code: @Override
      public Object readValue(final Attribute _attribute,
                              final List<Object> _objectList)
      {
          Double ret = null;
          final Object obj = _objectList.get(0);
          if (obj instanceof Number) {
              ret = ((Number) obj).doubleValue();
          } else if (obj != null) {
              ret = Double.parseDouble(obj.toString());
          }
          return ret;
      }
docs: {@inheritDoc}