code: public final URL toURL(String relativeUrl) {
      try {
        URL url = toURL();
        return new URL(url, relativeUrl);
      } catch (MalformedURLException e) {
        throw new IllegalArgumentException(e);
      }
    }
docs: Constructs the URL based on {@link URL#URL(URL, String)} with this URL representation from
  {@link #toURL()} and a relative url.
  
  <p>Any {@link MalformedURLException} is wrapped in an {@link IllegalArgumentException}.
  
  @return new URL instance
  @since 1.14