Java Do-While 循环 - 如何使用 Q 选项结束程序答案

【问题标题】：Java Do-While Loop - How to End the Program Using Q OptionJava Do-While 循环 - 如何使用 Q 选项结束程序
【发布时间】：2021-04-25 17:19:44
【问题描述】：

所以我有这个简单的银行程序。它的主菜单是：

B - 检查余额
D - 存款
W - 撤回
Q - 退出

它们的功能基本上就是它们的名字。并且有两种方法可以终止程序：通过输入“Q”和“N”。但我对 Q - Quit 选项有疑问。这是源代码：

Scanner scan = new Scanner (System.in);
        
    char anotherTransact = 0, option;
    int balance = 100000, deposit = 0, withdraw = 0;
    
    do {
    System.out.println("\nWelcome to ABC BANK \n");
    
    System.out.println("B - Check for Balance");
    System.out.println("D - Make Deposit");
    System.out.println("W - Make Withdraw");
    System.out.println("Q - Quit");
    
    
        System.out.print("\nSelect an option : ");
        option = scan.next().charAt(0);
        
        
        if ((option == 'B') || (option == 'b')) {
            System.out.println("\nYour current balance is " +balance);
        }
        
        else if ((option == 'D') || (option == 'd')) {
            System.out.print("\nEnter amount to Deposit : ");
            deposit = scan.nextInt();
            if (deposit > 1 && deposit <= 500000) {
                System.out.println("Deposit Transaction is successfully completed.");
                balance = balance + deposit; 
            }
            else if (deposit > 500000) {
                System.out.println("Deposit Amount must not be greater than 500,000");
            }
            else {
                System.out.println("Deposit must be greater than zero");
            }
        }
        
        else if ((option == 'W') || (option == 'w')) {
            System.out.print("\nEnter amount to Withdraw : ");
            withdraw = scan.nextInt();
            if ((withdraw % 100 == 0 && withdraw < 150000)) {
                System.out.println("Withdrawal Transaction is successfully completed.");
                balance = balance - withdraw;
            }
            else if (withdraw > 150000) {
                System.out.println("Withdrawal Amount must not be greater then 150,000");
            }
            else if (withdraw < 0) {
                System.out.println("Withdrawal Amount must be greater than zero");
            }
            else {
                System.out.println("Withdawal Amount must be divisible by 100");
            }
        }
        
        else if ((option == 'Q') || (option == 'q')) 
            break;
        
        else {
            System.out.println("\nInvalid entry, enter any valid option : (B/D/W/Q)");
        }
        
        
        System.out.print("\nWant to Transact another (Y/N?) ");
        anotherTransact = scan.next().charAt(0);
        
        System.out.println("\n======================================================"); 
    }
    
    
    while ((anotherTransact == 'Y') || (anotherTransact =='y'));
        if ((anotherTransact == 'N' || anotherTransact == 'n')) {
            System.out.println("\nThank you for using this program!");
        }
        else {
            System.out.println("Invalid entry, entery any valid option : (Y/N)");
        }
    scan.close();

它的输出是这样的：

Welcome to ABC BANK 

B - Check for Balance
D - Make Deposit
W - Make Withdraw
Q - Quit

Select an option :

当我输入Q选项时，输出应该是这样的：

Welcome to ABC BANK 

B - Check for Balance
D - Make Deposit
W - Make Withdraw
Q - Quit

Select an option : Q
======================================================

Thank you for using this program!

但是当我输入它时，它会说它是一个无效输入。我应该怎么做才能通过输入'Q'来结束程序？

【问题讨论】：

(anotherTransact == 'N' || anotherTransact == 'n') 用anotherTransact == 'Q'扩展它

标签： java loops if-statement do-while

【解决方案1】：

你的 Q 实现是正确的，它会退出 do while 循环，但在 while 语句之后你有一个 if：

 if ((anotherTransact == 'N' || anotherTransact == 'n')) {
            System.out.println("\nThank you for using this program!");
        }
        else {
            System.out.println("Invalid entry, entery any valid option : (Y/N)");
        }

当您键入 Q 退出循环时，anotherTransact 不等于 'N'，因此它将进入 else 并打印无效条目。如果我理解正确，您实际上不需要在 while 语句之后是：

 System.out.println("\n======================================================"); 
 System.out.println("\nThank you for using this program!");

【讨论】：

【解决方案2】：

你的代码有两个问题：

将以下代码置于do-while 循环范围之外：

if ((anotherTransact == 'N' || anotherTransact == 'n')) {
    System.out.println("\nThank you for using this program!");
} else {
    System.out.println("Invalid entry, entery any valid option : (Y/N)");
}

