Java ATM机应用NoSuchElementException错误及循环输入修复求助

问题场景 #

正在学习Java并开发ATM机应用，需求是循环询问用户What would you like to do，直到用户输入有效的操作（仅支持Withdraw和Deposit）。当前代码中，无效输入会正常循环提示Please select a valid choice，但执行一次有效操作后会抛出NoSuchElementException（无此元素异常），无法继续循环。

错误原因 #

核心问题是重复创建并关闭Scanner实例：

• 在App类中已经创建了关联System.in的Scanner对象
• 在Transactions类的Withdrawal()和Deposit()方法中，又各自新建Scanner实例，并在方法结束时调用scanner.close()
• 关闭Scanner会同时关闭底层的System.in输入流，后续App类中的Scanner尝试读取输入时，因输入流已关闭，直接抛出NoSuchElementException

另外原代码存在拼写错误：System.out.println("What woul you like to do?") 中的woul应为would。

修复方案 #

1. 统一使用一个Scanner实例，通过参数传递给Transactions的方法
2. 移除Transactions方法内的Scanner创建与关闭逻辑
3. 修正拼写错误
4. 优化Transactions实例创建逻辑，避免每次循环重置余额

修改后的代码 #

App.java #

import java.util.Scanner;

public class App {
    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in); 
        boolean done = false;       
        // 仅创建一次Transactions实例，保证余额持续更新
        Transactions newTransaction = new Transactions();

        while(!done) {
            System.out.println("What would you like to do?");
            String response = scanner.next().toLowerCase();

            if (response.equals("withdraw")) {
                newTransaction.Withdrawal(scanner);
            } else if (response.equals("deposit")) {
                newTransaction.Deposit(scanner);
            } else {
                System.out.println("Please select a valid choice");
            }   
        }
        scanner.close();
    }
}

Transactions.java #

import java.util.Scanner;

public class Transactions {
    private int currentBalance = 100;

    public void Withdrawal(Scanner scanner) {
        System.out.println("How much would you like to withdraw?");

        int withdrawAmount = scanner.nextInt();

        if (withdrawAmount > 0) {
            currentBalance -= withdrawAmount;
            System.out.println("Amount withdrawn: $" + withdrawAmount);
            System.out.println("Current Balance: $" + Balance());
            if (currentBalance < 0) {
                System.out.println(
                        "You have withdrawn more than you have in current balance.\nYou will be charged a overdraft fee");
            }
        } else {
            System.out.println("Can't remove 0 or negative amount from account");
        }
    }

    public void Deposit(Scanner scanner) {
        System.out.println("How much would you like to deposit?");

        int depositAmount = scanner.nextInt();

        if (depositAmount > 0) {
            currentBalance += depositAmount;
        }
        System.out.println("Amount deposited: $" + depositAmount);
        System.out.println("Current Balance: $" + Balance());
    }

    public int Balance() {
        return currentBalance;
    }
}

内容的提问来源于stack exchange，提问作者Brian Gomez