apache common io fileUtils listFiles: how to get list of files with no extension?

Question

I am trying to get a list of files with no extension using org.apache.commons.io.FileUtils.listFiles() like here http://www.avajava.com/tutorials/lessons/how-do-i-get-all-files-with-certain-extensions-in-a-directory-including-subdirectories.html

package test;

import java.io.File;
import java.io.IOException;
import java.util.List;

import org.apache.commons.io.FileUtils;

public class GetAllFilesInDirectoryBasedOnExtensions {

    public static void main(String[] args) throws IOException {

        File dir = new File("dir");
        String[] extensions = new String[] { "txt", "jsp" };
        System.out.println("Getting all .txt and .jsp files in " + dir.getCanonicalPath()
                + " including those in subdirectories");
        List<File> files = (List<File>) FileUtils.listFiles(dir, extensions, true);
        for (File file : files) {
            System.out.println("file: " + file.getCanonicalPath());
        }

    }

}

but I need a list of files with no extension. I've tried {".", ""} but that didn't help. Is it possible at all?