RFR: JDK-8319123 : Implementation of JEP-461: Stream Gatherers (Preview) [v2]

[Viktor Klang]

On Thu, 9 Nov 2023 09:39:33 GMT, Tagir F. Valeev <tvaleev at openjdk.org> wrote:

>> Ah, now I see what you mean. In this specific case you might be able to create a combiner (which would also need a Downstream) such that you can add the missing combinations (the end of the "left" and the beginning of the "right"). But imagine something like `windowFixed(3)`, where the presence of 1 or 2 elements to the "left" will skew all the elements to the "right", potentially rippling through the entire stream.
>> 
>> I think the Preview is a perfect place to see if the combiner would be made even better with a Downstream reference or not. Logically, the output of said Downstream would be placed *after* the "left"'s output, and *before* the "right"'s output (so in the middle of the two).
>> 
>> But then someone might want to be able to inject elements outside of that order, but I guess that remains to be seen.
>> 
>> Great thoughts, @amaembo 👍
>
> With windowFixed, true parallelization is not possible, unless the stream is SIZED/SUBSIZED, but this information is not available for Gatherer, so we cannot solve this at all (and this is probably a reasonable limitation of the API).
> 
>> But then someone might want to be able to inject elements outside of that order, but I guess that remains to be seen.
> 
> Not sure where one may want to inject other elements. If you want to add something after the right output, you can do this inside the finisher, or the next combiner, when the "right" will become the "left". There are no other places to insert. E.g., you cannot insert "before left", because downstream collector has no corresponding operation. To me it looks like inserting after the "left" part is the only possibility.

@amaembo Alright, then I think we are aligned. Experimentation with combiner-downstream should be possible during the Preview-cycle

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PR Review Comment: https://git.openjdk.org/jdk/pull/16420#discussion_r1387776261