HandlerCall.java

Observable
        .create(s -> {
            // do some crazy stuff in "IO_HANDLER"
            s.onNext(new Object());
            s.onCompleted();
        })
        .subscribeOn(HandlerScheduler.from(new CustomHandler("IO_HANDLER")))
        .observeOn(AndroidSchedulers.mainThread()) // don't forget to switch back to MainThread.
        .subscribe();