如何在输出中添加行号并检测是否找到文件？

如果我理解正确的要求，则希望为用户指定的文件的每一行打印行号。

如果是这样，那么您可以counter在逐行读取文件时添加一个变量：

import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.Scanner;
public class Q4 {
    public static void main(String[] args) throws IOException {
        try {

            Scanner scanner = new Scanner(System.in);
            System.out.print("Enter filename");
            String fileName = scanner.nextLine();
            File f = new File(fileName);

            if (!f.exists()) { 
                System.out.println(fileName + " doesn't exist!");
                return;
            }

            BufferedReader b = new BufferedReader(new FileReader(f));
            String readLine = null;
            System.out.println("");  //Intended to be empty as to allow the next line. So far that's the only way to get this part it to work.

            int counter = 1;
            while ((readLine = b.readLine()) != null) {
                System.out.println(counter + ": " + readLine);
                counter++;
            }

        } catch (IOException e) {
            System.out.println(e.getMessage());
        }

    }
}

我还添加了一个检查，以查看是否File存在使用该File.exists()方法。

回答来源：Stack Overflow