ConcurrentHashMap Vs HashTable

ConcurrentHashMap locking only applied for updates, only lock at segments level instead of the whole map.

HashTable locks the whole map.

• Assume Hashtable and ConcurrentHashMap are two types of Homes.
• Hashtable locks home's main door.
• ConcurrentHashMap locks specific room door instead of main door.

ConcurrentHashMap how it works

Concurrent hashmap by default has 16 segment of lock

• When setting the data in a particular segment, only the lock for that segment is obtained
• When getting data, no lock is used since it's volatile

However ConcurrentHashMap doesn't allow null key and value. See Lock Stripping