Composite Pattern

The Composite Pattern allows you to compose objects into tree structures to represent part-whole hierarchies. Composite lets client treat individual objects and composition of objects uniformly.

Bates and Sierra — Head First Design Patterns

Class Diagram

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Client uses the Component to manipulate objects.

Component defines an interface both for the composite and for the leaf which are element of the collection. It might implement a default behavior for methods.

Leaf has no children and inherits methods and override what make sense for the class itself.

Composite has children, the Components, which can be Component or Leaf type and inherits methods and override what make sense for the class itself.

Key Points

• New Approach. Not necessarily related to iterators.
• Trees. Build object structures in the form of trees containing both nodes and leaves.
  • Same operations are applied over both composite and leaves.
  • Ignore the differences between nodes and leaves.
• Part-whole Hierarchy. Node with children and leaves are in the same structure.
  • Tree of objects made of parts, nodes and leaves, but threaten as a whole.