Java Stream#flatMap minimal example.

Main.java

import java.util.Arrays;
import java.util.Collection;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<List<Integer>> nested = Arrays.asList(
                Arrays.asList(1, 2, 3),
                Arrays.asList(4, 5, 6),
                Arrays.asList(7, 8, 9)
        );

        nested.forEach(System.out::println);

        nested.stream()
              .flatMap(Collection::stream)
              .forEach(System.out::println);
    }
}

Output:

[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
1
2
3
4
5
6
7
8
9