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