Java中如何通过用户输入退出循环？代码输入END报错求优化

一、输入END报错的原因及修复 #

输入END报错是因为你用sc.nextDouble()尝试读取字符串类型的退出指令，这会触发InputMismatchException。同时嵌套的双层for循环会导致退出指令无法及时中断流程，需要调整循环结构并修改输入读取逻辑。

修复后的完整代码：

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

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("-------Welcome to the Radius calculator-------");

        boolean keepRunning = true;
        int usageCount = 0;
        int remainingTries = 5;

        while (keepRunning && usageCount < 4 && remainingTries > 0) {
            System.out.println("-----------------------------");
            System.out.println("Hey you can use this calculator " + remainingTries + " more time(s) till you have to purchase our full version only 3.99");
            System.out.println("Please enter your Radius or type END to exit");
            
            // 先读取输入内容，判断是否为退出指令
            String input = sc.next();
            if ("END".equalsIgnoreCase(input)) {
                keepRunning = false;
                break;
            }

            // 尝试将输入转为半径数值
            double radius;
            try {
                radius = Double.parseDouble(input);
            } catch (NumberFormatException e) {
                System.out.println("Invalid input! Please enter a number or END to exit.");
                continue;
            }

            System.out.println("Enter 1 if you would like to get the area");
            System.out.println("Enter 2 if you would like to get the circumference");
            
            // 读取用户选择，处理无效输入
            int choice;
            try {
                choice = sc.nextInt();
            } catch (InputMismatchException e) {
                System.out.println("Invalid choice! Please enter 1 or 2.");
                sc.nextLine(); // 清空缓冲区的无效输入
                continue;
            }

            // 执行计算逻辑
            if (choice == 1) {
                double area = Math.PI * radius * radius;
                System.out.printf("The area of the circle with radius %.2f is: %.2f%n", radius, area);
            } else if (choice == 2) {
                double circumference = 2 * Math.PI * radius;
                System.out.printf("The circumference of the circle with radius %.2f is: %.2f%n", radius, circumference);
            } else {
                System.out.println("No answer for you boss");
                System.out.println("Try again!");
                continue; // 无效选择不消耗使用次数
            }

            // 更新计数
            usageCount++;
            remainingTries--;
            System.out.println("You have used this calculator efficiently " + usageCount + " time(s)");
            System.out.println("-----------------------------");
        }

        sc.close(); // 关闭Scanner释放资源
        System.out.println("Calculator exited.");
    }
}

核心修复点： #

• 先读取字符串输入，判断是否为END（支持大小写不敏感），触发直接终止循环
• 用try-catch捕获类型转换异常，避免程序崩溃并提示用户
• 移除冗余的双层for循环，改用单个while循环结合计数变量控制使用次数，逻辑更清晰
• 关闭Scanner资源，避免资源泄漏

二、代码效率与可维护性提升建议 #

• 提取独立方法：把计算面积、周长的逻辑拆成单独方法，简化主代码：

  private static double calculateArea(double radius) {
      return Math.PI * radius * radius;
  }
  
  private static double calculateCircumference(double radius) {
      return 2 * Math.PI * radius;
  }
  

• 规范变量命名：将num改为radius、num1改为choice，提升代码可读性
• 修正拼写错误：calcultor→calculator、yoh→you、circumfrence→circumference、effiently→efficiently
• 使用格式化输出：用printf替代字符串拼接，输出更美观且性能更优
• 增加边界校验：添加半径不能为负数的判断，提示用户输入有效数值
• 减少嵌套层级：原代码多层嵌套（while→for→for→if），拆分为线性逻辑后更易调试维护

内容的提问来源于stack exchange，提问作者Melvin Baldwin