RFR: 8302603: Use Set.of in java.nio.charset.Charset

[Naoto Sato]

On Wed, 15 Feb 2023 09:42:53 GMT, Glavo <duke at openjdk.org> wrote:

> A small patch, using `Set.of` instead of `Collections.unmodifiableSet()` of `Charset#aliasSet`.
> 
> The alias should not be null, and the aliasSet is an immutable copy, so it is safe to use `Set.of`.
> 
> I have investigated all charsets, most of charsets have no more than 4 aliases, and the maximum is only 17.
> 
> In this case, the performance difference between immutable sets and HashSet can be ignored, and the memory footprint of immutable sets is smaller.
> 
> I need someone to help me open an Issue on JBS. Thank you very much.

Good cleanup. Filed an issue: https://bugs.openjdk.org/browse/JDK-8302603

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PR: https://git.openjdk.org/jdk/pull/12570