4月22日，Stacks，每日20行。

import java.util.*;public class Stacks {    static String[] months = {"January","February","March","April","May","June","July","August","September","October","November","December"};    public static void main(String[] args) {        Stack stk = new Stack();        for(int i = 0; i < months.length; i++){            stk.push(months[i] + " ");        }        System.out.println("stk=" + stk);        stk.addElement("The last line");        System.out.println("element 5 = " + stk.elementAt(5));        System.out.println("popping elements:");        while(!stk.empty()){            System.out.println(stk.pop());        }    }}