private function __validateString($string, $minLength = 1, $maxLength = 0)
    {
        if (!is_string($string)) {
            $this->error = 'parameter is not string.';
            return false;
        } else if ($maxLength > 0 && strlen($string) > $maxLength) {
            $this->error = 'parameter is too long. string:' . $string;
            return false;
        } else if (strlen($string) < $minLength) {
            $this->error = 'parameter is too short. string:' . $string;
            return false;
        }
        return true;
    }