Java程序无法输出及InputMismatchException错误排查求助

Hey there, let's work through your Java program issues step by step. You've hit two distinct problems, so we'll tackle each one and fix the code properly.

1. 第一个问题：主方法缺失 #

你最初遇到的错误是因为主方法放在了独立的Driver类中，但你直接运行了Township类。Java要求被运行的类必须包含public static void main(String[] args)方法（JavaFX应用除外）。你把主方法移到Township类的方向是对的，但这引出了第二个问题。

2. 第二个问题：InputMismatchException #

这个异常发生在Scanner试图读取某一类型的值（比如int）但遇到了其他内容（字符串、空行或格式错误）时。我们来拆解问题所在并修复：

问题1：文件读取格式不匹配 #

你的readFile方法假设bicycledata.txt的格式是这样的：

TownshipName
100
50
AnotherTown
200
150

如果你的文件格式不同（比如每个镇的所有数据在一行），nextLine()和nextInt()的调用就会不同步，导致异常。另外，调用nextInt()后需要正确处理换行符，我们可以通过更安全的方式读取每行内容并解析值。

问题2：用户输入的Scanner同步问题 #

在getInput方法中，用sc.nextInt()读取家庭数和自行车数后，输入缓冲区中残留的换行符会被下一个sc.nextLine()读取，导致你回到菜单选择选项1时，得到的镇名是空字符串。

修复后的完整代码（带注释说明） #

以下是修复了所有问题的代码，注释标注了关键修改点：

import java.util.*;
import java.io.*;

public class Township {
    // 实例变量
    String name;
    int households;
    int bicycles;
    double average;
    String tier;
    double funding;

    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(System.in);
        Township town[] = new Township[10];
        int count = readFile(town);

        while (true) {
            System.out.println("1. Input\n2. Report\n3. Exit");
            System.out.print("Enter choice: ");
            
            // 增加输入校验，确保用户输入整数
            while (!sc.hasNextInt()) {
                System.out.println("Please enter a valid number (1-3)!");
                sc.next(); // 消耗无效输入
            }
            int choice = sc.nextInt();
            sc.nextLine(); // 消耗选择后的换行符

            switch (choice) {
                case 1:
                    town[count] = getInput(sc);
                    count++;
                    break;
                case 2:
                    printReport(town, count);
                    break;
                case 3:
                    writeFile(town, count);
                    System.out.println("Goodbye!");
                    return;
                default:
                    System.out.println("Invalid choice, please pick 1, 2, or 3.");
            }
        }
    }

    // 构造方法
    public Township(String name, int households, int bicycles) {
        this.name = name;
        this.households = households;
        this.bicycles = bicycles;
        // 创建对象时自动计算相关值，避免遗漏调用
        calculateAverageBikes();
        calculateFunding();
        calculateTier();
    }

    // 重命名方法以遵循Java驼峰命名规范
    private void calculateAverageBikes() {
        average = (double) bicycles / households;
    }

    private void calculateFunding() {
        if (average >= 3.0) funding = 50000.00;
        else if (average >= 2.0) funding = 40000.00;
        else if (average >= 1.0) funding = 30000.00;
        else if (average >= 0.5) funding = 20000.00;
        else funding = 0.00;
    }

    private void calculateTier() {
        if (average >= 3.0) tier = "One";
        else if (average >= 2.0) tier = "Two";
        else if (average >= 1.0) tier = "Three";
        else if (average >= 0.5) tier = "Four";
        else tier = "Five";
    }

    // 按镇名字母顺序排序
    public static void sort(Township[] townships, int count) {
        for (int i = 0; i < count - 1; i++) {
            for (int j = i + 1; j < count; j++) {
                if (townships[i].name.compareTo(townships[j].name) > 0) {
                    Township temp = townships[i];
                    townships[i] = townships[j];
                    townships[j] = temp;
                }
            }
        }
    }

    // 向控制台打印报告
    public static void printReport(Township[] townships, int count) {
        sort(townships, count);
        System.out.println("---------------------------------------------------------------------------------------------------");
        System.out.printf("%-20s%-15s%-18s%-20s%-20s%s\n", 
            "Township Name", "Households", "Bicycles Reported", "Average Bikes/Household", "Proposed Funding", "Tier Designation");
        System.out.println("---------------------------------------------------------------------------------------------------");
        for (int i = 0; i < count; i++) {
            System.out.printf("%-20s%-15d%-18d%-20.2f%-20.2f%s\n", 
                townships[i].name, townships[i].households, townships[i].bicycles, 
                townships[i].average, townships[i].funding, townships[i].tier);
        }
    }

