protected function requirementCheck()
    {
        if(count($this->required) > 0)
        {
            // non-exists list
            $nonExist	= array();

            foreach($this->required as $k)
            {
                if(!isset($this->data[$k]))
                {
                    $nonExist[]	= $k;
                }
            }

            if(count($nonExist) > 0)
                return $nonExist;
        }

        return false;
    }