I am working these days on a dedicated course for learning how to use the Vaadin web framework. I have recently completed to develop detailed slides that explain how to install the Vaadin plugin for the Eclipse IDE and how to develop Vaadin based rich internet web applications using the Eclipse IDE. My course is available for free personal usage at www.abelski.com. The following video clips complement these slides.
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.