RFR: 8351443: Improve robustness of StringBuilder [v5]

[Jaikiran Pai]

On Mon, 5 May 2025 17:32:19 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.
>
> Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Refactor to consistently use `isLatin1(coder)` within AbstractStringBuilder.

src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 240:

> 238:      *     <li>The new coder is the different thn the old coder
> 239:      *     <li>The new length is greater than to the current length
> 240:      *     <li>The new length is non-negative, as it might have overflowed due to an increment

I think this is a typo too and `non-negative` should have been `negative`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24967#discussion_r2074505941