用字符串拼接实现一个零钱通，同样可以用多态数组实现

package com;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;

public class smallChange {
    public static void main(String[] args) {

        double money = 0;
        double balance = 0;
        String details = "=======零钱通明细=======";
        Date date = null;
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm");
        boolean loop = true;

        Scanner msc = new Scanner(System.in);
        String key = " ";
        String key1 = " ";

        do{

            System.out.println("========零钱通========");
            System.out.println("\t1.零钱通明细");
            System.out.println("\t2.收益入账");
            System.out.println("\t3.消费支出");
            System.out.println("\t4.退出");

            System.out.println("请输入你的选择：");
            key = msc.next();

            switch(key){

                case "1":
                    System.out.println(details);
                    break;
                case "2":
                    System.out.println("请输入你此次的收入");
                    money = msc.nextDouble();
                    if(money <= 0){
                        System.out.println("入账金额有误");
                    }
                    balance += money;
                    date = new Date();
                    details += "\n收益入账\t" + money + "\t" +  sdf.format(date) + "\t" + balance;
                    break;
                case "3":

                    System.out.println("请输入你此次的消费项目：");
                    key1 = msc.next();
                    System.out.println("请输入你此次的消费金额：");
                    money = msc.nextDouble();
                    if( money <= 0 || money > balance){
                        System.out.println("无法消费");
                        break;
                    }
                    balance -= money;
                    date = new Date();
                    details += "\n消费支出" + money +"\t-" + money + "\t" +  sdf.format(date) + "\t" + balance;
                    break;
                case "4":
                   while(key1 != "y" || key1 != "n" ){
                       System.out.println("你确定退出码");
                       key1 = msc.next();
                       if(key1.equals("y")){
                           loop =false;
                           break;
                       }else if(key1.equals("n")){
                           System.out.println("返回");
                           break;
                       }else{
                           System.out.println("请重新输入");
                       }
                   }
                    break;
                default:
                    System.out.println("输入错误");

            }

        }while(loop);
    }
}