RxJava is a Java based extension of ReactiveX. ReactiveX is a project which aims to provide reactive programming concept to various programming languages. Reactive Programming refers to the scenario where program reacts as and when data appears. It is a event based programming concept and events can propagate to registers observers. As per the Reactive, they have combined the best of Observer pattern, Iterator pattern and functional pattern. The Observer pattern done right. ReactiveX is a combination of the best ideas from the Observer pattern, the Iterator pattern, and functional programming.