How can PublishSubject and BehaviorSubject be unsubscribed from?

Subjects are essentially are both Observables and Observers.

An Observable is essentially a thing that has a function that takes an Observer and returns a subscription. So, for example, given simple observable:

    Observable<Integer> observable = Observable.create(new Observable.OnSubscribeFunc<Integer>() {
        @Override
        public Subscription onSubscribe(Observer<? super Integer> observer) {
            observer.onNext(3);
            observer.onNext(2);
            observer.onNext(1);
            observer.onCompleted();

            return Subscriptions.empty();
        }
    });

And here we would subscribe to it, to print out a line for each integer:

    Subscription sub = observable.subscribe(new Action1<Integer>() {
        @Override
        public void call(Integer integer) {
            System.out.println(integer);
        }
    });

You would unsubscribe on the above by calling sub.unsubscribe().

Here is a PublishSubject that does roughly the same thing:

    PublishSubject<Integer> publishSubject = PublishSubject.create();
    Subscription subscription = publishSubject.subscribe(new Action1<Integer>() {
        @Override
        public void call(Integer integer) {
            System.out.println(integer);
        }
    });

    publishSubject.onNext(3);
    publishSubject.onNext(2);
    publishSubject.onNext(1);

You would unsubscribe from it the same way, by calling subscription.unsubscribe().

All Subjects extend Observable which you can subscribe to using any of the multiple subscribe(...) methods. Call to any of the subscribe(...) methods returns a Subscription.

Subscription subscription = anySubject.subscribe(...);

Use this subscription instance's unsubscribe() method when you want to stop listening to the events from the Subject.

subscription.unsubscribe();

A Subject is an Observable and an Observer at the same time, it can be unsubscribed from just like normal observables. What makes subject special is that it is sort of bridge between observables and observers. It can pass through the items it observes by reemitting them, and it can also emit new items. Subjects are to observables as promises are to futures.

Here is a brief description of the subjects family:

AsyncSubject: only emits the last value of the source Observable

BehaviorSubject: emits the most recently emitted item and all the subsequent items of the source Observable when a observer subscribe to it.

PublishSubject: emits all the subsequent items of the source Observable at the time of the subscription.

ReplaySubject: emits all the items of the source Observable, regardless of when the subscriber subscribes.

the official doc comes with some nice marble diagrams which makes it more easier to understand