Composite pattern

Motivation

When dealing with Tree-structured data, programmers often have to discriminate between a leaf-node and a branch. This makes code more complex, and therefore, error prone. The solution is an interface that allows treating complex and primitive objects uniformly. In object-oriented programming, a composite is an object designed as a composition of one-or-more similar objects, all exhibiting similar functionality. This is known as a "has-a" relationship between objects. The key concept is that you can manipulate a single instance of the object just as you would manipulate a group of them. The operations you can perform on all the composite objects often have a least common denominator relationship. For example, if defining a system to portray grouped shapes on a screen, it would be useful to define resizing a group of shapes to have the same effect (in some sense) as resizing a single shape.

When to use

Composite should be used when clients ignore the difference between compositions of objects and individual objects. If programmers find that they are using multiple objects in the same way, and often have nearly identical code to handle each of them, then composite is a good choice; it is less complex in this situation to treat primitives and composites as homogeneous.

Structure

Component

is the abstraction for all components, including composite ones

declares the interface for objects in the composition

(optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate

Leaf

represents leaf objects in the composition

implements all Component methods

Composite

represents a composite Component (component having children)

implements methods to manipulate children

implements all Component methods, generally by delegating them to its children

Variation

As it is described in Design Patterns, the pattern also involves including the child-manipulation methods in the main Component interface, not just the Composite subclass. More recent descriptions sometimes omit these methods.

Example

The following example, written in Java, implements a graphic class, which can be either an ellipse or a composition of several graphics. Every graphic can be printed. In Backus-Naur form,

Graphic ::= ellipse | GraphicList GraphicList ::= empty | Graphic GraphicList

It could be extended to implement several other shapes (rectangle, etc.) and methods (translate, etc.).

C#

The following example, written in C#.