Difference between Concurrent hash map and Synchronized hashmap in Java

JavaObject Oriented ProgrammingProgramming

Concurrent Hashmap is a class that was introduced in jdk1.5.  Concurrent hash map applies locks only at bucket level called fragment while adding or updating the map. So, a concurrent hash map allows concurrent read and write operation to the map. 

Synchronized hashmap(Collection.syncronizedHashMap()) is a method of Collection framework. This method applies a lock on the entire collection. So, if one thread is accessing the map then no other thread can access the same map. 

Sr. No.KeyConcurrent hash mapSynchronized hashmap
1
Implementation
It is a class that implements a Concurrent hash map and serializable interface. 
It is a method in Collection class.  
2
Lock mechanism
Locks the portion
Locks the whole map. 
3
Performance
Concurrent hashmap allows concurrent read and write. So performance is relatively better than a synchronized map. 
Multiple threads can't access the map concurrently. So, performance is relatively less than a concurrent hash map.
4
Null key
It doesn't allow null as a key or value. 
It allows null as a key.

Concurrent modification exception
It doesn't throw concurrent modification exception. 
Iterator return by synchronized map throws concurrent modification exception

Example of SynchronizedMap

public class SynchronizedMapExample {
   public static void main(String[] args) {
      Map<Integer,String> laptopmap = new HashMap<Integer,String>();
      laptopmap.put(1,"IBM");
      laptopmap.put(2,"Dell");
      laptopmap.put(3,"HCL");
      // create a synchronized map
      Map<Integer,String> syncmap = Collections.synchronizedMap(laptopmap);
      System.out.println("Synchronized map is : "+syncmap);
   }
}

Example of ConcurrentHashMap

public class ConcurrentHashMapExample {
   public static void main(String[] args) {
      //ConcurrentHashMap
      Map<Integer,String> laptopmap = new ConcurrentHashMap<Integer,String>();
      laptopmap.put(1,"IBM");
      laptopmap.put(2,"Dell");
      laptopmap.put(3,"HCL");
      System.out.println("ConcurrentHashMap is: "+laptopmap);
   }
}
raja
Published on 18-Nov-2019 06:41:47
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