RxJava as a reactive framework

For some time, there was a standard library for reactive programming on Java platforms—namely RxJava 1.x (see https://github.com/ReactiveX/RxJava for more details). That library paved the way for reactive programming as we know it in the Java world today. At the moment, it is not the only library of this kind; we also have Akka Streams and Project Reactor. The latter is covered in detail in Chapter 4, Project Reactor - the Foundation for Reactive Apps. So, currently, we have a few options from which we may choose. Furthermore, RxJava itself has changed a lot with the release of version 2.x. However, to understand the most basic concepts of reactive programming and the reasoning behind them, we will be focusing on the most fundamental part of the RxJava only, on the API, which has...