如果 while 循环中的分支没有按预期执行

Question

这是我正在使用的代码。此处引用的其他类已经过测试并且可以正常工作，但它们不应影响此处语句的功能。

import java.util.Scanner;

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

        double transferAmount;
        Account acct1 = new Account(100, null);
        Account acct2 = new Account(100, null);


        System.out.print("Choose an option: "
                + "\n1 <Enter>: to transfer from account 1 to account 2"
                + "\n2 <Enter>: to transfer from account 2 to account 1"
                + "\n3 <Enter>: to quit");
        int userOption = scan.nextInt();
        scan.nextLine();

        while(userOption != 3); 
        {
            if (userOption == 1)
            {
                System.out.print("You chose transfer from account 1 to account 2"
                        + "\nEnter an amount to transfer: ");
                transferAmount = scan.nextInt();
                acct1.transfer(acct2, transferAmount);
            }
            else if (userOption == 2)
            {
                System.out.print("You chose transfer from account 1 to account 2"
                        + "\nEnter an amount to transfer: ");
                transferAmount = scan.nextInt();
                acct2.transfer(acct1, transferAmount);
            }
            else if (userOption == 3)
            {
                System.out.println(acct1.toString());
                System.out.println(acct2.toString());
            }
            else
            {
                System.out.println("Invalid input");
            }

            System.out.print("Choose an option: "
                    + "\n1 <Enter> to transfer from account 1 to account 2?"
                    + "\n2 <Enter> to transfer from account 2 to account 1"
                    + "\n3 <Enter> to quit");
            userOption = scan.nextInt();
            scan.nextLine();
        }

        System.out.println(acct1.toString());
        System.out.println(acct2.toString());
        scan.close();
    }

}

我想要做的是提示用户选择由数值表示的三个选项之一。然后跟随那个值的 if 分支，在 1 或 2 的情况下向用户询问一个数量。在用户选择选项 3 之前不退出 while 循环。发生的事情是当输入 3 或指定范围之外的值时the program works as expected, but when 1 or 2 are chosen as options nothing happens, not even the print statements are output. 非常感谢您对这项工作的任何帮助。

Answer 1

你的 while 循环后有一个分号。

while(userOption != 3);

实际上等于：

while(userOption != 3) {
  // do nothing
}

所以在该行之后没有代码被执行。