> From: "Viktor Klang" <viktor.kl...@oracle.com>
> To: "Remi Forax" <fo...@univ-mlv.fr>
> Sent: Wednesday, January 17, 2024 6:01:38 PM
> Subject: Re: Seing a Collector as a Gatherer
> Hi Rémi,
> Yes, this was something I was hoping to get into the preview, but I wasn't
> sure
> where that conversion should end up.
> There are a few different places where it might go:
> Gatherer.of(Collector)
> Gatherers.collect(Collector)
> Collector.asGatherer()
> Collectors.gather(Collector)
> I wasn't really convinced where it should go, and I was hesitant to making any
> changes to existing public interfaces as a "nice to have", so I opted to leave
> it out for now.
> I think people would prefer to see it on Collector as a default method, but
> as I
> said before, making changes to Collector wasn't something I was ready to
> prioritize for the (first) JEP.
I think this method is also important pedagogically, there should be a place
that describe the relationship between a Collector and a Gatherer.
For Gatherer.of(), this one seems alien compared to the other overloads of
of()/ofSequential() and to a lesser extend I do not like too much to have
overloads with one parameter with two different interfaces, because someone can
comes with a class that implements both Collector and Integrator (as stupid as
it is),
For Gatherers.collect(Collector) is fine,
For Collector.asGatherer(), a default method has the disadvantage that usually
a Collector is typed from left to right so calling an instance method requires
an intermediary variable
Collector<String, ?, List<String>> collector = Collector.toList(); // ok
Gatherer<String, ?, List<String>> gatherer = Collector.toList().asGatherer();
// we are in trouble here
that's why Collectors.collectingAndThen() (aka compose the finisher) is a
static method in Collectors and not an instance method in Collector
(finishAndThen ?),
For Collectors.gather(), methods inside Collectors tend to return a Collector.
> Cheers,
> √
regards,
Rémi
> Viktor Klang
> Software Architect, Java Platform Group
> Oracle
> From: core-libs-dev <core-libs-dev-r...@openjdk.org> on behalf of Remi Forax
> <fo...@univ-mlv.fr>
> Sent: Wednesday, 17 January 2024 17:08
> To: core-libs-dev <core-libs-...@openjdk.java.net>
> Subject: Seing a Collector as a Gatherer
> Hello,
> I may have overlook that, but it seems there is no method to see a Collector
> as
> a Gatherer.
> A Gatherer is more general than a Collector and a Gatherer with a greedy
> integrator that does not call Downstream.push in the intergator and only once
> is the finisher is basicaly a Collector.
> In code:
> <E, A, T> Gatherer<E, A, T> asGatherer(Collector<? super E, A, ? extends T>
> collector) {
> var supplier = collector.supplier();
> var accumulator = collector.accumulator();
> var combiner = collector.combiner();
> var finisher = collector.finisher();
> return Gatherer.of(supplier,
> Gatherer.Integrator.ofGreedy((state, element, _) -> {
> accumulator.accept(state, element);
> return true;
> }),
> combiner,
> (state, downstream) -> downstream.push(finisher.apply(state)));
> }
> This is eaxctly how Gatherer.fold() works.
> Is there a reason why such method does not exist ?
> regards,
> Rémi
