Composite

Composite pattern

The Composite pattern is useful whenever one has a class which may contain instances of itself. Common computer science examples are nodes in a tree structure, or shapes in a graphical model. More concrete physical world examples include regions (which can potentially contain many smaller regions), schools of fish (where those fish may swim, eat, etc in a collective fashion), and divisions/teams in a workplace.

The key feature of this pattern is a subclass which both extends the superclass, and contains (though an aggregation or composition relationship) a number of instances of the superclass.

The advantage gained from this structure is that we can transparently use a Composite Instance just as with a Primitive instance. shape.draw(), for example could draw any shape, from a primitive to a highly complex diagram or model.

Features

Classes

• Interface (Abstract class) that represents Component.
• Concrete Composite class that implements Component.
• Concrete leaf class that implements Component.
• Client class.

Attribute

• Composite maintains a collection of Components. E.g, Collection<Component>

Methods

• Component has abstract methods to add and remove an Component to and from.
• Composite has concrete method to add an instance of Component that shared by Composites and leaves on the collection.
• Composite has concrete method to remove an instance of Component that shared by Composites and leaves from the collection.

Relationships

• Composite contains both Composite and leaf as Component.
• All Composites and leaves are seen as Component by Client.

Recognising the pattern

Classes: Composite, Component, multiple Leaves

• Inheritance hierarchy that has a Component interface
• Component interface has declarations for add() and remove() methods for adding/removing Components.
• Composite class has private collection of Components.