code: public static String getFileExtension(String fullName) {
      checkNotNull(fullName);
      String fileName = new File(fullName).getName();
      int dotIndex = fileName.lastIndexOf('.');
      return (dotIndex == -1) ? "" : fileName.substring(dotIndex + 1);
    }
docs: Returns the <a href="http://en.wikipedia.org/wiki/Filename_extension">file extension</a> for
  the given file name, or the empty string if the file has no extension. The result does not
  include the '{@code .}'.
  
  @since 11.0