二、 编写一个类，用两个栈实现队列，支持队列的基本操作（add,poll,peek）

请指教交流！

package com.it.hxs.c01;

import java.util.Stack;

/*
 编写一个类，用两个栈实现队列，支持队列的基本操作（add,poll,peek） 
 */
public class HxsQueue {

    public static void main(String args[]) {
        HxsQueue demoQueue = new HxsQueue();
        demoQueue.add("111");
        demoQueue.add("222");
        demoQueue.add("333");
        demoQueue.poll();
        System.out.println(demoQueue.peek());
    }

    private Stack<String> pushStack;// 推入栈
    private Stack<String> popStack;// 推出栈

    public HxsQueue() {
        this.pushStack = new Stack<String>();
        this.popStack = new Stack<String>();
    }

    public void add(String content) {
        if ("".equals(content) || content == null) {
            throw new RuntimeException("添加的元素值不能为空！");
        } else {
            pushStack.push(content);
            this.popStack = new Stack<String>();
            for (int index = pushStack.size() - 1; index >= 0; index--) {
                popStack.push(pushStack.elementAt(index));
            }
        }
    }

    public String poll() {
        String result = "无元素";
        if (!popStack.isEmpty()) {
            result = popStack.pop();
            this.pushStack = new Stack<String>();
            for (int index = popStack.size() - 1; index >= 0; index--) {
                pushStack.push(popStack.elementAt(index));
            }
        }
        return result;
    }

    public String peek() {
        String result = "无元素";
        if (!popStack.isEmpty()) {
            result = popStack.peek();
        }
        return result;
    }

}