code: public String getNamespaceURI(String prefix) {
          if (prefix == null || prefix.equals(XMLConstants.DEFAULT_NS_PREFIX)) {
              return prefix2Uri.get(DEFAULT_NS);
          } else {
              return prefix2Uri.get(prefix);
          }
      }
docs: This method is called by XPath. It returns the default namespace, if the
  prefix is null or "".
  
  @param prefix
  to search for
  @return uri