one

FutureTask<Integer> f = new FutureTask<Integer>(() -> {
    Thread.sleep(2000);
    return 21;
});
new Thread(f).start();

Observable<Integer> values = Observable.from(f);

Subscription subscription = values.subscribe(
    v -> System.out.println("Received: " + v),
    e -> System.out.println("Error: " + e),
    () -> System.out.println("Completed")
);