The Composite pattern supports these characteristics by defining a class structure that supports extensibility. This structure is composed of a component, leaf, and Composite class.

The base component provides the core model, defining standard methods or variables to be used by all the objects in the Composite.

Leaf classes support terminal behavior. That is, they represent parts of the Composite, but they cannot contain other components.

Composite or branch classes can have other components added to them, permitting extensibility of the Composite structure.

A drawing created using graphical editing tools is a common example of the Composite pattern in action. With a drawing, a number of elementary shapes can be associated and treated as a whole; you also can define drawings so that they contain other drawings, or a mixture of drawings and shapes.

Additional possibilities for Composite pattern use include applications with organizational charts, task breakdowns, schedules, and outlining features. Applications that support grouping are also good candidates for the Composite pattern, provided that the grouping action can be performed recursively and that the final product, as well as its component elements, have the same functional behavior.

Implementation

The basic Composite class diagram is shown in Figure 3.4.

Figure 3.4. Composite class diagram

The Composite has three elements:

Composite supports a dynamic group of Components so it has methods to add and remove Component instances from its collection. The methods defined in the Component are implemented to execute the behavior specific for this type of Composite and to call the same method on each of its nodes. These Composite classes are also called branch or container classes.

Leaf – The class that implements the Component interface and that provides an implementation for each of the Component ’s methods. The distinction between a Leaf class and a Composite class is that the Leaf contains no references to other Components. The Leaf classes represent the lowest levels of the containment structure.

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