References: Xiaoping Jia, ObjectOriented Software Development Using JavaDouglas C'Schmidt, Design Pa

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Design Patterns

• References Xiaoping Jia, Object-Oriented
  Software Development Using JavaDouglas
  C.Schmidt, Design Pattern Case Studies with C

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Design Patterns

• Generic (reusable) solutions to a recurring
  problem
• Evolved from Christopher Alexanders 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 Patternsa 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 Patternsa 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() operation() getData
() static Singleton theInstance data
return theInstance
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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)
Random
Median of Three
Select First
Concrete Strategies
Strategy pattern resolves how to extend policies
for selecting a pivot value without modifying
main quicksort algorithm
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Dungeons Patterns
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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