The Extension Interface design pattern allows multiple interfaces to be exported by a component, to prevent bloating of interfaces and breaking of client code when developers extend or modify the functionality of the component.
Consider a telecommunication management network (TMN) [ITUT92] framework that can be customized to monitor and control remote network elements such as IP routers and ATM switches. Each type of network element is modeled as a multi-part framework component in accordance with the Model-View-Controller pattern [POSA1]. A view and a controller are located on a management application console. The view renders the current state of a network element on the console and the controller allows network administrators to manage the network element.
A model resides on the network element and communicates with the view and controller to receive and process commands, such as commands to send state information about the network element to the management application console. All components in the TMN framework are organized in a hierarchy. The UniversalComponent interface shown in the following figure provides the common functionality needed by every component, such as displaying key properties of a network element and accessing its neighbors.
In theory, this design might be appropriate if the UniversalComponent interface shown above is never ...