解析如何用两个栈来实现队列的方法

import java.util.Stack;
public class QueneWithTwoStacks<E> {
 private Stack<E> stack1;
 private Stack<E> stack2;
 public void appendTail(E e) {
  stack1.push(e);
 }
 public E deleteHead() throws Exception {
  if (stack2.size() <= 0) {
   while (!stack1.isEmpty()) {
    stack2.push(stack1.pop());
   }
  }
  if (stack2.size() == 0) {
   throw new Exception("Queue is empty!");
  }
  return stack2.pop();
 }
}