Using Composite Class

The main advantage of extending the Composite class instead of extending another widget is that using the Composite class will hide all of the methods and properties of the wrapped widget. For example, instead of extending a VerticalPanel where a widget can be added, extending Composite that is initialized with a VerticalPanel will prevent the user of the custom widget from adding to the panel.

http://www.ing.iac.es/gwt-linux-1.4.60/doc/javadoc/com/google/gwt/user/client/ui/Composite.html