More ObjectOriented Design Patterns

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CSC 335 Object-Oriented Programming and Design
Presentation 13 More Object-Oriented Design
Patterns
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Design Patterns

• 30 years ago, Christopher Alexander asked
• Are beauty and quality objective?
• Can we agree some things are beautiful and some
  are not?

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What is good design?

• Alexander is an architect (at UC Berkeley today)
• What makes a good architectural design?
• What makes bad architectural design?
• Can we recognize good design?
• Is there an objective basis for such a judgment?
• Is there a basis for describing common consensus
• Alexander believed that beauty can be described
  through an objective basis that can be measured.

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Cultural Anthropology

• Within a culture, individuals agree what is good
  design, what is beautiful
• There are patterns in cultures that serve as
  objective bases for judging design
• Proposed The quality of software can be measured
  objectively
• This is the idea behind design patterns
• Patterns are not invented, they are recognized as
  elegant solutions that have been used many times

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Two questions

• Alexander asks
• What is present in good quality design that is
  not present in a poor quality design?
• What is present in poor quality design that is
  not present in a good quality design?
• The task is to identify the things what makes a
  design good and what makes a design bad
• What are the commonalities in what is viewed as
  good (and what is viewed as bad)

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Look for commonalities

• What are the commonalities in what is viewed as
  good and what is viewed as bad
• A software system that is relatively easy to
  maintain is considered good
• A fragile software system is considered bad
• A software system that is easy to understand is
  considered good
• obfuscated spaghetti code is bad

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Architecture

• Alexander observed many buildings, towns,
  streets, gardens, and pieces of furniture
• Good constructs had things in common
• They can be different but still of high quality
• They may solve different problems
• Consider a porch
• A porch may be used to provide shade on a hot
  sunny day
• Another porch provides a transition from the
  outside to the inside and from the inside to the
  outside
• Did you ever live where there was no roof over
  the door?

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Can solve the same problem in different ways

• Or two different porches might solve the same
  problem in two different ways
• Both may provide a pleasing way to enter or exit
  a house and still look different
• Alexander was trying to identify and describe the
  consistency of quality in design
• having a transition from room to room or from
  inside to outside is important
• those transitions can take many forms

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Patterns A definition

• He looked at different structures that solved the
  same problem
• Found similarities between designs of high
  quality
• Alexander called these similarities patterns
• A pattern is a solution to a problem in a context
• 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.

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OOPSLA 96 Alexander PicDoug Lea quote

• "Alexander's central premise, driving over thirty
  years of thoughts, actions, and writings, is
  that there is something fundamentally wrong with
  twentieth century architectural design methods
  and practices. ... Alexander illustrates failures
  in the sensitivity of contemporary methods to the
  actual requirements and conditions surrounding
  their development. He argues that contemporary
  methods fail to generate products that satisfy
  the true requirements placed upon them by
  individuals and society, and fail to meet the
  real demands of real users, and ultimately fail
  in the basic requirement that design and
  engineering improve the human condition."

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Alexander's Request

• Alexander believes
• architects failed (our living spaces are
  filled--too late)
• the ubiquitous computer and the software that
  runs on it may be another chance to improve the
  quality of life
• At OOPSLA 1996, Alexander asked the audience of
  2,800 software engineers to take up the challenge
  of making software beautiful. He talked of
  wholeness and living structure
• Received with a rare standing ovation

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Alexander"Quality without a name" Quote from
Dick Gabriel (Sun Fellow)

• "Its modules and abstractions are not too big. If
  they were too big, their size and inflexibility
  would have created forces that would overgovern
  the overall structure of the software every
  module, function, class, and abstraction is small
  and named so that I know what it is without
  looking at its implementation.
• If I look at any small part of it, I can see what
  is going on, I don't need to refer to other parts
  to understand what something is doing. This tells
  me that the abstractions make sense for
  themselves -- they are whole.
• If I look at any large part in overview, I can
  see what is going on, I don't need to know all
  the details to get it.
• Everything about it seems familiar.
• I can imagine changing it, adding some
  functionality.
• I am not afraid of it, I will remember it."

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Does this relate to Software?

• Early 1990s software developers wondered if the
  same approach could be used in our discipline
• Are there problems that occur over and over again
  that could be solved in somewhat the same way?
• Could software be developed in terms of patterns,
  writing solutions after the pattern was
  identified?
• People started looking for patterns
• Then came one of the biggest selling books in
  CS...

