为何Java程序输入错误类型时未触发预期的异常捕获？

我编写了一段计算矩形面积的Java代码，使用Scanner接收用户输入。当输入实数类型的数值时程序运行正常，但输入随机字符时程序直接终止，控制台显示finished with non-zero exit value 1，并未执行catch块中的错误提示信息。疑惑为何这段代码的异常处理机制未生效？

控制台输出：

finished with non-zero exit value 1

原代码：

import java.util.Scanner;

public class NewClass {
    public static void main(String[] args) {
        NewClass newClass = new NewClass();
        Scanner scanner = new Scanner(System.in);
        try {
            System.out.println("Enter the first side of the rectangle: ");
            double firstValue = scanner.nextDouble();
            System.out.println("Enter the second side of the rectangle: ");
            double secondValue = scanner.nextDouble();
            double result = newClass.calculateRectangleArea(firstValue, secondValue);
            System.out.println("Area of ​​a rectangle with sides " + firstValue + " " + "and" + secondValue + " " + "are" + result);
        } catch (NumberFormatException exception) {
            System.out.println("Please enter the correct type!");
        }
    }

    public double calculateRectangleArea(double a, double b) {
        return a * b;
    }
}

原因分析 #

你捕获的NumberFormatException并不是Scanner.nextDouble()抛出的异常类型。当输入无法解析为double类型时，nextDouble()方法会抛出**InputMismatchException**，而不是NumberFormatException。因为你没有捕获这个实际抛出的异常，它会直接向上传播，导致JVM终止程序，所以catch块的代码根本不会执行。

解决方法 #

1. 修改捕获的异常类型：将catch块的异常改为InputMismatchException，记得导入java.util.InputMismatchException包；
2. 处理错误输入残留：捕获异常后调用scanner.next()跳过当前错误的输入内容，避免后续输入操作陷入死循环；
3. 优化输出语句：原输出语句的语法略有问题，调整后更通顺。

修改后的代码示例：

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

public class NewClass {
    public static void main(String[] args) {
        NewClass newClass = new NewClass();
        Scanner scanner = new Scanner(System.in);
        try {
            System.out.println("Enter the first side of the rectangle: ");
            double firstValue = scanner.nextDouble();
            System.out.println("Enter the second side of the rectangle: ");
            double secondValue = scanner.nextDouble();
            double result = newClass.calculateRectangleArea(firstValue, secondValue);
            System.out.println("Area of a rectangle with sides " + firstValue + " and " + secondValue + " is " + result);
        } catch (InputMismatchException exception) {
            System.out.println("Please enter the correct numeric type!");
            // 跳过错误输入，防止后续输入操作出错
            scanner.next();
        }
    }

    public double calculateRectangleArea(double a, double b) {
        return a * b;
    }
}

内容的提问来源于stack exchange，提问作者diho472