RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v11]

[Viktor Klang]

On Fri, 17 Nov 2023 11:28:23 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> This Pull-Request implements [JEP-461](https://openjdk.org/jeps/461)
>
> Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Adding @implSpec for Gatherer.andThen, plus adds tests to verify the default impls for Gatherer-related interfaces

src/java.base/share/classes/java/util/stream/Gatherer.java line 263:

> 261:      *           which is semantically equivalent to the combination of
> 262:      *           {@code this} and {@code that} gatherer.
> 263:      *

@AlanBateman @PaulSandoz `Gatherer#andThen` was missing an @implSpec, so this commit addresses that.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16420#discussion_r1397125423