RFR: 8376698: Add Spliterator tests for TreeMap sub-maps

[Oli Gillespie]

On Thu, 29 Jan 2026 12:40:42 GMT, Oli Gillespie <ogillespie at openjdk.org> wrote:

> Add missing cases to SpliteratorTraversingAndSplittingTest. This came up when I was fixing https://bugs.openjdk.org/browse/JDK-8372946, and I noticed the tests do not cover these sub-maps.
> 
> Two interesting parts:
> 
> 1. These tests failed when first added because `SubMapKeyIterator` and `DescendingSubMapKeyIterator` do not eagerly throw `NullPointerException` for `null` action arguments. The spec says "Throws: NullPointerException - if the specified action is null", so I updated the implementation to match other spliterators.
> 2. Since the descending maps have the reverse expected iteration order, I added support in the test harness for descending maps.

Thanks!

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PR Comment: https://git.openjdk.org/jdk/pull/29485#issuecomment-3820025823