code: public static String getAbsoluteURI(String urlString, String base)
            throws TransformerException
    {
      if (base == null)
        return getAbsoluteURI(urlString);
  
      String absoluteBase = getAbsoluteURI(base);
      URI uri = null;
      try
      {
        URI baseURI = new URI(absoluteBase);
        uri = new URI(baseURI, urlString);
      }
      catch (MalformedURIException mue)
      {
        throw new TransformerException(mue);
      }
  
      return replaceChars(uri.toString());
    }
docs: Take a SystemID string and try to turn it into a good absolute URI.
  
  @param urlString SystemID string
  @param base The URI string used as the base for resolving the systemID
  
  @return The resolved absolute URI
  @throws TransformerException thrown if the string can't be turned into a URI.