'flatMap()' ≠ 'Flat Map' in Parallel-Stream

Introduce

public class Main {
  public static void main(String[] args) {    
    Stream.of("0123456789".split("")) // ["0", "1", ..., "9"]
      .parallel() // parallel stream
      .map(Main::genList) // Generate List
      .forEach(System.out::println);
  }
  
  private static List<String> genList(String contents) {
    return Arrays.asList(
      String.valueOf(new char[3]).replaceAll("\0", contents).split(""));
  }
}

// output (parallel)

[5, 5, 5]
[2, 2, 2]
[6, 6, 6]
[3, 3, 3]
[4, 4, 4]
[8, 8, 8]
[9, 9, 9]
[7, 7, 7]
[0, 0, 0]
[1, 1, 1]

public class Main {
  public static void main(String[] args) {
    Stream.of("0123456789".split(""))
        .parallel()
        .flatMap(Main::genStringListStream) // <-- #2
        .forEach(System.out::println); // <-- #3
  }

  private static List<String> genList(String contents) {
    return Arrays.asList(
      String.valueOf(new char[10]).replaceAll("\0", contents).split(""));
  }

  private static Stream<String> genStringListStream(String contents) {
    return genList(contents).parallelStream();
  }
}

// output

5
5
5
2
2
2
6
6
6
8
8
8
9
9
9
7
7
7
3
3
3
4
4
4
0
0
0
1
1
1

Why

@Override
public final <R> Stream<R> flatMap(
	Function<? super P_OUT, ? extends Stream<? extends R>> mapper
) {
  // ...

	@Override
  public void accept(P_OUT u) {
      try (Stream<? extends R> result = mapper.apply(u)) {
          if (result != null) {
              if (!cancellationRequestedCalled) {
                  result.sequential().forEach(downstream); // <-- #1
              }
              else {
                  Spliterator<? extends R> s = result.sequential().spliterator();
                  do { } while (!downstream.cancellationRequested() && s.tryAdvance(downstream));
              }
          }
      }
  }

  // ...
}

public class Main {
  public static void main(String[] args) {
    Stream.of("0123456789".split(""))
        .parallel()
        .flatMap(Main::genStringListStream)
        .parallel()
        .forEach(System.out::println);
  }

  private static List<String> genList(String contents) {
    return Arrays.asList(
      String.valueOf(new char[10]).replaceAll("\0", contents).split(""));
  }

  private static Stream<String> genStringListStream(String contents) {
    return genList(contents).parallelStream();
  }
}

// output

5
5
5
6
6
6
8
8
8
9
9
9
7
7
7
2
2
2
3
3
3
4
4
4
0
0
0
1
1
1

public class Main {
  public static void main(String[] args) {
    Stream.of("0123456789".split(""))
        .parallel()
        .flatMap(Main::genListStream)
        .forEach(System.out::println);
  }

  private static List<String> genList(String contents) {
    return Arrays.asList(
      String.valueOf(new char[10]).replaceAll("\0", contents).split(""));
  }

	private static Stream<List<String>> genListStream(String contents) {
		return Stream.of(genList(contents)).parallel();
  }
}

// output

[2, 2, 2]
[5, 5, 5]
[6, 6, 6]
[8, 8, 8]
[9, 9, 9]
[7, 7, 7]
[0, 0, 0]
[1, 1, 1]
[3, 3, 3]
[4, 4, 4]

Conclusion