Gatherer: spliterator characteristics are not propagated

[Viktor Klang]

Hi Rémi,

You can find some of my benches here: https://github.com/openjdk/jdk/tree/master/test/micro/org/openjdk/bench/java/util/stream/ops/ref

Initially I had Characteristics such as ORDERED etc on Gatherer but it just didn't end up worth it when looking at the bench results over a wide array of stream sizes and number of operations.

Cheers,
√


Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Remi Forax <forax at univ-mlv.fr>
Sent: Wednesday, 17 January 2024 16:48
To: core-libs-dev <core-libs-dev at openjdk.java.net>
Subject: Gatherer: spliterator characteristics are not propagated

While doing some benchmarking of the Gatherer API, i've found that the characteristics of the spliterator was not propagated by the method Stream.gather() making the stream slower than it should.

As an example, there is no way when reimplementing map() using a Gatherer to say that this intermediate operation keep the size, which is important if the terminal operation is toList() because if the size is known, toList() will presize the List and avoid the creation of an intermediary ArrayList.

See https://github.com/forax/we_are_all_to_gather/blob/master/src/main/java/com/gihtub/forax/wearealltogather/bench/MapGathererBenchmark.java

I think that adding a way to propagate the spliterator characteristics through a Gatherer would greatly improve the performance of commons streams (at least all the ones that end with a call to toList).

I have some idea of how to do that, but I prefer first to hear if i've overlook something and if improving the performance is something worth changing the API.

regards,
Rémi
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