SDSU CS 635 Advanced Object-Oriented Design & Programming
Spring Semester, 2002
Composite & Visitor
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San Diego State University -- This page last updated 12-Feb-02

Contents of Doc 5, Composite & Visitor


References
Design Patterns: Elements of Resuable Object-Oriented Software, Gamma, Helm, Johnson, Vlissides, Addison-Wesley, 1995, pp. 163-174, 331-344

The Design Patterns Smalltalk Companion, Alpert, Brown, Woolf, Addison-Wesley, 1995, pp. 371-386


Doc 5, Composite & Visitor Slide # 2

Composite

Example - Motivation

GUI Windows and GUI elements


How does the window hold and deal with the different items it has to manage?


Widgets are different that WidgetContainers


Doc 5, Composite & Visitor Slide # 3
Bad News Implementation

class Window
   {
   Buttons[] myButtons;
   Menus[] myMenus;
   TextAreas[] myTextAreas;
   WidgetContainer[] myContainers;
   
   public void update()
      {
      if ( myButtons != null )
         for ( int k = 0; k < myButtons.length(); k++ )
            myButtons[k].refresh();
      if ( myMenus != null )
         for ( int k = 0; k < myMenus.length(); k++ )
            myMenus[k].display();
      if ( myTextAreas != null )
         for ( int k = 0; k < myButtons.length(); k++ )
            myTextAreas[k].refresh();
      if ( myContainers != null )
         for ( int k = 0; k < myContainers.length(); k++ )
            myContainers[k].updateElements();
      etc.
      }
   public void fooOperation()
      {
      if ( blah ) etc.
      }
   }


Doc 5, Composite & Visitor Slide # 4
A Better Idea - Program to an interface


class Window
   {
   GUIWidgets[] myWidgets;
   WidgetContainer[] myContainers;
   
   public void update()
      {
      if ( myWidgets != null )
         for ( int k = 0; k < myWidgets.length(); k++ )
            myWidgets[k].update();
      if ( myContainers != null )
         for ( int k = 0; k < myContainers.length(); k++ )
            myContainers[k].updateElements();
      etc.
      }
   }


Doc 5, Composite & Visitor Slide # 5

The Composite Pattern



Component implements default behavior for widgets when possible

Button, Menu, etc overrides Component methods when needed

WidgetContainer will have to overrides all widgetOperations

class WidgetContainer
   {
   Component[] myComponents;
   
   public void update()
      {
      if ( myComponents != null )
         for ( int k = 0; k < myComponents.length(); k++ )
            myComponents[k].update();
      }
   }

Doc 5, Composite & Visitor Slide # 6

Issue: WidgetContainer Operations


WidgetContainer operations tend to relate to adding, deleting and managing widgets

Should the WidgetContainer operations be declared in Component?

Pro - Transparency

Declaring them in the Component gives all subclasses the same interface

All subclasses can be treated alike. (?)


Con - Safety

Declaring them in WidgetContainer is safer

Adding or removing widgets to non-WidgetContainers is an error

What should be the proper response to adding a TextArea to a button? Throw an exception?

One out is to check the type of the object before using a WidgetContainer operation

Doc 5, Composite & Visitor Slide # 7

Explicit Parent References


Aid in traversing the structure

class WidgetContainer
   {
   Component[] myComponents;
   
   public void update()
      {
      if ( myComponents != null )
         for ( int k = 0; k < myComponents.length(); k++ )
            myComponents[k].update();
      }
   public add( Component aComponent )
      {
      myComponents.append( aComponent );
      aComponent.setParent( this );
      }
   }
   
class Button extends Component
   {
   private Component parent;
   public void setParent( Component myParent)
      {
      parent = myParent;
      }
   
   etc.
   }

Doc 5, Composite & Visitor Slide # 8

More Issues


Should Component implement a list of Components?

The button etc. will have a useless data member

Child ordering is important in some cases

Who should delete components?

