Recuperar un número a partir de una expresión XPath

public static Double evaluador(InputStream stream, String expression){
   Double number = null;
   try{
      // creación del source
      InputSource source = new InputSource(stream);
 
      // creación del XPath 
      XPathFactory factory = XPathFactory.newInstance();
      XPath xpath = factory.newXPath();
 
      // evaluar la expresión XPath
      XPathExpression exp = xpath.compile(expression);
      number = (Double)exp.evaluate(source,XPathConstants.NUMBER);
   }catch(XPathExpressionException xpee){
      xpee.printStackTrace();
   }
   return number;
}