Parallelism and Flatmap in Java 8 Streams

Question

Consider the following example:

    IntStream.of(-1, 1)
             .parallel()
             .flatMap(i->IntStream.range(0,1000).parallel())
             .forEach(System.out::println);

Does it matter whether I set the inner flag to parallel? The results look very similar if I leave it away or not.

Also why does the code (ReferencePipeline) sequentialize the mapping?

I am confused by the line:

result.sequential().forEach(downstream);

Answer 1

In the current JDK (jdk1.8.0_25), the answer is no, it doesn't matter you set the inner flag to parallel, because even you set it, the .flatMap() implementation set's back the stream to sequential here:

result.sequential().forEach(downstream);

("result" is the inner stream and it's sequential() method's doc says: Returns an equivalent stream that is sequential. May return itself, either because the stream was already sequential, or because the underlying stream state was modified to be sequential.)

In most cases there could be no effort to make the inner stream parallel; if outer stream has at least same number of items as number of threads that can run parallel (ForkJoinPool.commonPool().getParallelism() = 3 in my computer).