但是，仅仅纠正这个问题是不够的。检查下一点/问题以了解需要做什么才能使其按照您的预期工作。

在输入无效的情况下不循环：你需要另一个循环（我推荐do-while循环如下所示）来纠正这个问题。

boolean valid;
do {
    valid = true;
    System.out.print("\nWant to Transact another (Y/N?) ");
    anotherTransact = scan.next().charAt(0);

    System.out.println("\n======================================================");

    if (anotherTransact == 'N' || anotherTransact == 'n') {
        System.out.println("\nThank you for using this program!");
    } else if (!(anotherTransact == 'Y' || anotherTransact == 'y')) {
        System.out.println("Invalid entry, entery any valid option : (Y/N)");
        valid = false;
    }
} while (!valid);

完整更新的代码：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        char anotherTransact = 0, option;
        int balance = 100000, deposit = 0, withdraw = 0;
        do {
            System.out.println("\nWelcome to ABC BANK \n");

            System.out.println("B - Check for Balance");
            System.out.println("D - Make Deposit");
            System.out.println("W - Make Withdraw");
            System.out.println("Q - Quit");

            System.out.print("\nSelect an option : ");
            option = scan.next().charAt(0);

            if ((option == 'B') || (option == 'b')) {
                System.out.println("\nYour current balance is " + balance);
            } else if ((option == 'D') || (option == 'd')) {
                System.out.print("\nEnter amount to Deposit : ");
                deposit = scan.nextInt();
                if (deposit > 1 && deposit <= 500000) {
                    System.out.println("Deposit Transaction is successfully completed.");
                    balance = balance + deposit;
                } else if (deposit > 500000) {
                    System.out.println("Deposit Amount must not be greater than 500,000");
                } else {
                    System.out.println("Deposit must be greater than zero");
                }
            } else if ((option == 'W') || (option == 'w')) {
                System.out.print("\nEnter amount to Withdraw : ");
                withdraw = scan.nextInt();
                if ((withdraw % 100 == 0 && withdraw < 150000)) {
                    System.out.println("Withdrawal Transaction is successfully completed.");
                    balance = balance - withdraw;
                } else if (withdraw > 150000) {
                    System.out.println("Withdrawal Amount must not be greater then 150,000");
                } else if (withdraw < 0) {
                    System.out.println("Withdrawal Amount must be greater than zero");
                } else {
                    System.out.println("Withdawal Amount must be divisible by 100");
                }
            } else if ((option == 'Q') || (option == 'q')) {
                break;
            } else {
                System.out.println("\nInvalid entry, enter any valid option : (B/D/W/Q)");
            }

            boolean valid;
            do {
                valid = true;
                System.out.print("\nWant to Transact another (Y/N?) ");
                anotherTransact = scan.next().charAt(0);

                System.out.println("\n======================================================");

                if (anotherTransact == 'N' || anotherTransact == 'n') {
                    System.out.println("\nThank you for using this program!");
                } else if (!(anotherTransact == 'Y' || anotherTransact == 'y')) {
                    System.out.println("Invalid entry, entery any valid option : (Y/N)");
                    valid = false;
                }
            } while (!valid);
        } while (anotherTransact == 'Y' || anotherTransact == 'y');
    }
}

示例运行：

Welcome to ABC BANK 

B - Check for Balance
D - Make Deposit
W - Make Withdraw
Q - Quit

Select an option : b

Your current balance is 100000

Want to Transact another (Y/N?) d

======================================================
Invalid entry, entery any valid option : (Y/N)

Want to Transact another (Y/N?) d

======================================================
Invalid entry, entery any valid option : (Y/N)

Want to Transact another (Y/N?) y

======================================================

Welcome to ABC BANK 

B - Check for Balance
D - Make Deposit
W - Make Withdraw
Q - Quit

Select an option : d

Enter amount to Deposit : 200
Deposit Transaction is successfully completed.

Want to Transact another (Y/N?) y

======================================================

Welcome to ABC BANK 

B - Check for Balance
D - Make Deposit
W - Make Withdraw
Q - Quit

Select an option : b

Your current balance is 100200

Want to Transact another (Y/N?) n

======================================================

Thank you for using this program!

一些补充说明：

您应该永远不要关闭Scanner 的System.in，因为它也会关闭System.in，并且在不重新启动JVM 的情况下无法再次打开它。但是，请确保为文件关闭 Scanner。
您应该养成将块的主体包含在{...} 中的习惯，即使主体中只有一条语句。这将使您避免某些语句意外超出范围，例如
```
if (1 == 2)
    System.out.print("Hello");
    System.out.println("World");

System.out.println("What's up?"); 
```

将打印

World
What's up?

不只是

What's up?

保持代码格式正确，因为它可以帮助您轻松快速地找到问题。您的 IDE 可以在单击按钮或按下组合键时格式化代码。

【讨论】：