Here's an example of how to use the MagicNumberFileFilter class from the Apache Commons IO library to filter files based on their magic numbers:

import java.io.File;
import org.apache.commons.io.filefilter.MagicNumberFileFilter;

public class Main {
    public static void main(String[] args) {
        // The magic number to search for
        byte[] magicNumber = new byte[]{0x50, 0x4B, 0x03, 0x04};

        File dir = new File("/path/to/directory");
        File[] matchingFiles = dir.listFiles((FileFilter) new MagicNumberFileFilter(magicNumber));

        for (File file : matchingFiles) {
            System.out.println(file.getName());
        }
    }
}

In this example, /path/to/directory should be replaced with the path to the directory you want to search through. The listFiles method is called on the dir object, and it takes an instance of MagicNumberFileFilter as an argument. The MagicNumberFileFilter constructor takes a byte array as an argument, which is the magic number to search for. The filtered matching files are stored in the matchingFiles array, and their names are printed out using a for loop.

Note that the magic number must be an exact match to the file contents, it is not a string, it is a sequence of bytes that identifies the file type.