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

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();
      list = path.list(filter(args[0]));
    for (int i = 0; i < list.length; i++)

5.15.Anonymous inner class
5.15.1.Demonstrate an inner class.
5.15.2.Define an inner class within a for loop.
5.15.3.Use anonymous inner classes
5.15.4.Building the anonymous inner class in-place
5.15.5.Anonymous inner class cannot have a named constructor, only an instance initializer
5.15.6.Creating a constructor for an anonymous inner class
5.15.7.Using 'instance initialization' to perform construction on an anonymous inner class
5.15.8.Argument must be final to use inside anonymous inner class
5.15.9.A method that returns an anonymous inner class
5.15.10.An anonymous inner class that calls the base-class constructor
5.15.11.An anonymous inner class that performs initialization
5.15.12.Demonstrates method-scoped inner classes
5.15.13.Demonstrates anonymous classes
5.15.14.Demonstration of some static nested classes
5.15.15.Access inner class from outside
5.15.16.Accessing its enclosing instance from an inner class