In addition, operators requiring a predicate no longer use `Func1<T, Boolean>` but have a separate, primitive-returning type of `Predicate<T>` (allows better inlining due to no autoboxing).

The Reactive-Streams specification has its own Subscriber as an interface. This interface is lightweight and combines request management with cancellation into a single interface `org.reactivestreams.Subscription` instead of having `rx.Producer` and `rx.Subscription` separately. This allows creating stream consumers with less internal state than the quite heavy `rx.Subscriber` of 1.x.

.doFinally(() -> System.out.println("Finally"))

.take(2) // cancels the above after 2 elements

.subscribe(System.out::println);