Java 中的 try/catch

有人可以给我提示为什么这个尝试和捕获不起作用吗？ 它抛出扫描仪异常，而不是打印我期望的消息。

import java.util.*;
import java.io.*;
import java.math.*;
import javax.swing.*;

public class Main {
    public static void main(String[] args) {
        Boolean test = true;
        while (test == true) {
            try {
                double x, y;
                String operator;
                Scanner scan = new Scanner(System.in);
                Scanner scan_2 = new Scanner(System.in);
                Scanner ScanOperator = new Scanner(System.in);
                System.out.println(" Enter a double value: ");
                x = scan.nextDouble();
                System.out.println(" Enter another double value: ");
                y = scan_2.nextDouble();
                System.out.println(" Enter a operator for the operation you want to execute, or X if you want to quit: ");
                operator = ScanOperator.nextLine();
                if (operator.equals("x") || operator.equals("X")) {
                    test = false;
                    System.out.println("No calculation was made!!!");
                }
                System.out.println(Calculation(operator, x, y));
            } catch (NumberFormatException nfe) {
               JOptionPane.showMessageDialog(null,"Input must be a number.");
            }
        }
    }

    public static double Calculation(String operator, double x, double y) {
        double result = 0;
        double myAdd = 0;
        double mySub = 0;
        double myMult = 0;
        double myDiv = 0;
        double myPower = 0;
        double myMod = 0;

        if (operator.equals("+")) {
            myAdd = x + y;
            result = myAdd;
        } else if (operator.equals("-")) {
            mySub = x - y;
            result = mySub;
        } else if (operator.equals("*")) {
            myMult = x * y;
            result = myMult;
        } else if (operator.equals("/")) {
            myDiv = x / y;
            result = myDiv;
        } else if (operator.equals("^")) {
            myPower = Math.pow(x, y);
            result = myPower;
        } else if (operator.equals("%")) {
            myMod = x % y;
            result = myMod;
        } else {
        }

        return result;
    }
}

很简单，程序抛出了ScannerException，但是你的try catch只能捕获NumberFormatException，你需要添加另一个catch子句才能捕获ScannerException，或者只捕获通用的Exception。

例如，当你说：

 } catch (NumberFormatException nfe) {     
     JOptionPane.showMessageDialog(null,"Input must be a number.");
 }

这只是指定如何捕获 NumberFormatException。
为了捕获所有异常，您需要这样做：

 } catch (NumberFormatException nfe) {     
     JOptionPane.showMessageDialog(null,"Input must be a number.");
 }catch (Exception e){
     JOptionPane.showMessageDialog(null,"Generic exception caught");
 }

在这种情况下，第二个 catch 将获取第一个 catch 中未捕获的所有内容，因为所有异常都扩展了 Exception 类，您可以使用该语句捕获所有派生类。

然而，由于捕获异常本身是不受欢迎的，你也可以这样做：

 } catch (NumberFormatException, ScannerException e) {     
     JOptionPane.showMessageDialog(null,"Input must be a number.");
 }

在同一块中捕获两个异常。