Casting gatherer

[Chen Liang]

Hi Nir, I think currently the most similar code pattern in stream that enjoys a good performance is:

.<MyClass>mapMulti((x, sink) -> {
    if (x instanceof MyClass myClass) {
        sink.accept(myClass);
    }
})

>From the language point of view, such pattern-match-and-map-0/1 operations may be common.
That said, can we provide a language feature in the future to convert a pattern declaration to BiConsumer<T, XxxConsumer<args>>? I believe that is a better way to address the question you have raised here.

Regards,
Chen Liang
________________________________
From: core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Nir Lisker <nlisker at gmail.com>
Sent: Saturday, April 26, 2025 1:55 PM
To: core-libs-dev at openjdk.org <core-libs-dev at openjdk.org>
Subject: Casting gatherer

Hi,

instanceof has been refitted to include an auto-cast ("pattern matching on instanceof"). Unfortunately, doing this as an intermediate operation on a stream requires first to filter via instanceof and then map via a cast. This is because

    x instanceof MyClass myClass

returns a boolean, not myClass.

I've asked for an easier way of doing it long ago directly on Stream and was declined, but now with Gatherers I'm bringing this up again. I think it would be reasonable to put such an operation in the Gatherers class. I imagine that many Gatherer libraries, or utility classes, will include it since it's a common operation, and having it in the JDK means that it'll be done the best way possible (you can optimize where others can't, if applicable here).

-- Nir
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