private function listHashTags(array $tweet)
    {
        $list = [];

        if (array_key_exists('entities', $tweet) and array_key_exists('hashtags', $tweet['entities']))
        {
            $hashTags = $tweet['entities']['hashtags'];

            foreach ($hashTags as $hashTag)
            {
                if (array_key_exists('text', $hashTag))
                {
                    $list[] = $hashTag['text'];
                }
            }
        }

        return $list;
    }