For http://stackoverflow.com/a/32767282/4856258

output.txt

JDK flatMap
Thread[ForkJoinPool.commonPool-worker-7,5,main]=156250
Thread[ForkJoinPool.commonPool-worker-4,5,main]=31250
Thread[ForkJoinPool.commonPool-worker-1,5,main]=125000
Thread[ForkJoinPool.commonPool-worker-6,5,main]=218750
Thread[main,5,main]=62500
Thread[ForkJoinPool.commonPool-worker-3,5,main]=125000
Thread[ForkJoinPool.commonPool-worker-2,5,main]=125000
Thread[ForkJoinPool.commonPool-worker-5,5,main]=156250
Holger's flatMap
Thread[ForkJoinPool.commonPool-worker-7,5,main]=136980
Thread[ForkJoinPool.commonPool-worker-4,5,main]=130624
Thread[ForkJoinPool.commonPool-worker-1,5,main]=133932
Thread[ForkJoinPool.commonPool-worker-6,5,main]=169549
Thread[main,5,main]=1
Thread[ForkJoinPool.commonPool-worker-3,5,main]=147063
Thread[ForkJoinPool.commonPool-worker-2,5,main]=142912
Thread[ForkJoinPool.commonPool-worker-5,5,main]=138939

snippet.java

{
	System.out.println("JDK flatMap");
  	Map<Thread, Integer> threads = new ConcurrentHashMap<>();
	IntStream.range(0, 1000000).boxed().flatMap(Stream::of).parallel()
			.peek(e -> threads.merge(Thread.currentThread(), 1, Integer::sum))
			.collect(Collectors.summingInt(Integer::intValue));
	threads.entrySet().forEach(System.out::println);
}
{
	System.out.println("Holger's flatMap");
	Map<Thread, Integer> threads = new ConcurrentHashMap<>();
	flatMap(IntStream.range(0, 1000000).boxed(), Stream::of).parallel()
	.peek(e -> threads.merge(Thread.currentThread(), 1, Integer::sum))
	.collect(Collectors.summingInt(Integer::intValue));
	threads.entrySet().forEach(System.out::println);
}