如何用栈结构实现队列结构

package com.harrison.class02;
import java.util.Stack;
public class Code07_TwoStacksImplementQueue {
  public static class TwoStacksToQueue{
    public Stack<Integer> pushStack;
    public Stack<Integer> popStack;
    public TwoStacksToQueue() {
      pushStack=new Stack<Integer>();
      popStack=new Stack<Integer>();
    }
    public void pushToPop() {
      if(popStack.isEmpty()) {
        while(!pushStack.isEmpty()) {
          popStack.push(pushStack.pop());
        }
      }
    }
    public void add(int pushInt) {
      pushStack.push(pushInt);
      pushToPop();
    }
    public int poll() {
      if(pushStack.isEmpty() && popStack.isEmpty()) {
        throw new RuntimeException("队列空了！");
      }
      pushToPop();
      return popStack.pop();
    }
    public int peek() {
      if(pushStack.isEmpty() && popStack.isEmpty()) {
        throw new RuntimeException("队列空了！");
      }
      pushToPop();
      return popStack.peek();
    }
  }
  public static void main(String[] args) {
    TwoStacksToQueue test=new TwoStacksToQueue();
    test.add(1);
    test.add(2);
    test.add(3);
    System.out.println(test.peek());
    System.out.println(test.poll());
    System.out.println(test.peek());
    System.out.println(test.poll());
    System.out.println(test.peek());
    System.out.println(test.poll());
  }
}