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

• Hashtable and ConcurrentHashMap do not allow null keys or null values.

• Collections.synchronizedMap(Map) synchronizes all operations (get, put, size, etc).

• ConcurrentHashMap supports full concurrency of retrievals, and adjustable expected concurrency for updates.

As usual, there are concurrency--overhead--speed tradeoffs involved. You really need to consider the detailed concurrency requirements of your application to make a decision, and then test your code to see if it's good enough.