If there is no garbage collection Container is best bet


Doc 5, Composite & Visitor Slide # 9

Applicability


Use Composite pattern when you want




Doc 5, Composite & Visitor Slide # 10
Java Use of Composite - AWT Widgets



Doc 5, Composite & Visitor Slide # 11
Specialized Java Containers



Doc 5, Composite & Visitor Slide # 12

Visitor

Example - Trees


What about preorder visit, postorder visit?
What about an HTML print?
What about printing a 2D representation?
What if this was an expression tree - evaluation?
What if this was a binary search tree - adding, deleting
What about balancing the binary search tree -
AVL
Red-Black



Doc 5, Composite & Visitor Slide # 13
Solution 1

Build all the operations into the tree structure

This works in many situations

If you need to add a lot of operations the classes can become cluttered

class BinaryTree {
   public String htmlPrint() { blah}
   public String 2DPrint() { blah}
   public String preorderTraversal() { blah}
   public String postorderTraversal() { blah}
   public String avlAdd() { blah}
   public String RedBlackAdd() { blah}
   etc.
}


Doc 5, Composite & Visitor Slide # 14
Solution 2

Use different classes or subclasses for the different types of trees

Does not work in classes we want to be able to mix all combinations

class BinaryTree {
   blah
}
   
class AVLTree extends BinaryTree {
   blah
}
   
class RedBlackTree extends BinaryTree {
   blah
}
class PreorderTree extends BinayrTree {
   blah
}
   
etc.

Doc 5, Composite & Visitor Slide # 15
Solution 3





class BinaryTreeNode extends Node {
   public void accept(Visitor aVisitor) {
      aVisitor.visitBinaryTreeNode( this );
   }
   etc.
}

class BinaryTreeLeaf extends Node {
   public void accept(Visitor aVisitor) {
      aVisitor.visitBinaryTreeLeaf( this );
   }
   etc.
}

abstract class Visitor {
   abstract void visitBinaryTreeNode( BinaryTreeNode );
   abstract void visitBinaryTreeLeaf( BinaryTreeLeaf );
}
   
class HTMLPrintVisitor extends Visitor {
   public void visitBinaryTreeNode( BinaryTreeNode ) {
      HTML print code here
   }
   etc.
}

Doc 5, Composite & Visitor Slide # 16
Double Dispatch

Note that a visit to one node requires two method calls


Node example = new BinaryTreeLeaf();
   
Visitor traveler = new HTMLPrintVisitor();
      
example.accept( traveler );

example.accept() calls aVisitor.visitBinaryTreeNode(this);


The first method selects the correct method in the Visitor class

The second method selects the correct Visitor class

Doc 5, Composite & Visitor Slide # 17

Structure




This is more complex than solutions 1 & 2

The structure, an iterator, or the visitor can do the traversal



The visitor is told what type it is acting on, so using the wrong visitor will be a compile error

Doc 5, Composite & Visitor Slide # 18

When to Use Visitor






Doc 5, Composite & Visitor Slide # 19

Consequences



If the structure involves many different classes then adding a new operation to the structure requires changing all those classes



To add a new ConcreteElement you need to change all existing visitors


Text claims iterators cannot iterate through structure containing unrelated classes

However, the visitor assumes that each element in the structure contains a visit method, which implies at least a common visit interface for all elements


Visitor can accumulate information

The visitor may force you to provide public operations in the elements that you would not otherwise make public

C++ friends are useful here

Doc 5, Composite & Visitor Slide # 20
Avoiding the Accept Method

Visitor pattern requires elements to have an accept method

Sometimes this is not possible

You don’t have the source for the elements

Smalltalk Example

Use reflection to avoid the accept method in the elements

Visitor>>visit: anElement
   self
      perform: ('visit' , anElement class name , ':') asSymbol
      with: anElement


Aspect Oriented Programming

AspectJ provides a way around the use of Element accept methods in aspect oriented Java

AspectS provides a similar process for Smalltalk

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2002 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
OpenContent license defines the copyright on this document.

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