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

Collections.synchronizedMap() method synchronizes all the methods of the HashMap and effectively reduces it to a data structure where one thread can enter at a time because it locks every method on a common lock.

In ConcurrentHashMap synchronization is done a little differently. Rather than locking every method on a common lock, ConcurrentHashMap uses separate lock for separate buckets thus locking only a portion of the Map. By default there are 16 buckets and also separate locks for separate buckets. So the default concurrency level is 16. That means theoretically any given time 16 threads can access ConcurrentHashMap if they all are going to separate buckets.