The Java Stream flatMap() method is an intermediate operation.

The Stream.flatMap() function, as the name suggests, is the combination of a map and a flat operation. This means you first apply the map function and then flatten the result.

Java Stream flatMap() Example

To understand what flattening a stream consists in, consider a structure like [ [1,2,3],[4,5,6],[7,8,9] ] which has "two levels". It's basically a big List containing three more List. Flattening this means transforming it in a "one level" structure e.g. [ 1,2,3,4,5,6,7,8,9 ] i.e. just one list.

For example: In the below program, you can see that we have three lists that are merged into one by using a flatMap() function:

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args)
    {
        List<Integer> evens = Arrays.asList(2, 4, 6);
        List<Integer> odds = Arrays.asList(3, 5, 7);
        List<Integer> primes = Arrays.asList(2, 3, 5, 7, 11);
        List<Integer> numbers = Stream.of(evens, odds, primes)
                .flatMap(list -> list.stream())
                .collect(Collectors.toList());
        System.out.println("flattend list: " + numbers);
    }
}

Output:

flattend list: [2, 4, 6, 3, 5, 7, 2, 3, 5, 7, 11]

Related Java Stream API's Examples

1. Java Stream filter() Example
2. Java Stream map() Example
3. Java Stream flatMap() Example
4. Java Stream distinct() Example
5. Java Stream limit() Example
6. Java Stream peek() Example
7. Java Stream anyMatch() Example
8. Java Stream allMatch() Example
9. Java Stream noneMatch() Example
10. Java Stream collect() Example
11. Java Stream count() Example
12. Java Stream findAny() Example
13. Java Stream findFirst() Example
14. Java Stream forEach() Example
15. Java Stream min() Example
16. Java Stream max() Example
17. Java Stream reduce() Example
18. Java Stream toArray() Example