Difference between HashMap and ConcurrentHashMap

HashMap is the Class which is under Traditional Collection and ConcurrentHashMap is a Class which is under Concurrent Collections, apart from this there are various differences between them which are:

• HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature.
• HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously. But ConcurrentHashMap performance is low sometimes because sometimes Threads are required to wait on ConcurrentHashMap.
• While one thread is Iterating the HashMap object, if other thread try to add/modify the contents of Object then we will get Run-time exception saying ConcurrentModificationException.Whereas In ConcurrentHashMap we wont get any exception while performing any modification at the time of Iteration.

Using HashMap








// Java program to illustrate // HashMap drawbacks import java.util.HashMap;    class HashMapDemo extends Thread {     static HashMap<Integer,String> l=new HashMap<Integer,String>();        public void run()     {                           try         {             Thread.sleep(1000);             // Child thread trying to add             // new element in the object             l.put(103,"D");         }         catch(InterruptedException e)         {             System.out.println("Child Thread going to add element");         }     }        public static void main(String[] args) throws InterruptedException     {         l.put(100,"A");         l.put(101,"B");         l.put(102,"C");         HashMapDemo t=new HashMapDemo();         t.start();                    for (Object o : l.entrySet())          {             Object s=o;             System.out.println(s);             Thread.sleep(1000);         }         System.out.println(l);     } }

Output:

100=A
Exception in thread "main" java.util.ConcurrentModificationException

Using ConcurrentHashMap

Output:

100=A
101=B
102=C
103=D
{100=A, 101=B, 102=C, 103=D}

Using HashMap

output:

{null=World, 100=Hello, 101=Geeks, 102=Geeks}

Using ConcurrentHashMap

Output:

Exception in thread "main" java.lang.NullPointerException