code: @Override
      protected URL toURL(String filename) {
          try {
              return new File(filename).toURI().toURL();
          } catch (MalformedURLException e) {
              throw new IllegalArgumentException(e);
          }
      }
docs: Returns a file URL pointing to the given file
  
  @param filename path to the file
  @return a file URL poiting to the file