Composite pattern

In computer science, the composite pattern is a partitioning design pattern. Composite allows a group of objects to be treated in the same way as a single instance of an object. The intent of composite is to "compose" objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions uniformly.^[1]

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 (e.g., a shape) designed as a composition of one-or-more similar objects (other kinds of shapes/geometries), 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 can be used when clients should ignore the difference between compositions of objects and individual objects.^[1] 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

Composite pattern in UML.

Composite pattern in LePUS3.

Component

• is the abstraction for all components, including composite ones
• declares the interface for objects in the composition
• implements default behavior for the interface common to all classes, as appropriate
• declares an interface for accessing and managing its child components
• (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

Example

Common Lisp

The following example, written in Common Lisp, and translated directly from the Java example below it, implements a method named print-graphic, which can be used on either an ellipse, or a list whose elements are either lists or ellipses.

(defstruct ellipse) ;; An empty struct.
 
;; For the method definitions, "object" is the variable,
;; and the following word is the type.
 
(defmethod print-graphic ((object null))
  NIL)
 
(defmethod print-graphic ((object cons))
  (print-graphic (first object))
  (print-graphic (rest object)))
 
(defmethod print-graphic ((object ellipse))
  (print 'ELLIPSE))
 
(let* ((ellipse-1 (make-ellipse))
       (ellipse-2 (make-ellipse))
       (ellipse-3 (make-ellipse))
       (ellipse-4 (make-ellipse)))
 
  (print-graphic (cons (list ellipse-1 (list ellipse-2 ellipse-3)) ellipse-4)))

Java

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 algebraic form,

       Graphic = ellipse | GraphicList
       GraphicList = empty | Graphic GraphicList

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

import java.util.List;
import java.util.ArrayList;
 
/** "Component" */
interface Graphic {
 
    //Prints the graphic.
    public void print();
 
}
 
/** "Composite" */
class CompositeGraphic implements Graphic {
 
    //Collection of child graphics.
    private List<Graphic> mChildGraphics = new ArrayList<Graphic>();
 
    //Prints the graphic.
    public void print() {
        for (Graphic graphic : mChildGraphics) {
            graphic.print();
        }
    }
 
    //Adds the graphic to the composition.
    public void add(Graphic graphic) {
        mChildGraphics.add(graphic);
    }
 
    //Removes the graphic from the composition.
    public void remove(Graphic graphic) {
        mChildGraphics.remove(graphic);
    }
 
}
 
/** "Leaf" */
class Ellipse implements Graphic {
 
    //Prints the graphic.
    public void print() {
        System.out.println("Ellipse");
    }
 
}
 
/** Client */
public class Program {
 
    public static void main(String[] args) {
        //Initialize four ellipses
        Ellipse ellipse1 = new Ellipse();
        Ellipse ellipse2 = new Ellipse();
        Ellipse ellipse3 = new Ellipse();
        Ellipse ellipse4 = new Ellipse();
 
        //Initialize three composite graphics
        CompositeGraphic graphic = new CompositeGraphic();
        CompositeGraphic graphic1 = new CompositeGraphic();
        CompositeGraphic graphic2 = new CompositeGraphic();
 
        //Composes the graphics
        graphic1.add(ellipse1);
        graphic1.add(ellipse2);
        graphic1.add(ellipse3);
 
        graphic2.add(ellipse4);
 
        graphic.add(graphic1);
        graphic.add(graphic2);
 
        //Prints the complete graphic (four times the string "Ellipse").
        graphic.print();
    }
}

Python

class Component(object):
    def __init__(self, *args, **kw):
        pass
 
class Leaf(Component):
    def __init__(self, *args, **kw):
        Component.__init__(self, *args, **kw)
 
class Composite(Component):
    def __init__(self, *args, **kw):
        Component.__init__(self, *args, **kw)
        self.children = []
 
    def append_child(self, child):
        self.children.append(child)
 
    def remove_child(self, child):
        self.children.remove(child)
 
c = Composite()
l = Leaf()
c.append_child(l)

See also

• Design Patterns (book) - the book that started it all
• Mixin
• Facade pattern
• Decorator pattern
• Law of Demeter
• Delegation pattern
• Builder pattern
• Abstract factory pattern

External links

• Composite pattern description from the Portland Pattern Repository
• Composite pattern in UML and in LePUS3, a formal modelling language
• Class::Delegation on CPAN
• Chinese Ring Puzzle Applet
• "The End of Inheritance: Automatic Run-time Interface Building for Aggregated Objects" by Paul Baranowski
• PerfectJPattern Open Source Project, Provides componentized implementation of the Composite Pattern in Java
• [1] A persistent Java-based implementation

References

This article's citation style may be unclear. The references used may be made clearer with a different or consistent style of citation, footnoting, or external linking. (September 2009)

Parts of this article originated from the Perl Design Patterns Book