Locks are cheap performance-wise (or at least they can be) but they’re easy to screw up and they can be difficult to performance test.
replyConcurrentHashMap has the advantage of hiding the locking from me and more importantly has the advantage of being correct, and it can still use the same Map interface so if it’s eventually used downstream somewhere stuff like `compute` will work and it will be thread safe without and work with mutexes.
The argument I am making is that it is literally no extra work to use the ConcurrentHashMap, and in my benchmarks with JMH, it doesn’t perform significantly worse in a single-threaded context. It seems silly for anyone to try and save a nanosecond to use a regular HashMap in most cases.
As soon as you have a couple of those ConcurrentHashMaps in scope with relationships between their elements, your concurrency story is screwed unless you've really thought everything out. I'd encourage using just plain HashMap to signal that there's no thread safety so you know to rethink everything if/when it comes up.
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