Difference between HashMap and ConcurrentHashMap
Last Updated :
06 Aug, 2019
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
import java.util.HashMap;
class HashMapDemo extends Thread
{
static HashMap<Integer,String> l= new HashMap<Integer,String>();
public void run()
{
try
{
Thread.sleep( 1000 );
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
import java.util.HashMap;
import java.util.concurrent.*;
class HashMapDemo extends Thread
{
static ConcurrentHashMap<Integer,String> l =
new ConcurrentHashMap<Integer,String>();
public void run()
{
l.put( 103 , "D" );
try
{
Thread.sleep( 2000 );
}
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
101=B
102=C
103=D
{100=A, 101=B, 102=C, 103=D}
In HashMap, null values are allowed for key and values, whereas in ConcurrentHashMap null value is not allowed for key and value, otherwise we will get Run-time exception saying NullPointerException.
Using HashMap
import java.util.*;
class ConcurrentHashMapDemo
{
public static void main(String[] args)
{
HashMap m= new HashMap();
m.put( 100 , "Hello" );
m.put( 101 , "Geeks" );
m.put( 102 , "Geeks" );
m.put( null , "World" );
System.out.println(m);
}
}
|
output:
{null=World, 100=Hello, 101=Geeks, 102=Geeks}
Using ConcurrentHashMap
import java.util.concurrent.*;
class ConcurrentHashMapDemo
{
public static void main(String[] args)
{
ConcurrentHashMap m= new ConcurrentHashMap();
m.put( 100 , "Hello" );
m.put( 101 , "Geeks" );
m.put( 102 , "Geeks" );
m.put( null , "World" );
System.out.println(m);
}
}
|
Output:
Exception in thread "main" java.lang.NullPointerException
HashMap is introduced in JDK 1.2 whereas ConcurrentHashMap is introduced by SUN Microsystem in JDK 1.5.
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