    // 安全获取用户输入
    public static Township getInput(Scanner sc) {
        System.out.print("Enter the name of Township: ");
        String name = sc.nextLine();
        
        // 校验家庭数输入
        int households = 0;
        while (true) {
            System.out.print("Enter the number of households: ");
            if (sc.hasNextInt()) {
                households = sc.nextInt();
                if (households > 0) break; // 确保是正数
                else System.out.println("Households must be a positive number!");
            } else {
                System.out.println("Please enter a valid integer for households.");
                sc.next(); // 消耗无效输入
            }
        }

        // 校验自行车数输入
        int bicycles = 0;
        while (true) {
            System.out.print("Enter the number of bicycles reported: ");
            if (sc.hasNextInt()) {
                bicycles = sc.nextInt();
                if (bicycles >= 0) break; // 非负数
                else System.out.println("Bicycles can't be negative!");
            } else {
                System.out.println("Please enter a valid integer for bicycles.");
                sc.next(); // 消耗无效输入
            }
        }
        sc.nextLine(); // 消耗最后一个整数输入后的换行符

        return new Township(name, households, bicycles);
    }

    // 安全读取文件（处理常见格式问题）
    public static int readFile(Township[] townships) throws IOException {
        int count = 0;
        File file = new File("bicycledata.txt");
        
        // 读取前检查文件是否存在
        if (!file.exists()) {
            System.out.println("Warning: bicycledata.txt not found, starting with empty list.");
            return count;
        }

        Scanner fileScanner = new Scanner(file);
        while (fileScanner.hasNextLine() && count < townships.length) {
            String name = fileScanner.nextLine().trim();
            // 跳过文件中的空行
            if (name.isEmpty()) continue;

            // 读取家庭数
            if (!fileScanner.hasNextInt()) {
                System.out.println("Invalid data in file for households, skipping entry: " + name);
                // 跳过该条目剩余的行
                while (fileScanner.hasNextLine()) {
                    if (fileScanner.hasNextInt()) break;
                    fileScanner.nextLine();
                }
                continue;
            }
            int households = fileScanner.nextInt();
            fileScanner.nextLine(); // 消耗换行符

            // 读取自行车数
            if (!fileScanner.hasNextInt()) {
                System.out.println("Invalid data in file for bicycles, skipping entry: " + name);
                while (fileScanner.hasNextLine()) fileScanner.nextLine();
                continue;
            }
            int bicycles = fileScanner.nextInt();
            fileScanner.nextLine(); // 消耗换行符

            if (households > 0 && bicycles >= 0) {
                townships[count] = new Township(name, households, bicycles);
                count++;
            } else {
                System.out.println("Invalid values for " + name + ", skipping (households must be positive, bicycles non-negative).");
            }
        }
        fileScanner.close();
        return count;
    }

    // 写入报告到文件
    public static void writeFile(Township[] townships, int count) throws IOException {
        sort(townships, count);
        // 使用try-with-resources自动关闭写入流
        try (PrintWriter pout = new PrintWriter(new FileWriter("report.txt"))) {
            pout.println("---------------------------------------------------------------------------------------------------");
            pout.printf("%-20s%-15s%-18s%-20s%-20s%s\n", 
                "Township Name", "Households", "Bicycles Reported", "Average Bikes/Household", "Proposed Funding", "Tier Designation");
            pout.println("---------------------------------------------------------------------------------------------------");
            for (int i = 0; i < count; i++) {
                pout.printf("%-20s%-15d%-18d%-20.2f%-20.2f%s\n", 
                    townships[i].name, townships[i].households, townships[i].bicycles, 
                    townships[i].average, townships[i].funding, townships[i].tier);
            }
        }
    }
}

关键修改点： #

• 主方法位置：将主方法保留在public Township类中，确保Java运行时能找到它。
• 输入校验：为用户和文件输入添加了整数校验，防止InputMismatchException。
• Scanner同步：修复了nextInt()后的换行符处理问题，避免镇名输入为空。
• 文件读取鲁棒性：增加了文件存在性检查、空行跳过，遇到无效数据时优雅处理而非崩溃。
• Java命名规范：将AverageBikes()这类方法重命名为calculateAverageBikes()，遵循驼峰命名法。
• 自动计算：将平均值、资金、等级计算移到构造方法中，确保创建对象时自动执行，避免遗漏调用。
• 资源自动管理：使用try-with-resources处理文件写入，确保流能正确关闭。

bicycledata.txt要求的格式： #

确保你的输入文件采用以下结构（每个镇占3行）：

Springfield
200
450
Shelbyville
150
300

如果你偏好其他格式（比如每行逗号分隔数据），可以告诉我，我们再调整readFile方法。

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