RFR: 8348171: Refactor GenerationCounters and its subclasses [v4]

[Thomas Schatzl]

On Fri, 31 Jan 2025 12:30:30 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple refactoring of removing the use of `virtual` method and use concrete subclasses when needed.
>> 
>> Test: tier1-5
>
> Albert Mingkun Yang has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - review
>  - * some more refactoring

Lgtm. Related to @zhengyu123 's comment, not sure right now what is meant with "looking odd" here as the previous code did not update the counters either, but it might be useful to wait on Shenandoah team's input anyway.

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Marked as reviewed by tschatzl (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23209#pullrequestreview-2589340090