你已经宣布 subtract 以及 amounttaken 作为 boolean 变量，因此它们只能 true 或者 false 因为下面给出的值是一个示例运行：

Do you want to withdraw money?
true
How much would you like to withdraw?
true
Subtract

如果尝试输入不同于 true 或者 false ，你会得到 InputMismatchException .
我想您应该为变量输入一些十进制数（amount）， amounttaken . 如果是，声明为 double 或者 float 而不是 boolean 例如

import java.util.InputMismatchException;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        login();
    }

    public static void login() {
        double balance;
        try {
            boolean subtract;
            double amounttaken;// Changed to double
            // This is to see if you want to withdraw money
            System.out.println("Do you want to withdraw money?");
            Scanner SCaddOrWithdraw = new Scanner(System.in);
            subtract = SCaddOrWithdraw.nextBoolean();
            if (subtract) {
                System.out.println("How much would you like to withdraw?");
                Scanner SCamounttaken = new Scanner(System.in);
                amounttaken = SCamounttaken.nextDouble();// Changed for double
                System.out.println("Subtract");
            } else if (!subtract) {
                System.out.print("Ask for bal");
            }
        } catch (InputMismatchException e) {
            System.out.println("Invalid Input");
        }
    }
}

示例运行：

Do you want to withdraw money?
true
How much would you like to withdraw?
3000
Subtract

它是一个完整的结构。

try {
    // Statement Which May Throw Exceptions
} catch(Exception e) {
    // If any statement throws the exception
    // Do alternative flow here
}

如果try块中的代码可能引发异常，则可以执行另一种方法。就像上面的代码一样，如果任何人输入一个字符串而不是布尔值，它将抛出inputmismatchexception，它将被捕获在catch块中，您可以将错误消息打印到用户nice and clearly.：）