Benefit of using forEachOrdered with Parallel streams

Question

The official Oracle documentation says:

Note that you may lose the benefits of parallelism if you use operations like forEachOrdered with parallel streams. Oracle - Parallelism

Why would anyone use forEachOrdered with parallel stream if we are losing parallelism?

Answer 1

depending on the situation, one does not lose all the benefits of parallelism by using ForEachOrdered.

Assume that we have something as such:

stringList.parallelStream().map(String::toUpperCase)
                           .forEachOrdered(System.out::println);

In this case, we can guarantee that the ForEachOrdered terminal operation will print out the strings in uppercase in the encounter order but we should not assume that the elements will be passed to the map intermediate operation in the same order they were picked for processing. The map operation will be executed by multiple threads concurrently. So one may still benefit from parallelism but it's just that we’re not leveraging the full potential of parallelism. To conclude, we should use ForEachOrdered when it matters to perform an action in the encounter order of the stream.

edit following your comment:

What happens when you skip map operation? I am more interested in forEachOrdered right after parallelStream()

if you're referring to something as in:

 stringList.parallelStream().forEachOrdered(action);

there is no benefit in doing such thing and I doubt that's what the designers had in mind when they decided to create the method. in such case, it would make more sense to do:

stringList.stream().forEach(action);

to extend on your question "Why would anyone use forEachOrdered with parallel stream if we are losing parallelism", say you wanted to perform an action on each element with respect to the streams encounter order; in such case you will need to use forEachOrdered as the forEach terminal operation is non deterministic when used in parallel hence there is one version for sequential streams and one specifically for parallel streams.

Answer 2

I don't really get the question here. Why? because you simply have no alternative - you have so much data that parallel streams will help you (this still needs to be proven); but yet you still need to preserve the order - thus forEachOrdered. Notice that the documentation says may and not will lose that for sure - you would have to measure and see.