RFR: 8372946 - TreeMap sub-map entry spliterator is expensive

[Oli Gillespie]

On Tue, 2 Dec 2025 15:31:43 GMT, Oli Gillespie <ogillespie at openjdk.org> wrote:

> `TreeMap` sub-maps use the default `IteratorSpliterator` implementation for `TreeMap$EntrySetView` which is slow for some operations, because `EntrySetView.size()` iterates all elements. This is most trivially shown by something like `largeTreeMap.tailMap(0L, false).entrySet().limit(1).count()` taking a long time. This showed up in my application, where it was trivial to mitigate by switching to a for loop, but I think the fix is easy enough.
> 
> `keySet()` does not have the same problem, as it provides a custom `Spliterator` implementation which is not `Spliterator.SIZED`, and returns `Long.MAX_VALUE` for `estimateSize()` (which is the recommended approach when the size is expensive to compute). I'm *assuming* this optimization was simply missed for the EntryIterator in the original implementation, but I don't know for sure.
> 
> This patch fixes the issue by providing a custom spliterator for `EntrySetView`, which is not SIZED. The implementation is copied almost exactly from the equivalent `KeyIterator` classes in this file (`SubMapKeyIterator`, `DescendingSubMapKeyIterator`). The only difference is in `SubMapEntryIterator.getComparator`, for which I copied the implementation from `TreeMap$EntrySpliterator`.
> 
> 
> Basic performance test: `map.tailMap(0L, false).entrySet().stream().limit(1).count()` for a `TreeMap` with `10_000_000` entries.
> 
> Before (keySet is fast using `SubMapKeyIterator`, entrySet is slow using `IteratorSpliterator`):
> 
> class java.util.TreeMap$KeySet
>     .stream().limit(1).count() took 0.046ms
>     spliterator = SubMapKeyIterator, estimateSize() = 9223372036854775807
> class java.util.TreeMap$AscendingSubMap$AscendingEntrySetView
>     .stream().limit(1).count() took 218ms
>     spliterator = IteratorSpliterator, estimateSize() = 9999999 
> 
> 
> After (entrySet is now fast, using `SubMapEntryIterator`):
> 
> class java.util.TreeMap$KeySet
> 	.stream().limit(1).count() took 0.017ms
> 	spliterator = SubMapKeyIterator, estimateSize() = 9223372036854775807
> class java.util.TreeMap$AscendingSubMap$AscendingEntrySetView
> 	.stream().limit(1).count() took 0.013ms
> 	spliterator = SubMapEntryIterator, estimateSize() = 9223372036854775807

Tier2 test failure. I will investigate.


 java.util.ConcurrentModificationException
	at java.base/java.util.TreeMap$NavigableSubMap$SubMapIterator.prevEntry(TreeMap.java:2068)
	at java.base/java.util.TreeMap$NavigableSubMap$DescendingSubMapEntryIterator.next(TreeMap.java:2156)
	at java.base/java.util.TreeMap$NavigableSubMap$DescendingSubMapEntryIterator.forEachRemaining(TreeMap.java:2166)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:635)
	at java.base/java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:291)
	at java.base/java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:652)
	at java.base/java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:658)
	at org.openjdk.tests.java.util.stream.CollectionAndMapModifyStreamTest.testEntrySetSizeRemove(CollectionAndMapModifyStreamTest.java:164)
	at org.openjdk.tests.java.util.stream.CollectionAndMapModifyStreamTest.testMapEntriesSizeRemove(CollectionAndMapModifyStreamTest.java:155)

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PR Comment: https://git.openjdk.org/jdk/pull/28608#issuecomment-3602932282