Queue using stack

QueueStack.java

package dsa.stack;

import java.util.Stack;

/**
 * An implementation of Queue using two Stacks
 * @author Braga
 */

public class QueueStack<T>{
 private Stack<T> firstStack;
 private Stack<T> secondStack;
 
 public QueueStack(){
  firstStack = new Stack<T>();
  secondStack = new Stack<T>();
 }

 public QueueStack<T> enQueue(T item) {
  firstStack.push(item);
  return this;
 }

 public T deQueue() {
  if(!secondStack.isEmpty()){
   return secondStack.pop();
  }else if(!firstStack.isEmpty()){
   while(!firstStack.isEmpty()){
    secondStack.push(firstStack.pop());
   }
   return secondStack.pop();
  }
  return null;
 }

 public int size() {
  return firstStack.size() + secondStack.size();
 }
 
 public boolean isEmpty(){
  if (size() == 0) {
   return true;
  }
  return false;
 }

}