This subject does not have a public constructor by design; a new empty instance of this PublishSubject can be created via the create() method.

Since a Subject is conceptionally derived from the Processor type in the Reactive Streams specification, nulls are not allowed (Rule 2.13) as parameters to onNext(Object) and onError(Throwable). Such calls will result in a NullPointerException being thrown and the subject's state is not changed.

Since a PublishSubject is an io.reactivex.rxjava3.core.Observable, it does not support backpressure.

When this PublishSubject is terminated via onError(Throwable) or onComplete(), late io.reactivex.rxjava3.core.Observers only receive the respective terminal event.

Unlike a BehaviorSubject, a PublishSubject doesn't retain/cache items, therefore, a new Observer won't receive any past items.

Even though PublishSubject implements the Observer interface, calling onSubscribe is not required (Rule 2.12) if the subject is used as a standalone source. However, calling onSubscribe after the PublishSubject reached its terminal state will result in the given Disposable being disposed immediately.

Calling onNext(Object), onError(Throwable) and onComplete() is required to be serialized (called from the same thread or called non-overlappingly from different threads through external means of serialization). The toSerialized() method available to all Subjects provides such serialization and also protects against reentrance (i.e., when a downstream Observer consuming this subject also wants to call onNext(Object) on this subject recursively).

This PublishSubject supports the standard state-peeking methods hasComplete(), hasThrowable(), getThrowable() and hasObservers().

Scheduler:

PublishSubject does not operate by default on a particular io.reactivex.rxjava3.core.Scheduler and the Observers get notified on the thread the respective onXXX methods were invoked.

Error handling:

When the onError(Throwable) is called, the PublishSubject enters into a terminal state and emits the same Throwable instance to the last set of Observers. During this emission, if one or more Observers dispose their respective Disposables, the Throwable is delivered to the global error handler via io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable) (multiple times if multiple Observers cancel at once). If there were no Observers subscribed to this PublishSubject when the onError() was called, the global error handler is not invoked.

Example usage:

 PublishSubject<Object> subject = PublishSubject.create();
 // observer1 will receive all onNext and onComplete events
 subject.subscribe(observer1);
 subject.onNext("one");
 subject.onNext("two");
 // observer2 will only receive "three" and onComplete
 subject.subscribe(observer2);
 subject.onNext("three");
 subject.onComplete();

 // late Observers only receive the terminal event
 subject.test().assertEmpty();