flatMap

Learn to flatten nested streams with flatMap.

Flattening a stream: flatMap

map turns each element into a single new value. flatMap turns each element into a stream of multiple values and then merges those streams into one. You use it to flatten nested lists.

Example: lists.stream().flatMap(List::stream). Each inner list is turned into a stream, then the streams are merged into a single stream. The result is a Stream<Integer> containing all elements of the inner lists in order.

Common patterns with flatMap

Two cases come up most often with flatMap: flattening nested collections and producing several values from a single object.

For example, if an Order object holds an items list, orders.stream().flatMap(o -> o.items().stream()) gives all items of all orders in a single stream. Another example: turning a string into a stream of characters with str.chars().mapToObj(c -> (char) c); this is also a one-to-many transform.

Exercise Flatten nested lists

Create a List<List<Integer>> with three inner lists: List.of(1, 2, 3), List.of(4, 5), List.of(6). Flatten the stream with flatMap(List::stream) and collect with toList. Then print each element on its own line using System.out::println.

Show example solution

import java.util.List;

public class FlattenLists {
    public static void main(String[] args) {
        List<List<Integer>> nested = List.of(
            List.of(1, 2, 3),
            List.of(4, 5),
            List.of(6)
        );
        List<Integer> flat = nested.stream()
            .flatMap(List::stream)
            .toList();
        flat.forEach(System.out::println);
    }
}

Test yourself

Match the map vs flatMap difference

map

One-to-one transform
flatMap

One-to-many transform
List::stream

Opens an inner list
Stream.of

Creates a stream
Which method merges nested lists into a single stream?
- flatMap
- map
- filter
- reduce
Show answer

flatMap — FlatMap. It flattens nested streams.

Complete the flatMap call

List<Integer> flat = nested.stream()
    .flatMap(____)
    .toList();

Show answer

List<Integer> flat = nested.stream()
    .flatMap(List::stream)
    .toList();

Next lesson: Summary - Advanced stream

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