thread1

final BehaviorSubject<Integer> subject = BehaviorSubject.create();
subject.subscribe(i -> {
    System.out.println("Received " + i + " on " + Thread.currentThread().getId());
});

int[] i = {1}; // naughty side-effects for examples only ;)
Runnable r = () -> {
    synchronized(i) {
        System.out.println("onNext(" + i[0] + ") on " + Thread.currentThread().getId());
        subject.onNext(i[0]++);
    }
};

r.run(); // Execute on main thread
new Thread(r).start();
new Thread(r).start();