8072840: Add a method to Collector that returns a sized supplying mutable result container #23461
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Demo of Guava changes for openjdk/jdk#23461. This change pre-sizes collectors for which the size of the output collection must match the size of the input stream. It omits cases like `ImmutableSet` (which deduplicates), but it includes cases `ImmutableList` (obviously) and `ImmutableMap`/`ImmutableBiMap` (which rejects duplicate keys). RELNOTES=`collect`: Changed `toImmutableList`, `toImmutableMap`, and `toImmutableBiMap` to internally pre-size their collections when possible. PiperOrigin-RevId: 725756865
Demo of Guava changes for openjdk/jdk#23461. This change pre-sizes collectors for which the size of the output collection must match the size of the input stream. It omits cases like `ImmutableSet` (which deduplicates), but it includes cases `ImmutableList` (obviously) and `ImmutableMap`/`ImmutableBiMap` (which rejects duplicate keys). RELNOTES=`collect`: Changed `toImmutableList`, `toImmutableMap`, and `toImmutableBiMap` to internally pre-size their collections when possible. PiperOrigin-RevId: 725756865
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This changes introduces a new
LongFunction<A> sizedSupplier()method onCollectorthat is passed the exact size of the stream if known and-1else. This can greatly reduces the amount of allocations and copying for streams that reshape data structures rather than perform extensive computations.For convenience,
Collector.ofSizedfactory methods are added that accept an additionalIntFunction<A>that is called instead of the providedSupplier<A>if the stream size is known. Data structures that can have sizes that don't fit in anintcan still use the new feature by providing their own implementation ofCollector.The default implementation of
sizedSupplierreturns a function that ignores its argument and always returnssupplier().get(). ExistingCollectorimplementations outside the standard library can adoptsizedSupplierwhile remaining backwards compatible with older JDKs by overriding the method without an@Overrideannotation.Existing collectors in
Collectorsare updated to use the new functionality if the final size of the collection can be determined from the stream size (e.g.toMapwithout a merge function does, buttoSetdoesn't). For this purpose, a new constructor accepting an initial capacity is added toStringJoiner.Progress
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gitCheckout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/23461/head:pull/23461$ git checkout pull/23461Update a local copy of the PR:
$ git checkout pull/23461$ git pull https://git.openjdk.org/jdk.git pull/23461/headUsing Skara CLI tools
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https://git.openjdk.org/jdk/pull/23461.diff