CopyOnWriteArrayList Class in Java

Class declaration

public class CopyOnWriteArrayList
   extends Object
implements List, RandomAccess, Cloneable, Serializable

• CopyOnWriteArrayList is a thread-safe variant of ArrayList where operations which can change the ArrayList (add, update, set methods) creates a clone of the underlying array.

• CopyOnWriteArrayList is to be used in a Thread based environment where read operations are very frequent and update operations are rare.

• Iterator of CopyOnWriteArrayList will never throw ConcurrentModificationException.

• Any type of modification to CopyOnWriteArrayList will not reflect during iteration since the iterator was created.

• List modification methods like remove, set and add are not supported in the iteration. This method will throw UnsupportedOperationException.

• null can be added to the list.

CopyOnWriteArrayList Methods

Following is the list of important methods available in the CopyOnWriteArrayList class.

Example

The following program illustrates several of the methods supported by ArrayList −

Live Demo

import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;
public class Tester {

   public static void main(String args[]) {
      // create an array list
      CopyOnWriteArrayList al = new CopyOnWriteArrayList();
      System.out.println("Initial size of al: " + al.size());

      // add elements to the array list
      al.add("C");
      al.add("A");
      al.add("E");
      al.add("B");
      al.add("D");
      al.add("F");
      al.add(1, "A2");
      System.out.println("Size of al after additions: " + al.size());

      // display the array list
      System.out.println("Contents of al: " + al);

      // Remove elements from the array list
      al.remove("F");
      al.remove(2);
      System.out.println("Size of al after deletions: " + al.size());
      System.out.println("Contents of al: " + al);

      try {
         Iterator iterator = al.iterator();
         while(iterator.hasNext()) {
            iterator.remove();
         }
      }catch(UnsupportedOperationException e) {
         System.out.println("Method not supported:");
      }
      System.out.println("Size of al: " + al.size());
   }
}

This will produce the following result −

Output

Initial size of al: 0
Size of al after additions: 7
Contents of al: [C, A2, A, E, B, D, F]
Size of al after deletions: 5
Contents of al: [C, A2, E, B, D]
Method not supported:
Size of al: 5