Queue Using Multiple Stacks

QueueUsingMultipleStack.java

import java.util.Stack;

/**
 * Author: Deepak Singhvi
 */
public class QueueUsingMultipleStack {

  private Stack<Integer> s1 = new Stack<Integer>();
  private Stack<Integer> s2 = new Stack<Integer>();

  public static void main(String args[]) {
    QueueUsingMultipleStack q = new QueueUsingMultipleStack();
    q.enQueue(4);
    q.enQueue(2);
    q.enQueue(7);

    int counter = 1;
    System.out.println(counter++ + ": " + q.deQueue());
    q.enQueue(8);
    while (!q.s1.isEmpty()) {
      System.out.println(counter++ + ": " + q.deQueue());
    }
  }

  public Integer enQueue(Integer element) {
    return s1.push(element);
  }

  public Integer deQueue() {
    if (s1.isEmpty()) {
      System.out.println("Queue Empty");
      return null;
    }
    while (!s1.isEmpty()) {
      s2.push(s1.pop());
    }
    Integer firstElement = s2.pop();
    while (!s2.isEmpty()) {
      s1.push(s2.pop());
    }
    return firstElement;
  }
}