What is the difference between HashTable and ConcurrentHashMap, and which one is more efficient?

HashTable synchronizes every method call using lock on the entire object. Each access to the table requires acquiring a lock, which leads to poor performance in multi-threaded environments because only one thread can perform any operation (PUT, GET, etc.) at a time.

ConcurrentHashMap uses more granular locking. It divides the internal structure into segments (segment-based locking). Only the segment being accessed by a thread is locked. Different threads can access different segments simultaneously, significantly increasing concurrency.

ConcurrentHashMap also provides more efficient read performance, as reads can be lock-free.

Summary:

Feature	HashTable	ConcurrentHashMap
Synchronization	Lock on the entire object	Lock on segment level
Thread safety	Fully synchronized (slow)	High concurrency (fast)
Null keys/values	Not allowed	Not allowed (depends on version)
Collection inheritance	Implements Dictionary	Extends AbstractMap, implements ConcurrentMap

ConcurrentHashMap is more efficient, especially in multi-threaded applications, due to its improved concurrency model.