Composite pattern

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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]

Contents

[edit] 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 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.

[edit] 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.

Compose means a special thing: it refers to building objects using delegation. Delegation-composition hangs onto constituent parts-using references. By contrast, mixins inherit from each part. Mixins prevent returning a whole object in response to requests for information, and they prevent having more than one of any given part.

The composite pattern is an object-oriented pendant to algebraic data types.[citation needed]

[edit] Structure

Composite pattern in UML.
Composite pattern in UML.
Composite pattern in LePUS3.
Composite pattern in LePUS3.

[edit] 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

[edit] Leaf

  • represents leaf objects in the composition
  • implements all Component methods

[edit] Composite

  • represents a composite Component (component having children)
  • implements methods to manipulate children
  • implements all Component methods, generally by delegating them to its children

[edit] 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 algebraic form,

       Graphic = ellipse | GraphicList
       GraphicList = empty | ellipse GraphicList

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

List = empty_list | atom List | List List

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();
    }
}

C++ Example

#include <iostream>
#include <vector>
#include <string>
 
using std::cout;
using std::vector;
using std::string;
 
class Component
{
  public:
   virtual void list() const = 0;
   virtual ~Component(){};
};
 
class Leaf : public Component 
{
  public:
   explicit Leaf(int val) : value_(val) 
   {
   }
   void list() const 
   {
      cout << "   " << value_ << "\n";
   }
  private:
   int value_;
};
 
class Composite : public Component 
{
  public:
   explicit Composite(string id) : id_(id) 
   {
   }
   void add(Component *obj)
   {
      table_.push_back(obj);
   }
   void list() const
   {
      cout << id_ << ":" << "\n";
      for (vector<Component*>::const_iterator it = table_.begin(); 
       it != table_.end(); ++it)
      {
         (*it)->list();
      }
   }
  private:
   vector <Component*> table_;
   string id_;
};
 
int main()
{
   Leaf num0(0);
   Leaf num1(1);
   Leaf num2(2);
   Leaf num3(3);
   Leaf num4(4);
   Composite container1("Container 1");
   Composite container2("Container 2");
 
   container1.add(&num0);
   container1.add(&num1);
 
   container2.add(&num2);
   container2.add(&num3);
   container2.add(&num4);
 
   container1.add(&container2);
   container1.list();
   return 0;
}

[edit] See also

[edit] External links

[edit] References

  1. ^ a b Gamma, Erich; Richard Helm, Ralph Johnson, John M. Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley, 395. ISBN 0201633612. 
 The references in this article would be clearer with a different or consistent style of citation, footnoting, or external linking.

Parts of this article originated from the Perl Design Patterns Book

v • d • e
Design patterns in the book Design Patterns
Creational
Structural
Adapter · Bridge · Composite · Decorator · Façade · Flyweight · Proxy
Behavioral