Seing a Collector as a Gatherer

[Viktor Klang]

Yes, having the conversion in Gatherers would seem like the most sensible option.

Then the question becomes whether gather is the most sensible name for it―I'm thinking of readability under the use of static imports etc. gatherUsing(), viaCollector(), adapt(), etc.

Cheers,
√


Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: forax at univ-mlv.fr <forax at univ-mlv.fr>
Sent: Wednesday, 17 January 2024 21:16
To: Viktor Klang <viktor.klang at oracle.com>
Cc: core-libs-dev <core-libs-dev at openjdk.java.net>
Subject: [External] : Re: Seing a Collector as a Gatherer



________________________________
From: "Viktor Klang" <viktor.klang at oracle.com>
To: "Remi Forax" <forax at 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-retn at openjdk.org> on behalf of Remi Forax <forax at univ-mlv.fr>
Sent: Wednesday, 17 January 2024 17:08
To: core-libs-dev <core-libs-dev at 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

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