Hands-On Reactive Programming in Spring 5

Observable

RxJava 2's Observable type provides almost the same semantics as in RxJava 1.x, however, it no longer accepts null values. Also, Observable does not support backpressure and does not implement the Publisher interface. So, it is not directly compatible with the Reactive Streams specification. Consequently, be careful when using it for streams with many elements (more than a couple of thousand). On the other hand, the Observable type has less overhead than the Flowable type. It has the toFlowable method that transforms the stream to Flowable by applying the backpressure strategy of the user's choice.