RFR: 8351443: Improve robustness of StringBuilder

[Chen Liang]

On Wed, 30 Apr 2025 14:12:36 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

> Refactor AbstractStringBuilder to maintain consistency among count, coder, and value buffers while the buffer capacity is being expanded and/or inflated from Latin1 to UTF16 representations. 
> The refactoring pattern is to read and write AbstractStringBuilder fields once using locals for all intermediate values. 
> Support methods are static, designed to pass all values as arguments and return a value.
> 
> The value byte array is reallocated under 3 conditions:
> - Increasing the capacity with the same encoder
> - Increasing the capacity and inflation to change the coder from LATIN1 to UTF16
> - Inflation with the same capacity
> 
> Added StressSBTest to exercise public instance methods of StringBuilder.

src/java.base/share/classes/java/lang/StringLatin1.java line 58:

> 56: 
> 57:     public static byte coderFromChar(char cp) {
> 58:         return (byte)((0xff - cp) >>> Integer.SIZE - 1);

Suggestion:

        return (byte)((0xff - cp) >>> (Integer.SIZE - 1));

For clarity.

Or use `return (byte)((0xff - cp) >>> Character.SIZE);` as this bit is set in `0xFF - 0xFFFF`

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PR Review Comment: https://git.openjdk.org/jdk/pull/24967#discussion_r2068778745