Caching and invalidating replayable observables

RXAuthenticator.java

    Stack<AuthenticationRequestHolder> cachedRequest = new Stack<>();

    public Observable<AuthenticationResult> login(final AuthenticationRequest request) {
        
        AuthenticationRequestHolder cached = cachedRequest.isEmpty()?null:cachedRequest.pop();
        if (cached != null && cached.request.equals(request)) {
            cachedRequest.push(cached);
            return cached.result;
        }
        
        // make new request
        Observable<AuthenticationResult> result = Observable.create(new Observable.OnSubscribe<AuthenticationResult>() {
            @Override
            public void call(Subscriber<? super AuthenticationResult> observer) {
                try {
                    boolean ok = someAuthAPIRequest();
                    if (ok) {
                        observer.onNext(new AuthenticationResult(true, Collections.EMPTY_LIST));
                    } else {
                        observer.onNext(new AuthenticationResult(false, Collections.singletonList("Invalid username or password")));
                    }
                    observer.onCompleted();
                } catch (Exception e) {
                    observer.onError(e);
                }
            }
        }).doOnEach(new Action1<Notification<? super AuthenticationResult>>() {
            @Override
            public void call(Notification<? super AuthenticationService.AuthenticationResult> notification) {
                // determine if the result can remain cached to serve future requests
                switch (notification.getKind()) {
                    case OnNext:
                        if(!((AuthenticationResult)notification.getValue()).authenticated){
                            cachedRequest.pop();
                        }
                        break;
                    case OnError:
                        cachedRequest.pop();
                        break;
                }
            }
        }).replay().autoConnect();

        cachedRequest.push(new AuthenticationRequestHolder(request, result));

        return result;
    }