RFR: 8299852: Modernize ConcurrentHashMap

[Martin Buchholz]

On Tue, 10 Jan 2023 10:32:34 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> `java.util.concurrent.ConcurrentHashMap` is relatively old and has not been updated to reflect the current state of Java and can be modernized: 
> 
> * Add `@Serial` annotations 
> * Seal classes and restrict subclassing for internal classes 
> * Use pattern matching for instance 
> * Remove redundant modifiers (such as some final declarations) 
> * Use Objects.requireNonNull for invariant checking 
> * Use diamond operators instead of explicit types 
> * Simplify expressions using Math::max

ObMention @DougLea

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PR: https://git.openjdk.org/jdk/pull/11924