Use anonymous inner classes : Anonymous inner class « Class Definition « Java Tutorial

1. Java Tutorial
2. Class Definition
3. Anonymous inner class

import java.io.File;
import java.io.FilenameFilter;
import java.util.Arrays;
import java.util.regex.Pattern;

public class MainClass {
  public static FilenameFilter filter(final String regex) {
    // Creation of anonymous inner class:
    return new FilenameFilter() {
      private Pattern pattern = Pattern.compile(regex);

      public boolean accept(File dir, String name) {
        return pattern.matcher(new File(name).getName()).matches();
      }
    }; // End of anonymous inner class
  }

  public static void main(String[] args) {
    File path = new File(".");
    String[] list;
    if (args.length == 0)
      list = path.list();
    else
      list = path.list(filter(args[0]));
    Arrays.sort(list);
    for (int i = 0; i < list.length; i++)
      System.out.println(list[i]);
  }
}