What's the difference between ConcurrentHashMap and Collections.synchronizedMap(Map)?

Question

I have a Map which is to be modified by several threads concurrently.

There seem to be three different synchronized Map implementations in the Java API:

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Answer 1

ConcurrentHashMap is optimized for concurrent access.

Accesses don't lock the whole map but use a finer grained strategy, which improves scalability. There are also functional enhanvements specifically for concurrent access, e.g. concurrent iterators.