References: Xiaoping Jia, Object-Oriented Software Development
Using Java;Douglas C.Schmidt, Design Pattern Case Studies with
C++
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Design Patterns
• Generic (reusable) solutions to a recurring
problem
• Evolved from Christopher Alexander’s
patterns for architecture.
– “Each pattern describes a problem which
occurs over and over again in our
environment, and then describes the core
of the solution to that problem, in such a
way that you can use this solution a million
times over, without ever doing it the same
way twice.” – Alexander et. al., A Pattern Language (Oxford
U.Press,1977)
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Design Patterns–a ‘Hot Topic’
• OOPSLA Workshops – 17%
– Patterns for Software Architecture
– Killer Examples for Design Patterns &
Objects First
– Patterns in Distributed Real Time &
Embedded Systems
– Patterns for Customer Interaction &
Expectation Management
– Software Development Process Patterns
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Design Patterns–a ‘Hot Topic’
• OOPSLA Tutorials – 20%
– Pattern-oriented Software Architectures for Networked and
Concurrent Applications
– Patterns of Enterprise Application Architecture
– Patterns at Work
– Patterns for Writing Effective Use Cases
– Dungeons and Patterns!
– Patterns and Application Experiences for Real-time Object
Request Brokers
– Refactoring to Patterns
– Object-oriented Reengineering: Patterns & Techniques
– Patterns for EJB Development
– How to Use Design Patterns in Java and .NET
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Design Patterns
• Categories of Software Design Patterns
– Creational : deal with process of creation
– Structural : deal with static composition
and structure of classes and objects
– Behavioral : deal with dynamic interaction
among classes and objects
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Design Patterns
• Each pattern is described as follows:
Pattern name
Category : creational, structural or behavioral
Intent : short description of problem addressed
AKA: other names for the pattern
Applicability : where pattern can be applied
Structure : diagram that describes participants
in the pattern & relationships among them
Participants : list of classes and/or objects that
participate in the pattern
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Singleton Pattern
• Category: creational
• Intent: ensure that a class has only one
instance and provide global point of
access to it
• Applicability: use where there must be
exactly one instance of a class and it
must be accessible to clients from a
well-known access point (e.g. database)
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Singleton Pattern
• Structure
Singleton
static getInstance()
return theInstance
operation()
getData()
static Singleton theInstance
data
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Singleton Pattern
• Participants : only one
• Singleton declares the unique instance
of the class as a static variable and
defines a static method getInstance() for
clients to access the unique instance.
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Singleton Pattern
public class Singleton {
public static Singleton getInstance() {
return theInstance;
}
private Singleton() {
//initialize instance fields
}
//….
private static Singleton theInstance = new Singleton();
}
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Strategy Pattern
• Category : behavioral
• Intent: define a family of algorithms,
encapsulate each one and make them
interchangeable
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Strategy Pattern
• Applicability: use when
– many related classes differ only in behavior
• plotting different functions
– different variations of algorithm are needed
• sorts
– an algorithm uses data that clients should
not know about
• LayoutManager in the AWT
– a class defines many behaviors which
appear as multiple conditional statements
in its methods
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Structure of Strategy Pattern
STRATEGY
Context
contextInterface()
Strategy
algorithmInterface()
Concrete Strategy A
algorithmInterface()
Concrete Strategy C
algorithmInterface()
Concrete Strategy B
algorithmInterface()
Note: arrowheads should be unfilled
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Using the Strategy Pattern
Pivot Strategy
getPivot()
quicksort
pivotStrategy.getPivot(array,lo,hi)
Select
First
Random
Median
of
Three
Concrete
Strategies
Strategy pattern resolves how to extend policies for selecting a
pivot value without modifying main quicksort algorithm
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A tutorial presented by Steve Metsker and William Wake
At OOPSLA 2001
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Why Design Patterns?
• Patterns record previous successes
in a reusable form
• Design patterns are well-worn
solutions to problems at about a
class level
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How the Game Works
• Each table has a pattern master
who has a map of a dungeon
• You must adventure through the
dungeon to reclaim pearls of
wisdom from a dragon
• Your primary helper is an Ahobbit
– With capricious magical abilities
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Ahobbits
• Your Ahobbit has a magical ring that
– Cannot be removed
– You can activate by getting him/her to say
“Aha”
• Ahobbits are enlightened only when
they understand a design pattern
• Ahobbits learn from real-world
examples in the dungeon
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Adventuring
• Find a room in the dungeon
• When you enter
– Examine the device or contents therein
– Determine which pattern it shows
– Use the device to explain to your Ahobbit
the intent of the pattern (2 guesses)
• When the Ahobbit says “Aha,”
something magical will occur
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Beginning
• We shall begin as one huge
exploration party
• Before breaking into one party per
table or group
• You stand before a door that leads to
the dungeon
• What do you do?
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