This is the first time we see `subscribe` being used, so it is worth paying attention to how it was used. In this case, we used the overload which expects one [Function](http://reactivex.io/RxJava/javadoc/rx/functions/Function.html) for the case of onNext. That function takes an argument of type `Integer` and returns nothing. Functions without a return type are also called actions. We can provide that function in different ways:

* we can supply an instance of `Action1<Integer>`,

* implicitly create one using a [lambda expression](http://en.wikipedia.org/wiki/Anonymous_function#Java) or

* pass a reference to an existing method that fits the signature.

In this case, `System.out::println` has an overload that accepts `Object`, so we passed a reference to it. `subscribe` will call `println` with the arriving values as the argument.