public function getKey(): string
    {
        if ($this->key === null) {
            $this->key = md5($this->getContent());
        }
        return $this->key;
    }
public function getKey(): string
    {
        if ($this->key === null) {
            $this->key = md5($this->getContent());
        }
        return $this->key;
    }
public function getKey ( ) : string { if ( $ this -> key === null ) { $ this -> key = md5 ( $ this -> getContent ( ) ) ; } return $ this -> key ; }
Get the entity key, 304 response needs a proper key value
@return string
Get the entity key 304 response needs a proper key value