()
        .flatMap(p -> p.getChildren().stream())
        .forEach(this::invoke);

Alternatively you can use Stream.map() and Stream.flatMap():

parents.stream()
        .map(Parent::getChildren)
        .flatMap(Collection::stream)
        .forEach(this::invoke);

How to flatten a list of objects containing a list and pass each element into a method?

1 Answers1