CS 696: Advanced OO
Spring Semester, 1997
Doc 14 Composite and Decorator

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San Diego State University -- This page last updated Apr 10, 1997

Contents of Doc 14 Composite and Decorator

Composite	slide # 1
...Example - Motivation	slide # 1
...Issue: WidgetContainer Operations	slide # 6
...Explicit Parent References	slide # 7
...Applicability	slide # 8
...Java Use of Composite - AWT Widgets	slide # 9

Decorator	slide # 11
...Solution 1 - Use Strategy	slide # 12
...Solution 2 - Use Decorator	slide # 13
...Applicability	slide # 15
...Consequences	slide # 15
...Implementation Issues	slide # 16
...Java Example of Decorator - Streams	slide # 17

Doc 14 Composite and Decorator Slide # 1

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 14 Composite and Decorator Slide # 2

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.
          if ( blah ) etc.
          }
     }
Doc 14 Composite and Decorator Slide # 3

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 14 Composite and Decorator Slide # 4

The Composite Pattern - Version B

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();
          }
     }

Issue: WidgetContainer Operations

Doc 14 Composite and Decorator Slide # 5

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 added a TextArea to a button? throw an exception?

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

Doc 14 Composite and Decorator Slide # 6

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 14 Composite and Decorator Slide # 7

More Issues

Should Component implement a list of Components?

The button etc. 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

Applicability

Use Composite pattern when

you want to represent part-whole hierarchies of objects
you want clients to be able to ignore the difference between compositions of objects and individual objects

Doc 14 Composite and Decorator Slide # 8

Java Use of Composite - AWT Widgets

Doc 14 Composite and Decorator Slide # 9

Specialized Java Containers

Doc 14 Composite and Decorator Slide # 10

Decorator

Problem - text views

A text view has the following features:

side scroll bar

bottom scroll bar

3D border

Flat border

This gives 12 different options:

TextView

TextViewWithNoBorder&SideScrollbar

TextViewWithNoBorder&BottomScrollbar

TextViewWithNoBorder&Bottom&SideScrollbar

TextViewWith3DBorder

TextViewWith3DBorder&SideScrollbar

TextViewWith3DBorder&BottomScrollbar

TextViewWith3DBorder&Bottom&SideScrollbar

TextViewWithFlatBorder

TextViewWithFlatBorder&SideScrollbar

TextViewWithFlatBorder&BottomScrollbar

TextViewWithFlatBorder&Bottom&SideScrollbar

How to implement?

Doc 14 Composite and Decorator Slide # 11

Solution 1 - Use Strategy

class TextView
     {
     Border myBorder;
     ScrollBar verticalBar;
     ScrollBar horizontalBar;
     
     public  void draw()
          {
          myBorder.draw();
          verticalBar.draw();
          horizontalBar.draw();
          code to draw self
          }
          
     etc.
     }

But TextView know about all the variations!
New type of variations require changing TextView
(and any other type of view we have)

Doc 14 Composite and Decorator Slide # 12

Solution 2 - Use Decorator

Turn the Strategy Inside out

TextView has no borders or scrollbars!
Add borders and scrollbars on top of a TextView

class TextView
     {
     public  void draw()
          {
          code to draw self
          }
          
     etc.
     }

Doc 14 Composite and Decorator Slide # 13

class FlatBorder
     {
     VisualComponent myComponent;
     
     public FlatBorder( VisualComponent aComponent )
          {
          myComponent = aComponent
          }
          
     public void draw()
          {
          int width = myComponent.width();
          int height = myComponent.height();
          
          myComponent.draw();
          code to draw FlatBorder
          }
     }
     
main()
     {
     TextView taxSummary = new TextView();
     VisualComponent flatTax = new FlatBorder( taxSummary );
     VisualComponent longTax = new VerticalScrollBar( flatTax );
     
     now use longTax
     }

Doc 14 Composite and Decorator Slide # 14

Applicability

Use Decorator:

to add responsibilities to individual objects dynamically and transparently

for responsibilities that can be withdrawn

when subclassing is impractical - may lead to too many subclasses

Consequences

More flexible than static inheritance

Avoids feature laden classes

Lots of little classes

A decorator and its components are not identical

so checking object identification can cause problems

Doc 14 Composite and Decorator Slide # 15

Implementation Issues

Keep Decorators lightweight

Don't put data members in VisualComponent

Doc 14 Composite and Decorator Slide # 16

Java Example of Decorator - Streams

import java.io.*;
import sdsu.io.*;
class  ReadingFileExample
     {
     public  static  void  main( String  args[]  ) throws Exception
          {
          FileInputStream inputFile;
          BufferedInputStream bufferedFile;
          ASCIIInputStream  cin;
          
          inputFile = new FileInputStream( "ReadingFileExample.java" );
          bufferedFile = new BufferedInputStream( inputFile );
          cin = new ASCIIInputStream( bufferedFile );

          System.out.println(  cin.readWord()  );

          for  ( int  k = 1 ;  k  <  4;  k++ )
               System.out.println(  cin.readLine()  );
          }
     }

CS 696: Advanced OO Spring Semester, 1997 Doc 14 Composite and Decorator

Contents of Doc 14 Composite and Decorator

Example - Motivation

Issue: WidgetContainer Operations

Explicit Parent References

Applicability

Java Use of Composite - AWT Widgets

Decorator

Solution 1 - Use Strategy

Solution 2 - Use Decorator

Applicability

Consequences

Implementation Issues

Java Example of Decorator - Streams

CS 696: Advanced OO
Spring Semester, 1997
Doc 14 Composite and Decorator