Design patterns

Design Patterns – Eran Toch
Methodologies in Information System Development
Design Patterns:Design Patterns:
Talking in the Design LanguageTalking in the Design Language
Eran Toch
Methodologies in the Development
of Information Systems

2November 2003
AgendaAgenda
• What is a Design Pattern?
• Structural Patterns
– The Composite Pattern
• Behavioral Patterns
– The Strategy Pattern
• Creational Patterns
– The Singleton Pattern
• Human Interaction Patterns
– Go Back to a Safe Place
• Summary

3November 2003
What is a Design Pattern?What is a Design Pattern?
In Short, a solution
for a typical problem
“a description of a recurrent
problem and of the core of possible
solutions.“

4November 2003
Why do we need them?Why do we need them?
• Problems are not always unique. Reusing
existing experience might be proved useful.
• We need a common language to describe
problems and solutions.
• The Design of Object-Oriented systems limits
the scope of problems and solutions we can
talk about

5November 2003
History of Design PatternsHistory of Design Patterns
Christopher Alexander
A Pattern Language: Towns, Buildings Construction 1970’
1995’
2000’
Architecture
Object Oriented
Software Design
Other Areas:
HCI, Organizational Behavior…
Gang of Four (GoF)
Design Patterns: Elements of
Reusable Object-Oriented Software
Many Authors

6November 2003
Structure of a design patternStructure of a design pattern
• Pattern Name and Classification
• Intent
– a Short statement about what the pattern does
• Motivation
– A scenario that illustrates where the pattern would be
useful
• Applicability
– Situations where the pattern can be used

7November 2003
Structure of a design pattern – cont’dStructure of a design pattern – cont’d
• Structure
– A graphical representation of the pattern
• Participants
– The classes and objects participating in the pattern
• Collaborations
– How to do the participants interact to carry out their
responsibilities?
• Consequences
– What are the pros and cons of using the pattern?
• Implementation
– Hints and techniques for implementing the pattern

8November 2003
Classification of patternsClassification of patterns
• Structural
– Composite*, Decorator, Façade, Adapter…
• Behavioral
– Strategy*, Command, Observer, Chain of
Responsibilities…
• Creational
– Singleton*, Abstract Factory, Factory Method…
• Other
– User interface Patterns
• Go Back to a Safe Place

9November 2003
AgendaAgenda
• Summary

10November 2003
The Composite PatternThe Composite Pattern
• Intent:
– Compose objects into tree structures to represent
part-whole hierarchies.
– Composite lets clients treat individual objects and
compositions of objects uniformly. This is called
recursive composition.

11November 2003
MotivationMotivation
When Processing the hierarchal
collection, we need to query the
type of the object (composite or
primitive) and act differently
Diagram reference: Design
Patterns [Gof]

12November 2003
Composite: ApplicabilityComposite: Applicability
• Use the 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. Clients will treat all objects in
the composite structure uniformly.

13November 2003
Composite: StructureComposite: Structure
• Define an abstract base class that specifies the
uniform behavior.
• Subclass the Primitive and Composite classes of the
Component class.
Diagram reference: Design
Patterns [Gof]

14November 2003
Composite: ConsequencesComposite: Consequences
• Benefits
– It makes it easy to add new kinds of components
– It makes clients simpler, since they do not have to
know if they are dealing with a leaf or a composite
component
• Liabilities
– It makes it harder to restrict the type of components of
a composite

15November 2003
Composite: Known UsesComposite: Known Uses
1. A GUI system has window objects which can
contain various GUI components (widgets) such as,
buttons and text areas. A window can also contain
widget container objects which can hold other
widgets.
Concrete examples: Java Swing, MFC
2. Folder based file system
3. Arithmetic expressions

16November 2003
AgendaAgenda
• Summary

17November 2003
The Strategy PatternThe Strategy Pattern
• Intent:
– Define a family of algorithms, encapsulate each one,
and make them interchangeable.
– Strategy lets the algorithm vary independently from
clients that use it.

18November 2003
Strategy: MotivationStrategy: Motivation
The client wants to use the algorithm
without knowing which one to use in
advance. We want to add new
algorithms without changing the
client class.

19November 2003
Strategy: StructureStrategy: Structure
• Encapsulate the behavior of the context class in a
separate algorithm class.
• The context class keeps a reference to the algorithm
class and passes on the messages.
• The algorithm class can be sub-classed to support
multiple algorithms.

20November 2003
Strategy: ConsequencesStrategy: Consequences
• Benefits
– Provides an alternative to subclassing the Context
class to get a variety of algorithms or behaviors
– Keeps the client clean from large conditional
statements
– Provides a choice of implementations for the same
behavior
• Liabilities
– Increases the number of objects
– All algorithms must use the same Strategy interface
– We still need conditional statements!

