Structure:

The Key to the composite pattern is an abstract class that represents both primitive elements (concepts, text, and graphics) and their containers (subtopics). Following figure shows the general structure of the composite pattern:

Participants and Collaborations:

The abstract class in the figure is responsible for providing a common interface to all objects in the composition, both simple (conceps, text, and graphics) and complex (topics and subtopics). The add and remove functions are examples of functions from such a common interface. It also defines the interface for accessing the child components of a complex object (a topic or a subtopic), e.g. the functions Operations and Getchild. The concrete classes Leaf (corresponding to simple contents of a lesson in the example) and composite (corresponding to topics and subtopics) are derived from component. The Leaf class represents leaf objects in the composition and has no children.