不存在类型变量U的实例，因此Row符合Iterable <? extends U>

Question

I have a method that returns an Observable of ResultSet:

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);
}

Now I need to create a version of this method that returns an Observable of Row instead. This is what I've tried:

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());
}

But item -> item.one() is marked by an error:

no instance(s) of type variable(s) U exists so that Row conforms to Iterable<? extends U>

Answer 1

flatMapIterable has the wrong type for what you're trying to do at the point you're trying to do it. You could use map instead of flatMapIterable , or reconsider your use of merge .