Difference between Traditional Collections and Concurrent Collections in java

We all know about  Traditional Collections ( i.e. List, Set, Queue and its implemented Classes) and Concurrent Collection (i.e. ConcurrentMap interface, ConcurrentHashMap class, CopyOnWriteArrayList class etc). In these two Collections, there are few differences like:

• Most of the Classes which are present in Traditional Collections (i.e ArrayList, LinkedList, HashMap etc) are non-synchronized in nature and Hence there is no thread-safety. But All the classes present in Concurrent Collections are synchronized in nature. Therefore In Concurrent classes, we dont have to take care about Thread-safety.
• While Traditional Collections also have some classes (like Vector, Stack etc) which are synchronized in nature and Traditional Collections also have SynchronizedSet, SynchronizedList, SynchronizedMap methods through which we can get Synchronized version of non-synchronized objects. But these above Synchronized classes are not good in terms of performance because of wide-locking mechanism .Whereas Concurrent Collections classes performance are relatively high than Traditional Collections classes.
• In the Traditional Collections, if a thread is iterating a Collection object and if another thread try to add new element in that iterating object simultaneously then we will get RuntimeException ConcurrentModificationException. Whereas In the above case, we will not get any Runtime Exception if we are Working with Concurrent Collections Classes.
• Traditional Collections classes is good choice if we are not dealing with thread in our application. whereas because of the Concurrent/Synchronized Collection we can use multiple Threads which are dealing with Collections Object. Therefore Concurrent Collections are best choice if we are dealing Multiple Threads in our application.

JAVA

// Java program to illustrate Traditional
// Collections Problem
import java.util.*;
class ConcurrentDemo extends Thread {
    static ArrayList l = new ArrayList();
    public void run()
    {
        try {
            Thread.sleep(2000);
        }
        catch (InterruptedException e) {
            System.out.println("Child Thread"
                    + " going to add element");
        }

        // Child thread trying to add new
        // element in the Collection object
        l.add("D");
    }

    public static void main(String[] args)
        throws InterruptedException
    {
        l.add("A");
        l.add("B");
        l.add("c");

        // We create a child thread that is
        // going to modify ArrayList l.
        ConcurrentDemo t = new ConcurrentDemo();
        t.start();

        // Now we iterate through the ArrayList
        // and get exception.
        Iterator itr = l.iterator();
        while (itr.hasNext()) {
            String s = (String)itr.next();
            System.out.println(s);
            Thread.sleep(6000);
        }
        System.out.println(l);
    }
}

Output:

Exception in thread “main” java.util.ConcurrentModificationException

JAVA

// Java program to illustrate ConcurrentCollection uses
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.*;
class ConcurrentDemo extends Thread {
    static CopyOnWriteArrayList l =
                     new CopyOnWriteArrayList();
    public void run()
    {
        try {
            Thread.sleep(2000);
        }
        catch (InterruptedException e) {
            System.out.println("Child Thread"
                     + " going to add element");
        }

        // Child thread trying to add new
        // element in the Collection object
        l.add("D");
    }

    public static void main(String[] args)
        throws InterruptedException
    {
        l.add("A");
        l.add("B");
        l.add("c");

        // We create a child thread that is
        // going to modify ArrayList l.
        ConcurrentDemo t = new ConcurrentDemo();
        t.start();

        // Now we iterate through the ArrayList
        // and get exception.
        Iterator itr = l.iterator();
        while (itr.hasNext()) {
            String s = (String)itr.next();
            System.out.println(s);
            Thread.sleep(6000);
        }
        System.out.println(l);
    }
}

output:

A
B
c