ConcurrentHashMap vs SynchronizedMap

Difference between ConcurrentHashMap and SynchronizedMap in java.

ConcurrentHashMap

• You should use ConcurrentHashMap when you need very high concurrency in your project.
• It is thread-safe without synchronizing the whole map.
• Reads can happen very fast while the write is done with a lock.
• There is no locking at the object level.
• The locking is at a much finer granularity at a hashmap bucket level.
• ConcurrentHashMap doesn’t throw a ConcurrentModificationException if one thread tries to modify it while another is iterating over it.
• ConcurrentHashMap uses a multitude of locks.

SynchronizedHashMap

• Synchronization at the Object level.
• Every read/write operation needs to acquire a lock.
• Locking the entire collection is a performance overhead.
• This essentially gives access to only one thread to the entire map & blocks all the other threads.
• It may cause contention.
• SynchronizedHashMap returns Iterator, which fails-fast on concurrent modification.