protected function addDefaultClass()
    {
        $formDefaultClass = $this->formObject
            ->getConfiguration()
            ->getSettings()
            ->getDefaultClass();

        $class = $this->tag->getAttribute('class');

        if (false === empty($formDefaultClass)) {
            $class = (!empty($class) ? $class . ' ' : '') . $formDefaultClass;
            $this->tag->addAttribute('class', $class);
        }
    }