ConcurrentHashMap vs Synchronized HashMap

Synchronized HashMap：

1. Each method is synchronized using an object level lock. So the get and put methods on synchMap acquire a lock.

2. Locking the entire collection is a performance overhead. While one thread holds on to the lock, no other thread can use the collection.

ConcurrentHashMap was introduced in JDK 5.

1. There is no locking at the object level,The locking is at a much finer granularity. For a ConcurrentHashMap, the locks may be at a hashmap bucket level.

2. The effect of lower level locking is that you can have concurrent readers and writers which is not possible for synchronized collections. This leads to much more scalability.

3. ConcurrentHashMap does not throw a ConcurrentModificationException if one thread tries to modify it while another is iterating over it.

This article Java 7: HashMap vs ConcurrentHashMap is a very good read. Highly recommended.

ConcurrentHashMap is thread safe without synchronizing the whole map. Reads can happen very fast while write is done with a lock.

The short answer:

Both maps are thread-safe implementations of the Map interface. ConcurrentHashMap is implemented for higher throughput in cases where high concurrency is expected.

Brian Goetz's article on the idea behind ConcurrentHashMap is a very good read. Highly recommended.