RFR: 8366224: Introduce DecimalDigits.appendPair for efficient two-digit formatting and refactor DateTimeHelper [v5]

[Per Minborg]

On Mon, 15 Sep 2025 06:11:18 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> This PR introduces a new efficient API for appending two-digit integers to StringBuilders and refactors DateTimeHelper to leverage this new functionality.
>> 
>> Changes include:
>> 
>> 1. New `appendPair` method for efficient two-digit integer formatting (00-99):
>>    - Added `AbstractStringBuilder.appendPair(int i)` with core implementation
>>    - Added `JavaLangAccess.appendPair(StringBuilder, int)` for internal access
>>    - Added `System.JavaLangAccessImpl.appendPair(StringBuilder, int)` bridge
>>    - Added `DecimalDigits.appendPair(StringBuilder, int)` public static utility method
>>    - Enhanced Javadoc documentation for all new methods
>> 
>> 2. Refactored `DateTimeHelper` to use the new `DecimalDigits.appendPair`:
>>    - Updated `DateTimeHelper.formatTo` methods for `LocalDate` and `LocalTime`
>>    - Replaced manual formatting logic with the new efficient two-digit appending
>>    - Improved code clarity and consistency in date/time formatting
>> 
>> These changes improve code clarity and performance when formatting two-digit numbers, particularly in date/time formatting scenarios.
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update test/jdk/jdk/internal/util/DecimalDigitsTest.java
>   
>   Co-authored-by: Chen Liang <liach at openjdk.org>

We have, for long, established that changes like this should be discussed before putting forward any code. Yet, I am unable to find any discussion in the appropriate mailing list. Please send me the link to the mailing list discussion. I must have missed it for some reason.

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PR Comment: https://git.openjdk.org/jdk/pull/26911#issuecomment-3290708624