Java文件读写与异常处理从入门到实战指南

// Windows风格（注意转义）
File file1 = new File("D:\\temp\\test.txt");
// Unix风格（推荐）
File file2 = new File("D:/temp/test.txt");
// 分目录和文件名
File dir = new File("D:/myDir");
File file3 = new File(dir, "data.txt");

import java.io.*;
import java.util.Scanner;
public class ReadFileDemo {
    public static void main(String[] args) throws FileNotFoundException {
        File file = new File("names.txt");
        Scanner scanner = new Scanner(file);
        while (scanner.hasNext()) {
            String line = scanner.nextLine();
            System.out.println(line);
        }
        scanner.close();
    }
}

PrintWriter out = new PrintWriter("output.txt");
out.println("Hello, Java!");
out.println("File writing test.");
out.close();

FileWriter fw = new FileWriter("output.txt", true);
PrintWriter out = new PrintWriter(fw);
out.println("这行会被追加到文件末尾");
out.close();

try {
    File file = new File("data.txt");
    Scanner sc = new Scanner(file);
    while (sc.hasNext()) {
        System.out.println(sc.nextLine());
    }
    sc.close();
} catch (FileNotFoundException e) {
    System.out.println("文件未找到：" + e.getMessage());
}

try {
    // 可能抛出多种异常的代码
} catch (FileNotFoundException e) {
    System.out.println("文件不存在");
} catch (IOException e) {
    System.out.println("IO错误");
}

Scanner sc = null;
try {
    sc = new Scanner(new File("test.txt"));
    // 读取文件
} catch (FileNotFoundException e) {
    System.out.println("文件未找到");
} finally {
    if (sc != null) {
        sc.close(); // 保证资源被释放
    }
}

public void readFile(String path) throws FileNotFoundException {
    Scanner sc = new Scanner(new File(path));
    // ...
}

if (amount < 0) {
    throw new IllegalArgumentException("金额不能为负数");
}

class NegativeBalanceException extends Exception {
    public NegativeBalanceException(String msg) {
        super(msg);
    }
}

import java.io.*;
public class FileCopy {
    public static void main(String[] args) {
        String src = "source.txt";
        String dest = "dest.txt";
        try (BufferedReader reader = new BufferedReader(new FileReader(src));
             PrintWriter writer = new PrintWriter(new FileWriter(dest))) {
            String line;
            while ((line = reader.readLine()) != null) {
                writer.println(line);
            }
            System.out.println("复制成功");
        } catch (FileNotFoundException e) {
            System.out.println("源文件不存在");
        } catch (IOException e) {
            System.out.println("IO错误：" + e.getMessage());
        }
    }
}