Currently, there are three co-existing versions of RxJava: 1.x, 2.x, and 3.0. We will go through some of the major differences later in the section entitled RxJava 1.x, 2.x, 3.0 – which one do I use? and discuss which version you should use.
RxJava 3.0 is a fairly lightweight library and comes in at fewer than 4 megabytes (MBs) in size. This makes it practical for Android and other projects that require a low dependency overhead. RxJava 3.0 has only one dependency, called Reactive Streams ( http://www.reactive-streams.org/), which is a core library (made by the creators of RxJava) that sets a standard for asynchronous stream implementations, one of which is RxJava 3.0.
RxJava 2x is even smaller—closer to 2 MB—and has only one dependency on Reactive Streams too.
It may be used in other libraries beyond RxJava and is a critical effort in the standardization...
