Here is an example of how to use the ExtensionFileComparator class from the org.apache.commons.io package in Java:

import java.io.File;
import org.apache.commons.io.comparator.ExtensionFileComparator;

public class Example {
    public static void main(String[] args) {
        File[] files = ...
        ExtensionFileComparator comparator = new ExtensionFileComparator();
        Arrays.sort(files, comparator);
    }
}

In this example, we create an instance of the ExtensionFileComparator class and use it to sort an array of File objects. This comparator sorts files based on their file extensions. The Arrays.sort() method is used to sort the files array using the comparator instance.

You can also pass a boolean value as argument to the constructor of the ExtensionFileComparator class indicating whether the sort should be done in reverse order. For example:

ExtensionFileComparator comparator = new ExtensionFileComparator(true);

This will sort the files in reverse order of the file extension.

It's worth noting that the ExtensionFileComparator class is an extension of AbstractFileComparator class, that uses the File class's natural ordering to compares files and sorts them based on their extension, in the order specified in the constructor.