JDK-8072840: Presizing for Stream Collectors

[Paul Sandoz]

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