RFR: 8327858: Improve spliterator and forEach for single-element immutable collections [v3]

[Stuart Marks]

On Fri, 26 Apr 2024 22:27:21 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Please review this patch that:
>> 1. Implemented `forEach` to optimize for 1 or 2 element collections.
>> 2. Implemented `spliterator` to optimize for a single element.
>> 
>> The default implementations for multiple-element immutable collections are fine as-is, specializing implementation doesn't provide much benefit.
>
> Chen Liang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:
> 
>  - Add test to ensure reproducible iteration order
>  - Merge branch 'master' of https://github.com/openjdk/jdk into feature/imm-coll-stream
>  - Merge branch 'master' of https://github.com/openjdk/jdk into feature/imm-coll-stream
>  - Use the improved form in forEach
>  - Merge branch 'master' of https://github.com/openjdk/jdk into feature/imm-coll-stream
>  - Null checks should probably be in the beginning...
>  - mark implicit null checks
>  - Merge branch 'master' of https://github.com/openjdk/jdk into feature/imm-coll-stream
>  - Copyright year, revert changes for non-few element collections
>  - Merge branch 'master' of https://github.com/openjdk/jdk into feature/imm-coll-stream
>  - ... and 6 more: https://git.openjdk.org/jdk/compare/a920af23...70583024

src/java.base/share/classes/java/util/ImmutableCollections.java line 676:

> 674:                 return Collections.singletonSpliterator(e0);
> 675:             }
> 676:             return super.spliterator();

Not a big deal, but I prefer using if/else for situations like this where the then-part and else-part are equal in size and are conceptually at the same level. (As opposed to a check for a quick special case, where an early return is sensible, followed by the main logic at the method's top level.) Code in the rest of the file is similar, I think.

(yes, I'm aware that this takes an extra line)

src/java.base/share/classes/java/util/ImmutableCollections.java line 934:

> 932:                 return Collections.singletonSpliterator(e0);
> 933:             }
> 934:             return super.spliterator();

Prefer if/else, as noted above.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15834#discussion_r1775972767
PR Review Comment: https://git.openjdk.org/jdk/pull/15834#discussion_r1775973150