The Observable design pattern is found in many Java APIs and in reactive programming. Here's an introduction to the timeless Observable pattern in Java.

Java Observable Properties This project contains properties which are able to notify registered listeners about changes.

RxJava – Reactive Extensions for the JVM – a library for composing asynchronous and event-based programs using observable sequences for the Java VM. - Blocking Observable Operators · ReactiveX/RxJava ...

Free, hosted Observable notebooks provide an interactive experience and lots of free, open-source Observable JS code you can reuse and learn from. Here's how to get started.