RFR: 8366224: Introduce DecimalDigits.appendPair for efficient two-digit formatting and refactor DateTimeHelper [v21]

[Johannes Döbler]

On Thu, 13 Nov 2025 16:08:15 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.appendLatin1(char c1, char c2)` with core implementation
>>    - Added `JavaLangAccess.appendPair(StringBuilder, char c1, char c2)` for internal access
>>    - 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:
> 
>   add comments

src/java.base/share/classes/java/time/MonthDay.java line 758:

> 756:         buf.append('-');
> 757:         DecimalDigits.appendPair(buf, day);
> 758:         return buf.toString();

Why not

    byte[] bytes = new byte[7];
	bytes[0] = (byte)'-';
	bytes[1] = (byte)'-';
	DecimalDigits.appendPair(bytes, 2, month); // TBD
	bytes[4] = (byte)'-';
	DecimalDigits.appendPair(bytes, 5, month);
	return JLA.uncheckedNewStringWithLatin1Bytes(bytes);

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PR Review Comment: https://git.openjdk.org/jdk/pull/26911#discussion_r2524176126