RFR: 8368178: Add specialization of SequencedCollection methods to standard list factories [v3]

[Tagir F. Valeev]

On Wed, 24 Sep 2025 23:35:16 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> Tagir F. Valeev has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Add SequencedCollection specialization for immutable collections
>>  - Fix indentation
>
> OK, List12::getFirst and List12::getLast seem fine.
> 
> But yeah... AbstractImmutableList::reversed and List12::reversed are indeed debatable. I wrestled over this for a while. I don't think they're incorrect, but I'm also having trouble convincing myself they're cases worth optimizing. There are also weird tradeoffs.
> 
> With no override, calling reversed() on a List12 returns a ReverseOrderListView(orig) and reversed() on that returns orig again. But with the override, calling reversed() gives a new List12, and calling reversed() on that gives another new List12. Nothing here is really terrible, but it's also not clear to me this is an improvement.
> 
> With AbstractImmutableList I got kind of tangled up in analyzing where this would be used. It seems to me the optimization would take effect for the zero-size nulls-disallowed case, the zero-size nulls-allowed case, and the size-one nulls-allowed case. (The size-one nulls-disallowed case is covered by List12, unless we don't add the override there....) I don't think this is incorrect for any case, but I'm also having a hard time seeing the improvement. It might save the allocation of a small object, or it might save an indirection. And ... that's about it?
> 
> The reversed() overrides hardly seem worthwhile. Maybe don't include them.
> 
> Regarding the tests, is it necessary to add new test files for EmptyList and SingletonList? Maybe it's possible to add cases to test/jdk/java/util/Collection/MOAT.java. I'm also a bit surprised to see a new provider added to the ListFactories test. But I haven't looked too closely. If it's necessary, then of course we should include it.

@stuart-marks a gentle ping: could you please review after the latest changes? Thank you in advance!

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PR Comment: https://git.openjdk.org/jdk/pull/27406#issuecomment-3443221548