gistfile1.java

//usecase
api.user() //Observable<User>
    .lift(new SerialOperator<>(
            new Action2<Subscriber<? super List<Feed>>, User>() {
                @Override
                public void call(Subscriber<? super List<Feed>> subscriber, User user) {
                    api.userFeed(user).subscribe(subscriber);
                }
            }))
    .lift(new SerialOperator<>(
            new Action2<Subscriber<? super String>, List<Feed>>() {
                @Override
                public void call(final Subscriber<? super String> subscriber, List<Feed> feeds) {
                    Observable.from(feeds)
                            .reduce(null, new Func2<String, Feed, String>() {
                                @Override
                                public String call(String s, Feed feed) {
                                    return (s == null ? "" : s + ",") + feed.summarize();
                                }
                            })
                            .subscribe(new Action1<String>() {
                                @Override
                                public void call(String s) {
                                    subscriber.onNext(s);
                                    subscriber.onCompleted();
                                }
                            });
                }
            }))
    .subscribe(new Action1<String>() {
        @Override
        public void call(String summary) {
            Log.d("moge", "summary=" + summary);
        }
    });

SerialOperator.java

public class SerialOperator<R, T> implements Observable.Operator<R, T> {

  Action2<Subscriber<? super R>, T> then;

  T next;

  SerialOperator(Action2<Subscriber<? super R>, T> then) {
      this.then = then;
  }

  @Override
  public Subscriber<? super T> call(final Subscriber<? super R> subscriber) {
      return new Subscriber<T>() {
          @Override
          public void onCompleted() {
              then.call(subscriber, next);
          }

          @Override
          public void onError(Throwable throwable) {
              subscriber.onError(throwable);
          }

          @Override
          public void onNext(T t) {
              next = t;
          }
      };
  }
}