RxJava

Starting with version 8, Java began supporting reactivity features as inbuilt capability, yet they were not used widely and didn't become popular among developers. However, some third party implementations of Reactive Programming in Java showed its advantages and it grew in popularity in the Java community.

There is nothing but the set of tools called Reactive Extension (or simply ReactiveX) allowing implementation of Reactive Programming for composing asynchronous and event-based programs using observable sequences. It is a Java VM (Virtual Machine) implementation of Reactive Extension. Initially written on Microsoft platforms, Reactive Extension offers reactive capabilities to various other programming languages, and one of the most popular among them is RxJava for the Java programming language.

It was the first Reactive Extension API specific to the Java platform. RxJava is compatible with older versions of Java and provides a facility to write asynchronous, event-based programs for both Java and Android platforms, which is very convenient. ReactiveX also covers other programming languages with Reactive Extension, like RxJs, Rx.Net, UnixRx, RxScala, RxCloujure, RxCPP, Rx.rb, and RxKotlin, along with other platforms and frameworks like RxCocoa, RxAndroid, and RxNetty.