CS3340:  Intro OOP and Design
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Composite Pattern

The composite pattern describes that a group of objects are to be treated in the same way as a single instance of an object. The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.

 

composite pattern class hierarchy

 

To be able to treat individual objects and compositions in the same manner. You can manipulate a single instance of an object the same as you would a group of them.

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.

Motivation

When you have 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.

Solution --- How it works

Split into Component, Composition (and optional) Leaf classes. Each class has the same operation

Component
  • is the abstraction (an interface) 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

 

exampel

example

 
composite pattern general

 

 

Example in Java

Has a graphic class, which can be either an ellipse or a composition of several graphics. Every graphic can be printed (shared operation).

 

       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> childGraphics = new ArrayList<Graphic>();

         //Prints the graphic.
      public void print() {
         for (Graphic graphic : childGraphics) {
              graphic.print();          }
      }


         //Adds the graphic to the composition.
      public void add(Graphic graphic) { 
         childGraphics.add(graphic);
      }


         //Removes the graphic from the composition.
      public void remove(Graphic graphic) {
          childGraphics.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();
      }  
}

 
© Lynne Grewe