This subject does not have a public constructor by design; a new empty instance of this BehaviorSubject can be created via the create() method and a new non-empty instance can be created via createDefault(Object) (named as such to avoid overload resolution conflict with Observable.create that creates an Observable, not a BehaviorSubject).

Since a Subject is conceptionally derived from the Processor type in the Reactive Streams specification, nulls are not allowed (Rule 2.13) as default initial values in createDefault(Object) or 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 BehaviorSubject is an io.reactivex.rxjava3.core.Observable, it does not support backpressure.

When this BehaviorSubject is terminated via onError(Throwable) or onComplete(), the last observed item (if any) is cleared and late io.reactivex.rxjava3.core.Observers only receive the respective terminal event.

The BehaviorSubject does not support clearing its cached value (to appear empty again), however, the effect can be achieved by using a special item and making sure Observers subscribe through a filter whose predicate filters out this special item:

BehaviorSubject<Integer> subject = BehaviorSubject.create();

final Integer EMPTY = Integer.MIN_VALUE;

Observable<Integer> observable = subject.filter(v -> v != EMPTY);

TestObserver<Integer> to1 = observable.test();

subject.onNext(1);
// this will "clear" the cache
subject.onNext(EMPTY);

TestObserver<Integer> to2 = observable.test();

subject.onNext(2);
subject.onComplete();

// to1 received both non-empty items
to1.assertResult(1, 2);

// to2 received only 2 even though the current item was EMPTY
// when it got subscribed
to2.assertResult(2);

// Observers coming after the subject was terminated receive
// no items and only the onComplete event in this case.
observable.test().assertResult();

Even though BehaviorSubject 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 BehaviorSubject 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 BehaviorSubject supports the standard state-peeking methods hasComplete(), hasThrowable(), getThrowable() and hasObservers() as well as means to read the latest observed value in a non-blocking and thread-safe manner via hasValue() or getValue().

Scheduler:

BehaviorSubject 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 BehaviorSubject 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 BehaviorSubject when the onError() was called, the global error handler is not invoked.

Example usage:

 // observer will receive all 4 events (including "default").
 BehaviorSubject<Object> subject = BehaviorSubject.createDefault("default");
 subject.subscribe(observer);
 subject.onNext("one");
 subject.onNext("two");
 subject.onNext("three");

 // observer will receive the "one", "two" and "three" events, but not "zero"
 BehaviorSubject<Object> subject = BehaviorSubject.create();
 subject.onNext("zero");
 subject.onNext("one");
 subject.subscribe(observer);
 subject.onNext("two");
 subject.onNext("three");

 // observer will receive only onComplete
 BehaviorSubject<Object> subject = BehaviorSubject.create();
 subject.onNext("zero");
 subject.onNext("one");
 subject.onComplete();
 subject.subscribe(observer);

 // observer will receive only onError
 BehaviorSubject<Object> subject = BehaviorSubject.create();
 subject.onNext("zero");
 subject.onNext("one");
 subject.onError(new RuntimeException("error"));
 subject.subscribe(observer);