ReplaySubject takes an entirely different approach compared to Publish and BehaviorSubject instances . It records all the values that it received and, upon a new subscription to itself, it will replay all the captured values to the new subscription.

Take a look here:

Subject<String> subject = ReplaySubject.create();

Observable.interval(0, 1, TimeUnit.SECONDS)
        .map(Objects::toString)
        .subscribe(subject);

Thread.sleep(3100);

subject.subscribe(v -> log(v));

In this block, we have created a ReplaySubject with the following:

Subject<String> subject = ReplaySubject.create();

Also, we have made it receive values from Observable.interval():

Observable.interval(0, 1, TimeUnit.SECONDS)
        .map(Objects::toString)
        .subscribe(subject);

Next, we wait for three seconds with this:

Thread.sleep(3100);

We subscribe to the created subject with the following:

subject.subscribe(v -> log(v));

Upon subscription, it will immediately print all the three items that were emitted until then:

main:0
main:1
main:2
main:3
RxComputationThreadPool-1:4
RxComputationThreadPool-1:5
RxComputationThreadPool-1:6

Afterward, it will just continue printing the items received from the original Observer. ReplaySubject will be useful when there is a need to replay the entire history of received items. For example, it can come in handy if anyone is working with the Event Sourcing or a command pattern.