我有一個返回 ResultSet 的 Observable 的方法：public static Observable<ResultSet> queryAllAsObservable(Session session, String query, Object... partitionKeys) {    List<ResultSetFuture> futures = sendQueries(session, query, partitionKeys);    Scheduler scheduler = Schedulers.io();    List<Observable<ResultSet>> observables = Lists.transform(futures, (ResultSetFuture future) -> Observable.fromFuture(future, scheduler));    return Observable.merge(observables);}現在我需要創建此方法的一個版本，它返回一個 Observable of Row。這是我試過的：public static Observable<Row> queryAllAsRowObservable(Session session, String query, Object... partitionKeys) {    List<ResultSetFuture> futures = sendQueries(session, query, partitionKeys);    Scheduler scheduler = Schedulers.io();    List<Observable<ResultSet>> observables = Lists.transform(futures, (ResultSetFuture future) -> Observable.fromFuture(future, scheduler));    return Observable.merge(observables).flatMapIterable(item -> item.one());}但是item -> item.one()被錯誤標記：no instance(s) of type variable(s) U exists so that Row conforms to Iterable<? extends U>