Differences between 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 .

     Please  write in comments  in case if you have any doubts .