code: @Override
    public String getNamespaceURI(String prefix) {
      // per javax.xml.namespace.NamespaceContext doc
      if (prefix == null)
        throw new IllegalArgumentException("Cannot get namespace URI for null prefix");
      if (XMLConstants.XML_NS_PREFIX.equals(prefix))
        return XMLConstants.XML_NS_URI;
      if (XMLConstants.XMLNS_ATTRIBUTE.equals(prefix))
        return XMLConstants.XMLNS_ATTRIBUTE_NS_URI;
  
      String namespaceURI = bindings.get(prefix);
  
      // per javax.xml.namespace.NamespaceContext doc
      return (namespaceURI != null) ? namespaceURI : XMLConstants.NULL_NS_URI;
    }
docs: Returns the URI for a namespace binding.