Benefit of using forEachOrdered with Parallel streams

偏执的太偏执、 2021-06-24 16:12 376阅读 0赞

[https://stackoverflow.com/questions/47336825/benefit-of-using-foreachordered-with-parallel-streams][https_stackoverflow.com_questions_47336825_benefit-of-using-foreachordered-with-parallel-streams]

> 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?

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.

A more interesting example would be `stringList.parallelStream().sorted().forEachOrdered(action)`

[https_stackoverflow.com_questions_47336825_benefit-of-using-foreachordered-with-parallel-streams]: https://stackoverflow.com/questions/47336825/benefit-of-using-foreachordered-with-parallel-streams
[Oracle - Parallelism]: https://docs.oracle.com/javase/tutorial/collections/streams/parallelism.html