Recap: Composite Pattern

Composite gives single items and groups one interface, so a tree of any depth answers a call the way a single item does.

• Intent. Compose objects into trees to represent part-whole hierarchies, and treat single objects and compositions uniformly.
• Participants. The component interface, the leaf, the composite that holds components, and the client.
• The recursion. A composite calls the operation on each child and combines the results, so depth never appears in the code.
• In the wild. Swing's Container holds components, and the browser's DOM is the same tree.
• The trade-off. Child methods on the component give transparency and run-time refusals; on the composite alone they give safety and a client that must know the node's kind.
• Against Decorator. Both compose recursively; a decorator wraps one object, a composite holds many.