JDK-8072840: Presizing for Stream Collectors

[Fabian Meumertzheim]

Thanks for your feedback and for making me aware of the complications
around Gatherers.

I will also put more thought into how this could work for Gatherers.

On Thu, Feb 13, 2025, 23:31 Viktor Klang <viktor.klang at oracle.com> wrote:

> Indeed. I hope I didn't sound discouraging about the possibility to
> propagate the stream size information.
>
> I merely want to emphasize that it may necessitate a slightly broader take
> on the problem of propagation of stream-instance metadata, especially in
> the face of Gatherers becoming a finalized feature.
>
> It's great that you started this conversation, Fabian!
>
> Cheers,
> √
>
>
> *Viktor Klang*
> Software Architect, Java Platform Group
> Oracle
> ------------------------------
> *From:* core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Paul
> Sandoz <paul.sandoz at oracle.com>
> *Sent:* Thursday, 13 February 2025 20:18
> *To:* Fabian Meumertzheim <fabian at buildbuddy.io>
> *Cc:* core-libs-dev <core-libs-dev at openjdk.org>
> *Subject:* Re: JDK-8072840: Presizing for Stream Collectors
>
> Hi Fabian,
>
> Thanks for sharing and reaching out with the idea before getting too
> beholden to it.
>
> I logged this is quite a while ago. It seemed like a possible good idea at
> the time, although I never liked the duplication of suppliers. I have
> become less enthusiastic overtime, especially so as Gatherers have been
> added. (Gatherer is the underlying primitive we could not find when we were
> furiously developing streams and meeting the Java 8 deadline.) My sense is
> if we are going to address we need to think more broadly about Gatherers.
> And, Viktor being the lead on Gatherers has a good take on where this might
> head.
>
> Paul.
>
> > On Feb 12, 2025, at 2:09 AM, Fabian Meumertzheim <fabian at buildbuddy.io>
> wrote:
> >
> > As an avid user of Guava's ImmutableCollections, I have been
> > interested in ways to close the efficiency gap between the built-in
> > `Stream#toList()` and third-party `Collector` implementations such as
> > `ImmutableList#toImmutableList()`. I've found the biggest problem to
> > be the lack of sizing information in `Collector`s, which led to me to
> > draft a solution to JDK-8072840:
> > https://github.com/openjdk/jdk/pull/23461
> >
> > The benchmark shows pretty significant gains for sized streams that
> > mostly reshape data (e.g. slice records or turn a list into a map by
> > associating keys), which I've found to be a pretty common use case.
> >
> > Before I formally send out the PR for review, I would like to gather
> > feedback on the design aspects of it (rather than the exact
> > implementation). I will thus leave it in draft mode for now, but
> > invite anyone to comment on it or on this thread.
> >
> > Fabian
>
>
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