RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v9]

[Paul Sandoz]

On Wed, 15 Nov 2023 17:50:48 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> This Pull-Request implements [JEP-461](https://openjdk.org/jeps/461)
>
> Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improvements after feedback

Elegantly and thoroughly done (I mostly focused on the API and implementation). I have been following the work and providing ongoing feedback hence no specific comments here.

Going preview now will allow for some additional time and feedback on the ergonomics of Gatherer construction. There is also the intriguing prospect of further work to replace stream internals if the performance holds up (the ability to optimize when composing gathers seems key here as you already have explored in the performance tests - the runtime compiler should be able to see through the shorter paths more easily).

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Marked as reviewed by psandoz (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16420#pullrequestreview-1733265473