static double getOptionalDouble(final Element element, final String attributeName, double defaultValue) {
    final String str = element.getAttributeValue(attributeName);
    if (str == null) {
      return defaultValue;
    }
    try {
      return Double.parseDouble(str);
    }
    catch (NumberFormatException e) {
      throw new IllegalArgumentException("attribute '" + attributeName + "' is not a proper double: " + str);
    }
  }
static double getOptionalDouble(final Element element, final String attributeName, double defaultValue)