How to add or group one collection into another collection in Java ?.

Program to demonstrate how to add or group one collection into another collection in Java

package com.hubberspot.example;

//1. We have to import three Collections
//framework class and interface as 
//ArrayList, Collections and Collection

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;


public class GroupCollectionsDemo {

 public static void main(String[] args) {

  // 2. Creating a Collection reference variable
  // for holding the objects. Collection is interface
  // which is the base interface for all the collections
  // such as ArrayList, Queue etc. Here we have created 
  // an ArrayList object which is holding the Integer objects
  // and its object is been referenced by Collection 
  Collection<Integer> list = new ArrayList<Integer>();

  // 3. Adding six Integer objects to Collection
  list.add(1);
  list.add(2);
  list.add(3);
  list.add(4);
  list.add(5);
  list.add(6);

  // 4. Creating yet another array collection and adding six 
  //Integer objects to it
  Integer[] array = new Integer[6];
  array[0] = 7;
  array[1] = 8;
  array[2] = 9;
  array[3] = 10;
  array[4] = 11;
  array[5] = 12;

  // 5. Looping each Collection object and printing it on 
  // console
  System.out.println("Elements in Collection before adding :");
  for(Integer i : list) {
   System.out.print(i + " ");
  }
  System.out.println("\n");

  // 6. Using Collections class addAll() method to add array of
  // Integers to the list Collection 
  Collections.addAll(list, array);

  System.out.println("Elements in Collection after adding :");

  // 7. Looping each Collection object after adding 
  // Integer array to it and printing it on 
  // console
  for(Integer i : list) {
   System.out.print(i + " ");
  }
  System.out.println("\n");

  // 8. Using Collections class addAll() method to add some
  // more integers to it after adding an array of
  // Integers to the list Collection
  Collections.addAll(list, 13,14,15,16,17,18);


  // 9. Looping each Collection object after adding 
  // Integer array to it plus some more integers 
  // to it and printing it on 
  // console

  System.out.println("Elements in Collection after adding more elements :");
  for(Integer i : list) {
   System.out.print(i + " ");
  }
 }
}

Output of the program :