Here is a Java program to convert a file to a byte array and vice-versa:

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

public class ConvertFileToByteArrayAndByteArrayToFile {
    public static void main(String[] args) throws IOException {
        
        // Convert file to byte array
        File file = new File("example.txt");
        byte[] fileContent = Files.readAllBytes(file.toPath());
        System.out.println("File content as byte array: " + new String(fileContent));
        
        // Convert byte array to file
        Path path = Files.write(Path.of("newfile.txt"), fileContent);
        System.out.println("File content written to file: " + path);
    }
}

This program first creates a File object representing a text file, "example.txt". It then reads the contents of the file into a byte array using the Files.readAllBytes() method. The resulting byte array contains the contents of the file in binary format.

Next, the program writes the byte array to a new file using the Files.write() method. The Files.write() method writes the contents of the byte array to the specified file path, and returns a Path object representing the newly created file.

Note that this program assumes that the file contains text data, and so it converts the byte array to a string using the String constructor. If the file contains binary data, you would need to use a different method to convert the byte array to a more appropriate format, such as Base64 encoding. Similarly, if you want to convert a byte array back to a file, you will need to know the format of the original file so that you can create the new file with the correct extension and content type.