RFR: 8325679: Optimize ArrayList subList sort

[Attila Szegedi]

On Wed, 14 Feb 2024 23:37:03 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> Somewhat surprisingly, `ArrayList$Sublist.sort()` is not specialized and will thus fall back to slower default method of `List.sort()` instead of sorting a range of the array in-place in its backing root `ArrayList`. 
>> 
>> This doesn't change observable behavior, so haven't added tests, and `tier1` tests still all pass except for `test/jdk/java/util/Locale/LocaleProvidersFormat.java` which also currently fails on master too on the machine I tested on.
>
> (Discussion mainly of historical interest.)
> 
> @pavelrappo Correct, historically, `Collections.sort` would fail to sort `CopyOnWriteArrayList`. You have to go back to JDK 7 to see this. The sorting approach used by `Collections.sort` (still present in the default implementation of `List.sort`) gets an array using `toArray()`, sorts it, and then copies the sorted elements back using `ListIterator.set`. Since `CopyOnWriteArrayList` doesn't support modifications using `ListIterator`, it fails with `UnsupportedOperationException`. The overrides of `List.sort` have fixed this problem.
> 
> COWAL still has some problems with other things that use similar techniques, such as `Collections.shuffle`. That uses get/set to swap elements, which COWAL does support, but it copies the array on each set() operation. This results in N copies of the array being made for an N-element list.

@stuart-marks > Just to clarify, my comments about "sorting sublists is rare" is a response to "has this been there since day one?" from @JimLaskey, and it's not an argument against adding this.

That's absolutely understood :-) I was just trying to add some color by describing the use case I stumbled upon. 

> There appears to be some coverage of the List.sort() default method in `test/jdk/java/util/List/ListDefault.java'. It does seem to cover sublists. I guess the question is whether this test is focused on the default implementation of List.sort(), or whether it's intended to cover all implementations -- whether the default or overridden -- of the default methods that were added in JDK 8. I think this area is worth a closer look. If the new ArrayList.subList().sort() override is covered already, we might be able to get away without adding a new test. Or possibly some adjustment could be made to this test if necessary.

I confirmed that the code in [ListDefaults.java:403](https://github.com/openjdk/jdk/blob/master/test/jdk/java/util/List/ListDefaults.java#L403) exercises `ArrayList$SubList.sort()`. In the meantime, I also added a more exhaustive stochastic test, although it might be an overkill. I'm inclined to remove it.

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PR Comment: https://git.openjdk.org/jdk/pull/17818#issuecomment-1951481144