Composite Design Pattern

The Composite design pattern is a wonderful tool for dealing with sets of objects that can encapsulate other objects. Generally speaking, if you can describe a set of object compositions in terms of a tree, it is probably a good candidate for the Composite design pattern.

The idea is actually quite simple. You can have two types of object, a Leaf or primitive object, and a Composite object. The Leaf object does not encapsulate any other objects, but the Composite does.

An interface is provided that treats all the objects, Leaf or Composite, as the same type. Using this general interface, clients pass commands to the objects without respect to whether or not the given concrete object is a Leaf or a Composite. If the concrete object is a Leaf, then the command requested is executed directly. If it is a Composite, then the command is propagated to the children objects. This children, may be Leaf objects or they may be Composite objects themselves. In the latter case, the command is further propagated out through all of the Composite objects until it reaches all of the Leaf objects and thus traverses the entire tree of objects.

This pattern allows you to create very general code where the client of the interface you provide is spared any ugly details about what type of object it is dealing with. The pattern is often used in graphical applications, but is very general and can be applied to a large variety of problem domains.
It is a great pattern and one I will strive to use more frequently in my code.