code: @Override
      protected String toExternalForm(URL url) {
          StringBuilder sb = new StringBuilder();
          sb.append("jar:");
          sb.append(url.getFile());
          String ref = url.getRef();
          if (ref != null) {
              sb.append(ref);
          }
          return sb.toString();
      }
docs: Build and return the externalized string representation of url.
  
  @return String the externalized string representation of url
  @param url
  a URL