RxJava is a Java VM implementation of ReactiveX (Reactive Extensions): a library for composing asynchronous and event-based programs by using observable sequences.
For more information about ReactiveX, see the Introduction to ReactiveX page.
RxJava tries to be very lightweight. It is implemented as a single JAR that is focused on just the Observable abstraction and related higher-order functions. You could implement a composable Future that is similarly unbiased, but Akka Futures for example come tied in with an Actor library and a lot of other stuff.)
RxJava is meant for a more polyglot environment than just Java/Scala, and it is being designed to respect the idioms of each JVM-based language. (This is something we’re still working on.)
The following external libraries can work with RxJava: