Composite Pattern

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Composite Pattern

Intent

• The intent of this pattern is to compose objects into tree structures to represent part-whole hierarchies.
• Composite lets clients treat individual objects and compositions of objects uniformly.

Implementation

The figure below shows a UML class diagram for the Composite Pattern:

• Component - Component is the abstraction for leafs and composites. It defines the interface that must be implemented by the objects in the composition. For example a file system resource defines move, copy, rename, and getSize methods for files and folders.
• Leaf - Leafs are objects that have no children. They implement services described by the Component interface. For example a file object implements move, copy, rename, as well as getSize methods which are related to the Component interface.
• Composite - A Composite stores child components in addition to implementing methods defined by the component interface. Composites implement methods defined in the Component interface by delegating to child components. In addition composites provide additional methods for adding, removing, as well as getting components.
• Client - The client manipulates objects in the hierarchy using the component interface.

A client has a reference to a tree data structure and needs to perform operations on all nodes independent of the fact that a node might be a branch or a leaf. The client simply obtains reference to the required node using the component interface, and deals with the node using this interface; it doesn�t matter if the node is a composite or a leaf.

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