RFR: 8370013: Refactor Double.toHexString to eliminate regex and StringBuilder [v14]

[Joe Darcy]

On Tue, 21 Oct 2025 11:29:07 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> This PR refactors the Double.toHexString method to improve its performance by eliminating the use of StringBuilder and regex operations. The new implementation uses a direct byte array approach to construct the hex string representation, which avoids the overhead of StringBuilder operations and regex pattern matching.
>> 
>> Existing tests in `java/lang/Double/ToHexString.java`.
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   from @rgiulietti

Marked as reviewed by darcy (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27811#pullrequestreview-3367060092