通过字符串拼接实现的一个小demo,用于巩固基础

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

public class moneysy {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        new Scanner(System.in);
        boolean loop = true;
        String input = "";
        Date date = null;
        String details = "----明细----";
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm");
        double money = 0;
        double balance = 0;
        do {
            System.out.println("\n====零钱通系统====");
            System.out.println("\t1.查询余额");
            System.out.println("\t2.存   款");
            System.out.println("\t3.消   费");
            System.out.println("\t5.退   出");
            System.out.print("请选择：");
            input = scanner.next();
            switch (input)
            {
                case "1":
                    System.out.println(details);
                    break;
                case "2":
                    System.out.print("请输入存款金额：");
                    money = scanner.nextDouble();
                    if(money > 0)
                    {
                        balance += money;
                        date = new Date();
                        details += "\n存款：" + money + "元"+ "\t"+ sdf.format(date) +"\t"+"余额" +"\t"+ balance + "元";
                        System.out.println("存款成功！");
                        break;
                    }
                    else {
                        System.out.println("存款金额不能为负数");
                        break;
                    }
                case "3":
                    //消费
                    System.out.print("请输入消费金额：");
                    money = scanner.nextDouble();
                    if (balance != 0)
                    {
                        if(money < 0)
                        {
                            System.out.println("请输入正确的操作！");
                            break;
                        }
                        else{

                            if (balance -money >=0)
                            {
                                balance -= money;
                                details += "\n消费：" + money + "元"+ "\t"+ sdf.format(date) +"\t"+"余额" +"\t"+ balance + "元";
                                System.out.println("\n消费：" + money + "元"+ "\t"+ sdf.format(date) +"\t"+"余额" +"\t"+ balance + "元");
                                System.out.println("消费成功！");
                                break;
                            }
                            else {
                                System.out.println("您的余额为:"+balance+",还需充值:"+(money-balance)+"元,才能完成消费!");//消费金额大于余额
                                break;
                            }
                        }
                    }
                case "4":
                    System.out.println("正在退出系统...");
                    loop = false;
                    break;
                case "5":
                    System.out.println("你在干嘛,输入5干嘛\n请重新输入");
                    break;
                default:
                    System.out.println("输入错误");
            }
        }while (loop);
        System.out.println("已退出系统");
    }
}