在Java中尝试/捕获

Question

有人可以给我一个提示,为什么这个尝试和捕获不起作用？它会抛出扫描程序异常,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;
    }
}

Answer 1

很简单,程序抛出ScannerException,但是你的try catch只能捕获NumberFormatException,你需要添加另一个catch子句以捕获ScannerException,或者只捕获一般的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,您需要进行以下操作:

 } 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.

但是,由于自身捕获异常是不受欢迎的,您还可以这样做:

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

在同一个块中捕获两个异常.

Answer 2

您捕获了错误的异常。