RFR: 8362958: Unnecessary copying / sorting in Streams using Comparator.naturalOrder() [v4]

[Archie Cobbs]

On Tue, 11 Nov 2025 17:56:19 GMT, Patrick Strawderman <duke at openjdk.org> wrote:

>> When `Comparator.naturalOrder()` was explicitly supplied to a collection such as `TreeSet`, or passed into the `sorted` method of a stream, the sorted characteristic was not preserved, causing unnecessary buffering and duplicate sorting.
>> 
>> Example:
>> 
>> 
>> TreeSet<Integer> sortedSet = new TreeSet<>(Comparator.naturalOrder());
>> sortedSet.add(1);
>> sortedSet.add(2);
>> // SortedOps.OfRef.opWrapSink is not a no-op
>> sortedSet.stream().sorted().forEach(System.out::println);
>> 
>> or
>> 
>> TreeSet<Integer> sortedSet = new TreeSet<>();
>> sortedSet.add(1);
>> sortedSet.add(2);
>> // SortedOps.OfRef.opWrapSink is not a no-op
>> sortedSet.stream().sorted(Comparator.naturalOrder()).forEach(System.out::println);
>> 
>> 
>> This PR updates `SortedOps.makeRef` and `StreamOpFlag.fromCharacteristics` to handle the above cases and avoid the unnecessary sort step.
>
> Patrick Strawderman has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix bad merge

src/java.base/share/classes/java/util/stream/StreamOpFlag.java line 753:

> 751:         int characteristics = spliterator.characteristics();
> 752:         if ((characteristics & Spliterator.SORTED) != 0 &&
> 753:                 (spliterator.getComparator() instanceof Comparator c &&

Not sure if the performance of `instanceof` is meaningfully faster than `!=`, but the `instanceof` test could be replaced with a null test:

if ((characteristics & Spliterator.SORTED) != 0) {
    Comparator<?> c = spliterator.getComparator();
    if (c != null && !c.equals(Comparator.naturalOrder()) { ...

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PR Review Comment: https://git.openjdk.org/jdk/pull/28226#discussion_r2515182450