(Rename files) Suppose you have a lot of files in a directory named Exercisei_j, where i and j are digits. Write a program that pads a 0 before i if i is a single digit. For example, a file named Exercise2_1 in a directory will be renamed to Exercise02_1. In Java, when you pass the symbol * from the command line, it refers to all files in the directory (see Supplement III.V). Use the following command to run your program: java Exercise12_28 *

Sure, I can help you with that! Here's a possible implementation in Java:

import java.io.File;

public class Exercise12_28 {
    public static void main(String[] args) {
        if (args.length == 0) {
            System.out.println("Usage: java Exercise12_28 *");
            return;
        }
        String pattern = "Exercise%d_%d";
        for (String filename : args) {
            File file = new File(filename);
            if (file.isFile() && file.getName().startsWith("Exercise")) {
                String[] parts = file.getName().split("_");
                if (parts.length == 2) {
                    try {
                        int i = Integer.parseInt(parts[0].substring(8));
                        int j = Integer.parseInt(parts[1]);
                        String newFilename = String.format(pattern, i < 10 ? 0 : "", i, j);
                        File newFile = new File(file.getParentFile(), newFilename);
                        if (file.renameTo(newFile)) {
                            System.out.printf("Renamed %s to %s%n", file.getName(), newFilename);
                        } else {
                            System.out.printf("Failed to rename %s to %s%n", file.getName(), newFilename);
                        }
                    } catch (NumberFormatException e) {
                        System.out.printf("Invalid filename format: %s%n", file.getName());
                    }
                } else {
                    System.out.printf("Invalid filename format: %s%n", file.getName());
                }
            }
        }
    }
}

Here's how it works:

- The program takes command line arguments as filenames to be renamed, using the wildcard symbol `*` to match all files in the directory.
- It checks each file to see if it has a name starting with "Exercise" and containing an underscore character, which separates the two digits `i` and `j`.
- If the file name is valid, it parses `i` and `j` as integers and uses them to format a new filename with a leading zero before `i` if `i` is less than 10.
- Finally, it uses the `renameTo` method of the `File` class to rename the file to the new name, and prints a message to indicate success or failure.

To run this program, you can save it as `Exercise12_28.java` in the directory containing the files to be renamed, and then open a command prompt or terminal window in that directory and type:

javac Exercise12_28.java
java Exercise12_28 *

This will compile and run the program, renaming all matching files in the current directory.