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Lecture 23: Design Patterns
Kenneth M. Anderson
CSCI 6448 - Spring Semester, 2002Object-Oriented Analysis and Design
April 9, 2002 © Kenneth M. Anderson, 2002
Last Lecture
• Database Design
– Data Models
• Relational, Object-Relational, Object
– Object Mapping
• OO concepts to (potentially) non-OO concepts
April 9, 2002 © Kenneth M. Anderson, 2002 3
Quiz
• Is it possible to determine if an individual
particular component?extended during design) is a member of aclass (developed during analysis and
• What “route” through analysis and design
answer this question?models would you have to take in order to
April 9, 2002 © Kenneth M. Anderson, 2002
Goals of Lecture
• Cover Design Patterns
– Examples– Background and Core Concepts
• Plus Implementations
April 9, 2002 © Kenneth M. Anderson, 2002 5
Pattern Resources
• Pattern Languages of Programming
– Technical conference on Patterns
• The Portland Pattern Repository
– http://c2.com/ppr/
• Patterns Homepage
– Go to page then click on “Patterns tab”– http://hillside.net/
April 9, 2002 © Kenneth M. Anderson, 2002
Design Patterns
– Addison-Wesley book published in 1995
Erich Gamma
Richard Helm
Ralph Johnson
John Vlissides
ISBN 0-201-63361-
Known as “The Gang of Four”
Presents 23 Design Patterns
Publishing Companybook, and is thus copyright © 1995 by Addison-WesleyMaterial in this lecture and lecture 24 is drawn from this
April 9, 2002 © Kenneth M. Anderson, 2002 7
What are Patterns?
– Christopher Alexander talking about buildings and towns
twice”solution a million times over, without ever doing it the same waysolution to that problem, in such a way that you can use thisagain in our environment, and then describes the core of the“Each pattern describes a problem which occurs over and over
1977 Alexander, et al., A Pattern Language. Oxford University Press,
April 9, 2002 © Kenneth M. Anderson, 2002
Patterns, continued
Patterns can have different levels of abstraction
– Instead, Patterns are descriptions of communicating– Patterns are not frameworks– Patterns are not classesIn Design Patterns (the book),
general design problem in a particular contextobjects and classes that are customized to solve a
April 9, 2002 © Kenneth M. Anderson, 2002 13
Design Pattern Template
– ClassificationPattern Name and
Creational
Structural
Behavioral
Intent
Also Known As
Motivation
Applicability
Structure
Participants
Collaborations
Consequences
Implementation
Sample Code
Known Uses
Related Patterns
April 9, 2002 © Kenneth M. Anderson, 2002
Examples
• Adapter• Factory Method• Singleton
April 9, 2002 © Kenneth M. Anderson, 2002 15
Singleton
• Intent
– Ensure a class has only one instance, and
provide a global point of access to it
• Motivation
– Some classes represent objects where multiple
security risk (e.g. Java security managers)instances do not make sense or can lead to a
April 9, 2002 © Kenneth M. Anderson, 2002
Singleton, continued
• Applicability
– Use the Singleton pattern when
• there must be exactly one instance of a class, and it
access pointmust be accessible to clients from a well-known
• when the sole instance should be extensible by
extended instance without modifying their codesubclassing, and clients should be able to use an
April 9, 2002 © Kenneth M. Anderson, 2002 17
Singleton Structure
static Instance() Singleton
{return uniqueInstance}
private singletonDataprivate static uniqueInstancepublic GetSingletonData() public SingletonOperation()
April 9, 2002 © Kenneth M. Anderson, 2002
Singleton, continued
– Just the Singleton classParticipants
– Clients access a Singleton instance solely throughCollaborations
Singleton’s Instance operation
– Permits a variable number of instances (if desired)– Reduced name space (versus global variables)– Controlled access to sole instanceConsequences
April 9, 2002 © Kenneth M. Anderson, 2002 19
Implementation
import (^) java.util.Date; public (^) class (^) Singleton (^) { private (^) static (^) Singleton (^) theOnlyOne; private (^) Date (^) d (^) = (^) new (^) Date(); public} private Singleton() { (^) static (^) Singleton (^) instance() { if (^) (theOnlyOne == (^) null) (^) { theOnlyOne = (^) new (^) Singleton(); return} (^) theOnlyOne; public} (^) Date (^) getDate() { return d;
April 9, 2002 © Kenneth M. Anderson, 2002
Using our Singleton Class
public (^) class (^) useSingleton { public (^) static (^) void (^) main(String[] args) (^) { Singleton a (^) = (^) Singleton.instance(); Singleton b (^) = (^) Singleton.instance(); System.out.println("" + (^) a.getDate()); System.out.println("" + (^) b.getDate()); System.out.println("" + (^) a); System.out.println("" + (^) b);
Singleton@136646Singleton@136646Sun Apr 07 13:03:34 MDT 2002Sun Apr 07 13:03:34 MDT 2002Output:}
April 9, 2002 © Kenneth M. Anderson, 2002 25
Factory Method, continued
– ProductParticipants
- Defines the interface of objects the factory method creates
– Concrete Product
- Implements the Product Interface
– Creator
- declares the Factory method which returns an object of type
Product
– Concrete Creator
- overrides the factory method to return an instance of a
Concrete Product
April 9, 2002 © Kenneth M. Anderson, 2002
Factory Method Structure
Product
ConcreteProduct
Creator
FactoryMethod()^ ConcreteCreator
AnOperation() FactoryMethod()
«instantiate»
product = FactoryMethod()
return new ConcreteProduct()
April 9, 2002 © Kenneth M. Anderson, 2002 27
Factory Method Consequences
application-specific classes into your codeFactory methods eliminate the need to bind
– In single-inherited systems, this constrains yourConcreteProductsubclassing in order to create a particularPotential disadvantage is that clients must use
partitioning choices
Provides hooks for subclasses
– See page 110 of the design patterns bookConnects parallel class hierarchies
April 9, 2002 © Kenneth M. Anderson, 2002
Implementation
• See code example (available on class
website)
• A factory can return balloons of different
– The factory hides several specific creators andcolors
cycles among them to create balloons
• A client retrieves multiple balloons and
displays their colors
April 9, 2002 © Kenneth M. Anderson, 2002 29
Adapter
– Intent
because of incompatible interfacesexpect. Adapter lets classes work together that could not otherwiseConvert the interface of a class into another interface clients
– Also Known As
Wrapper
– Motivation
an application requiresbecause its interface does not match the domain-specific interfaceSometimes a toolkit class that is designed for reuse is not reusable
Page 139-140 of Design Patterns provides an example
April 9, 2002 © Kenneth M. Anderson, 2002
Adapter, continued
• Applicability
– Use the Adapter pattern when
• you want to use an existing class, and its interface
does not match the one you need
• you want to create a reusable class that cooperates
with unrelated or unforeseen classes
April 9, 2002 © Kenneth M. Anderson, 2002 31
Adapter, continued
– TargetParticipants
- defines the domain-specific interface that Client uses
– Client
- collaborates with objects conforming to the Target interface
– Adaptee
- defines an existing interface that needs adapting
– Adapter
- adapts the interface of Adaptee to the Target interface April 9, 2002 © Kenneth M. Anderson, 2002
Adapter Structure
Client
Target
Request()
Adaptee
SpecificRequest()
Request() Adapter
Class Adapter
SpecificRequest()
