HashMap Vs ConcurrentHashMap : Java 

HashMap vs ConcurrentHashMap

1.  Thread -Safe : 

     ConcurrentHashMap is thread-safe that is the code can be accessed by single thread at a time .    
     while HashMap is not thread-safe .

2.  Synchronization Method :

    HashMap can be synchronized by using    
    synchronizedMap(HashMap)  method .  By using this  

    method we get a HashMap object which is equivalent 
    to the HashTable object . So every modification  is performed    
    on  Map is locked on Map object.

import java.util.*;

public class HashMapSynchronization {
    public static void main(String[] args) {
        // create map
        Map<String,String> map = new HashMap<String,String>();
        
        // populate the map
        map.put("1","ALIVE ");
        map.put("2","IS");
        map.put("3","AWESOME");
        
        // create a synchronized map
        Map<String,String> syncMap = Collections.synchronizedMap(map);
        
        System.out.println("Synchronized map :"+syncMap);
    }
}

   ConcurrentHashMap synchronizes or locks on the certain portion of the Map . To optimize
   the performance of ConcurrentHashMap , Map is divided into different partitions depending
   upon the Concurrency level . So that we do not need to synchronize the whole Map Object.


3.  Null Key


     ConcurrentHashMap does not allow NULL values . So the key can not be null in
     ConcurrentHashMap .While In HashMap there can only be one null key .

  
4.  Performance

     In multiple threaded environment HashMap is usually faster than ConcurrentHashMap . As   
     only single thread can access the certain portion of the Map and thus reducing the performance .
     While in HashMap any number of threads can access the code at the same time