ArrayList retainAll(Collection<?> c) method in java

import java.util.ArrayList;
public class ArrayListRetainAllMethod
{
   public static void main(String[] args)
   {
      ArrayList<String> al1 = new ArrayList<String>();
      al1.add("red");
      al1.add("blue");
      al1.add("green");
      // create another ArrayList
      ArrayList<String> al2 = new ArrayList<String>();
      al2.add("red");
      al2.add("green");
      al2.add("indigo");
      al2.add("yellow");
      // printing ArrayList before using retainAll() method
      System.out.println("ArrayList 1: " + al1);
      System.out.println("ArrayList 2: " + al2);
      // applying retainAll() method to al2 passing al1 as parameter
      al2.retainAll(al1);
      System.out.println("After Applying retainAll() method to al2: ");
      System.out.println("ArrayList 1: " + al1);
      System.out.println("ArrayList 2: " + al2);
   }
}