code: public void setDefaultNamespaceURI(String namespaceURI) {
      if (namespaceURI == null)
        throw new IllegalArgumentException("Cannot set default prefix to null namespace URI");
  
      bindings.put(XMLConstants.DEFAULT_NS_PREFIX, namespaceURI);
    }
docs: Specifies the namespace URI bound to the empty prefix.
  @param namespaceURI	the namespace URI