Diff. between HashMap and HashTable

HashTable
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1) is synchronized, thread-safe.
2) each time a thread is required to acquire a lock before using it.
3) does not allow null value as key.

HashMap
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1) is not synchronized or thread-safe.
2) lock is not required by threads.
3) allows one null value as key
4) it can be made thread-safe by using --> Collections.synchronizedMap() method.
5) better performance in single-threaded applications. -- no lock is required to acquire.
6) better performance in multi-threaded applications. -- you can implement a synchronize block at more broad level, so at each entry point lock is not required.