code: public final String evaluateString( Message message ) throws JMSException
      {
          Object value = evaluate(message);
          if (value == null)
              return null;
          
          if (value instanceof String)
              return (String)value;
          
          throw new FFMQException("Expected a string but got : "+value.toString(),"INVALID_SELECTOR_EXPRESSION");
      }
docs: Evaluate this node as a string