ReplaySubject is similar to PublishSubject followed by a cache() operator. It immediately captures emissions regardless of the presence of downstream Observers and optimizes the caching to occur inside the Subject itself:

import io.reactivex.subjects.ReplaySubject;
import io.reactivex.subjects.Subject;

public class Launcher {
    public static void main(String[] args) {

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

        subject.subscribe(s -> System.out.println("Observer 1: " + s));

        subject.onNext("Alpha");
        subject.onNext("Beta");
        subject.onNext("Gamma");
        subject.onComplete();

        subject.subscribe(s -> System.out.println("Observer 2: " + s));
    }
}

The output is as follows:

Observer 1: Alpha
Observer 1: Beta
Observer 1: Gamma
Observer 2: Alpha
Observer 2: Beta
Observer 2: Gamma

Obviously, just like using a parameterless replay() or a cache() operator, you need to be wary of using this with a large volume of emissions or infinite sources because it will cache them all and take up memory.