功能

package change2;

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

public class ChangeOOPFunction {
    // 状态
    boolean loop = true;
    Scanner scanner = new Scanner(System.in);
    String key = "";
     // 明细
    String details = "-------------明细--------------\n";
     // 入账
    double money = 0;
    double balance = 0;
    Date date = null;
     // 将获取的时间格式化成我们想要的格式
    SimpleDateFormat tf = new SimpleDateFormat("yyyy-MM-dd HH:mm");
     // 消费
    String note = "";

    // 菜单
    public void menu() {
        do {
            System.out.println("=========零钱通=========");
            System.out.println("\t\t\t1 明细");
            System.out.println("\t\t\t2 入账");
            System.out.println("\t\t\t3 消费");
            System.out.println("\t\t\t4 退出");
            System.out.print("请选择(1-4): ");
            key = scanner.next();

            switch(key){
                case "1" :
                    this.details();
                    break;
                case "2" :
                    this.income();
                    break;
                case "3" :
                    this.pay();
                    break;
                case "4" :
                    this.exit();
                    break;
                default:
                    System.out.println("输入有误，请重新输入");
            }
            System.out.println("++++++++++++++++++++++++++++++++");
        }while(loop);
    }

    // 明细
    public void details() {
        System.out.println(details);
    }

    // 入账
    public void income() {
        System.out.print("请输入您的金额：");
        money = scanner.nextDouble();
        if (money <= 0) {
            System.out.println("金融应该大于0哦");
            return;
        }
        balance += money;
        date = new Date();
        details += "入账金额" + "\t" + money + "\t"
                + tf.format(date) + "\t" + "余额"
                + "\t" + balance + "\n";
    }

    // 消费
    public void pay() {
        System.out.print("消费金额: ");
        money = scanner.nextDouble();
        if (money <= 0 || money > balance) {
            System.out.println("金额的范围是 0 - " + balance);
            return;
        }
        balance -= money;
        System.out.print("消费说明：");
        note = scanner.next();
        date = new Date();
        details += note + "\t" + money + "\t"
                + tf.format(date) + "\t" + "余额"
                + "\t" + balance + "\n";
    }

    // 退出
    public void exit() {
        String choice = "";
        while (true){
            System.out.println("您确定要退出吗？y/n");
            choice = scanner.next();
            if ("y".equals(choice) || "n".equals(choice)){
                break;
            }
        }
        if (choice.equals("y")){
            loop = false;
        }
    }
}

使用功能

package change2;

public class ChangeAction {
    public static void main(String[] args) {
        new ChangeOOPFunction().menu();
    }
}