this method add a request to a queue but what is done in second line of code ? :      public <T> void addToRequestQueue(Request<T> req, String tag) {
        req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
        getRequestQueue().add(req);
    }

what is done in this second line i cant understand  what is this (tag) ?TAG :tag ...I mean what is this code done 