Composite

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.