Simply Patrick Hopes can always go up, tears can only come down.

private static <T> Subject<T, List<T>> createAggregateSubject() {
    final List<Subscriber<? super List<T>>> observers = new LinkedList<>();
    final Observable.OnSubscribe<List<T>> onSubscribe = o -> {
        o.onStart();
        observers.add(o);
    };
    return new Subject<T, List<T>>(onSubscribe) {
        private List<T> list = new LinkedList<>();

        @Override
        public boolean hasObservers() {
            return !observers.isEmpty();
        }

        @Override
        public void onCompleted() {
            Iterables.removeIf(observers, Subscriber::isUnsubscribed);

            for (Subscriber<? super List<T>> o: observers) {
                o.onNext(list);
            }
            list.clear();
        }

        @Override
        public void onError(Throwable e) {
            for (Subscriber<? super List<T>> o: observers) {
                o.onError(e);
            }
        }

        @Override
        public void onNext(T t) {
            list.add(t);
        }
    };
}