<i18n dev> RFR: 8372460: Use EnumMap instead of HashMap for DateTimeFormatter parsing to improve performance [v7]

[Roger Riggs]

On Tue, 9 Dec 2025 10:02:36 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> This PR optimizes the parsing performance of DateTimeFormatter by replacing HashMap with EnumMap in scenarios where the keys are exclusively ChronoField enum values.
>> 
>> When parsing date/time strings, DateTimeFormatter creates HashMaps to store intermediate parsed values. HashMap has more overhead for operations compared to specialized map implementations.
>> 
>> Since ChronoField is an enum and all keys in these maps are ChronoField instances, we can use EnumMap instead, which provides better performance for enum keys due to its optimized internal structure.
>> 
>> Parsing scenarios show improvements from 12% to 95%
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove redundant checkField

This PR has been through too many incremental changes.
I suspect a better solution is to implement a fit-for-purpose Map, optimized for ChronoFields but taking into account the possibility of unknown TemporalFields. All within the implementation of a Map<TemporalField, Long>.
I'd like to see this PR closed and take a fresh look with all that is learned by the attempt.

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Changes requested by rriggs (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28471#pullrequestreview-3601320108