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Six: 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.^[2] 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.

[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.

Structure

Composite pattern in UML.

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
  
  
  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();
      }
  }

posted on 2012-11-01 17:10 Chan Chen 阅读(360) 评论(0)  编辑  收藏 所属分类: Design Pattern