21November 2003
Strategy: Known UsesStrategy: Known Uses
• QSIA Learning System
• Layout Manager in Java
Swing
• Sorting algorithms,
memory allocation
algorithms and more…

22November 2003
AgendaAgenda
• Summary

23November 2003
The Singleton PatternThe Singleton Pattern
• Intent.
– Ensure a class only has one instance, and provide a
global point of access to it.
• Motivation.
– Sometimes, an application needs one, and only one,
instance of an object. For example, we want just one
window manager. Or just one factory for a family of
products.
– We need to have that one instance easily accessible.
– And we want to ensure that additional instances of the
class can not be created.

24November 2003
Singleton: StructureSingleton: Structure
• Declare the single instance as a private static data
member.
• Provide a public static member function that
encapsulates all initialization code, and provides
access to the instance.

25November 2003
Singleton: Example CodeSingleton: Example Code
public class Singleton {
// The private reference to the one and only instance.
private static Singleton uniqueInstance = null;
// An instance attribute.
private int data = 0;
/**
* Returns a reference to the single instance.
* Creates the instance if it does not yet exist.
*/
public static Singleton instance() {
if(uniqueInstance == null)
uniqueInstance = new Singleton();
return uniqueInstance;
}
/**
* The Singleton Constructor. Note that it is private!
* No client can instantiate a Singleton object!
*/
private Singleton() {}
}

26November 2003
Singleton: Example Code – cont’dSingleton: Example Code – cont’d
public class TestSingleton {
public static void main(String args[]) {
// Get a reference to the single instance of Singleton.
Singleton s = Singleton.instance();
// Set the data value.
s.setData(34);
System.out.println("First reference: " + s);
System.out.println("Singleton data value is: " + s.getData());
// Get another reference to the Singleton.
// Is it the same object?
s = null;
s = Singleton.instance();
System.out.println("nSecond reference: " + s);
System.out.println("Singleton data value is: " + s.getData());
}
}
What will be the output?

27November 2003
AgendaAgenda
• Summary

28November 2003
Human Interaction PatternsHuman Interaction Patterns
• A Pattern language to describe human
interaction situations
• Examples:
– Go Back to a Safe Place
– Form
– Control Panel
– WYSIWYG Editor
– Hierarchical Set
– Map of Navigable Spaces
– More…

29November 2003
Go Back to a Safe PlaceGo Back to a Safe Place
• Problem:
– How can the artifact make navigation easy, convenient, and
psychologically safe for the user?
• Forces:
– A user may forget where they were, if they stop using the artifact
while they're in the middle of something and don't get back to it
for a while.
– If the user gets into a space or a state that they don't want to be
in, they will want to get out of it in a safe and predictable way.
– The user is more likely to explore an artifact if they are assured
that they can easily get out of an undesired state or space; that
assurance engenders a feeling of security.

30November 2003
Go Back to a Safe PlaceGo Back to a Safe Place
• Solution:
– Provide a way to go back to a checkpoint of the user's
choice.
• Examples:
– The "Home" button on a Web browser
– Turning back to the beginning of a chapter in a
physical book or magazine
– The “undo" feature on some computer applications

31November 2003
AgendaAgenda
• Summary

32November 2003
SummarySummary
• Patterns learned:
– Composite, Strategy, Singleton, Go Back to a Safe
Place.
• Advantages of Design Patterns:
– They capture expertise and make it accessible to non-
experts.
– Their names collectively form a vocabulary that helps
developers communicate better.
– They help people understand a system more quickly
when it is documented with the patterns it uses.

33November 2003
ReferencesReferences
• Design Patterns: Elements of Reusable Object-Oriented
Software, Gamma E. et el., 1995, Addison-Wesley.
• A course from Bob Tarr from UMBC University
http://www.research.umbc.edu/~tarr/dp/fall00/cs491.html
• The Design Patterns Java Companion, James W. Cooper (an
online version of the book)
http://www.patterndepot.com/put/8/JavaPatterns.htm
• A Site dedicated to Design Patterns by Vince Huston
http://home.earthlink.net/~huston2/dp/patterns.html
• Seven Habits of Successful Pattern Writers, John Vlissides
http://hillside.net/patterns/papers/7habits.html
• COMMON GROUND: A Pattern Language for Human-
Computer Interface Design, Jenifer Tidwell,
http://www.mit.edu/~jtidwell/common_ground_onefile.html

34November 2003
Doesn’t it go too far?Doesn’t it go too far?
• Taken from Vince Huston’s site about Design
Patterns:

Methodologies in Information System Development
Thanks!Thanks!

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