Adapt
You can make RxJava3 and Reactor 3 types interact without a single external library.
In the first two examples we will adapt from Flux to Flowable, which implements Publisher,
and vice-versa.
This is straightforward as both libraries provide a factory method to do that conversion from
any Publisher. The checker below runs the two opposite conversions in one go:
The next two examples are a little trickier: we need to adapt between Flux and Observable,
but the later doesn't implement Publisher.
In the first case, you can transform any publisher to Observable.
In the second case, you have to first transform the Observable into a Flowable, which
forces you to define a strategy to deal with backpressure (RxJava 3 Observable doesn't
support backpressure).
Next, let's try to transform a Mono to a RxJava Single, and vice-versa. You can
simply call the firstOrError method from Observable. For the other way around, you'll
once again need to transform the Single into a Flowable first.
Finally, you can easily transform a Mono to a Java 8 CompletableFuture and vice-versa.
Notice how these conversion methods all begin with from (when converting an external type
to a Reactor one) and to (when converting a Reactor type to an external one).
