	CS 635: Advanced Object-Oriented Design & Programming

Spring Semester, 1998
Decorator
To Lecture Notes Index
© 1998, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 21-Apr-98

Contents of Doc 23, Decorator

CS 635 Doc 23 Decorator

References	slide # 1

Decorator	slide # 2
...Solution 2 - Use Decorator	slide # 5
...Applicability	slide # 6
...Consequences	slide # 6
...Implementation Issues	slide # 7
...Java Example of Decorator - Streams	slide # 8

References

Design Patterns: Elements of Reusable Object-Oriented Software, Gamma, Helm, Johnson, Vlissides, Addison Wesley, 1995, pp 175-184

Doc 23, Decorator Slide # 2

Decorator

Changing the Skin of an Object

Class Structure
Decorator Class Structure

Runtime Structure
Decorator Runtime Structure

Doc 23, Decorator Slide # 3

Motivation - 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 23, Decorator Slide # 4

Solution 1 - Use Object Composition

Text View classes using Strategy like Composition

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 23, Decorator Slide # 5

Solution 2 - Use Decorator

Object Composition Inside out
Change the skin of an object not it guts

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

Text View using Decorator Pattern

Runtime Structure
Text View Decorator Objects

Doc 23, Decorator Slide # 6

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for responsibilities that can be withdrawn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 high up in hierarchy

Lots of little objects

A decorator and its components are not identical

so checking object identification can cause problems

     if ( aComponent instanceof TextView ) blah

Doc 23, Decorator Slide # 7

Implementation Issues

Keep Decorators lightweight

Don't put data members in VisualComponent

Doc 23, Decorator Slide # 8

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()  );
          }
     }

visitors since 21-Apr-98