Comment by [email protected]: In the context of the Widget class, Bob's point is even stronger -- failure to properly implement certain methods, which have very subtle requirements, will not only lead to failure, but will do so in a way that is insidious and difficult to detect. Things such as (often very large) memory leaks, failure to attach/detach later for arcane reasons, etc. These are unfortunately constraints that are imposed upon us by the mess that is the web browser.
For more information: http://code.google.com/p/google-web-toolkit/wiki/WhyWidgetIsAClass -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