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Gang of Four The GoF Book

• The book with the greatest influence is
• Design Patterns Elements of Reusable
  Object-Oriented Software
• Gamma (who wrote JUnit with Kent Beck), Helm,
  Johnson, and Vlissides
• This book catalogs 23 patterns (some you have had
  some exposure to)
• They offer advice for developing quality software

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Patterns in the GoF book

• The Gang of Four did not invent the patterns
• They found patterns that already existed in the
  software community 23 from book liner are
  attached
• These are the similarities found in the design of
  software viewed to be of high quality
• They made a rule to list three different systems
  that solved a problem in the same manner
• Thousands of patterns have been documented since
• telecommunications patterns, pedagogical
  patterns, analysis patterns, indexing patterns

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What a pattern has

• Formats of pattern writers vary, but a pattern
  usually has at least these four things
• A name (Iterator, Stategy, Observer, Composite)
• The purpose of the pattern, the problem it solves
• A guide for how to accomplish the solution
• The constraints and forces to be considered in
  order to accomplish the solution

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Why Study Patterns?

• Can reuse solutions
• gives us a head start
• avoids the gotchas later (unanticipated things)
• no need to reinvent the wheel
• Establish common terminology
• Design patterns provide a common point of
  reference
• Easier to say, "We need some Strategies here"
• Provide a higher level prospective
• frees us from dealing with the details too early

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Carpenter's ConversationAdapted from Ralph
Johnson

• How should we build the cabinet drawers?
• Cut straight down into the wood, cut back up 45
  degrees a specific length, then go straight back
  down a specific length, the cut back up at 45
  degrees until you reach, .....

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A Higher Level Discussion

• A high level discussion could have been
• "Should we use a miter joint or a dovetail
  joint?"
• This is a higher, more abstract level
• Avoids getting bogged down in details
• Which level of detail is more efficient?

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Consequences of which joint

• Dovetail joints
• are more complex, more expensive to make
• withstands climate conditions dovetail joint
  remains solid as wood contracts and expands
• independent of fastening system
• more pleasing to look at
• Thoughts underneath this question are
• Should we make a beautiful durable joint or a
  cheap and dirty one that lasts until the check
  clears?

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Consequences

• Carpenters, patterns writers, and software
  developers discuss consequences
• consequences simply refer to cause and effect
• If we do this, what will happen both good and
  bad
• also known as the forces that patterns consider
• Example If we use Mediator to add and drop
  courses
• Add an extra class that needs reference to
  several objects
• All of the logic and process is confined to one
  class so any change to the "rules" would be
  handled there
• Reduces dependencies between others objects
  (simpler design when student does NOT tell the
  scheduled course to change)

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Other advantages

• Most design patterns make software more
  modifiable, less brittle
• we are using time tested solutions
• Helps increase the understanding of basic
  object-oriented design principles
• encapsulation, inheritance, interfaces,
  polymorphism
• Designs of large systems can avoid large
  inheritance hierarchies (there are alternatives)
• learned the hard way about big inheritance
  hierarchies

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A Pattern

• One pattern you may have seen from 227 or 127B is
  the Iterator Pattern

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Pattern Iterator

• Name Iterator (a.k.a Enumeration)
• Problem How can you loop over all objects in any
  collection. You dont want to change client code
  when the collection changes. You also want the
  same interface (methods)
• Solutions 1) Have each of your classes implement
  an interface. 2) Have an Iterator interface that
  works with all collections
• Consequences Can change collection class without
  changing code to traverse the collection

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One Java version of an Iterator

• interface Enumeration
• boolean hasMoreElements()
• Tests if this enumeration contains more elements.
• Object nextElement()
• Returns the next element of this enumeration if
  this enumeration object has at least one more
  element to provide.

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Another Java version of an Iterator

• interface Iterator
• boolean hasNext()
• Returns true if the iteration has more elements.
• Object next()
• Returns the next element in the iteration

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Could change to ArrayList to LinkedList

• // The Client code
• List bank new ArrayList( )
• bank.add( new BankAccount("one", 0.01 ) )
• // ...
• bank.add(new BankAccount("nine thousand",
  9000.00) )
• String ID ???
• BankAccount searchAcct new BankAccount( ID,
  0.00 )
• Iterator i bank.iterator( )
• while( i.hasNext( ) )
• BankAccount ref (BankAccount)i.next( )
• if( ref.compareTo( searchAcct ) 0 )
• System.out.println( "Found " ref.getID()
  )

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UML Diagram of Java's Iterator and Collections
ltltinterfacegtgt List iterator()
ltltinterfacegtgt Iterator hasNext() next()
Vector iterator()
LinkedList iterator()
ArrayList iterator()
Iterator hasNext() next()
Context
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Pattern Strategy

• Name Strategy (a.k.a Policy)
• Problem You want to encapsulate a family of
  algorithms and make them interchangeable.
  Strategy lets the the algorithm vary
  independently from the clients that use it (GoF)
• Solution Create an abstract strategy class (or
  interface) and extend (or implement) it in
  numerous ways. Each subclass defines the same
  method names in different ways

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Pattern Strategy

• Consequences
• Allows families of algorithms. Known uses
• Layout managers in Java
• Different Poker Strategies in 335 Project Fall
  2000
• Different Pakman chase strategies 335 Fall 2001
• TextField validators in dBase and Borland OWL
• Will use different algorithms to verify if the
  user input is a valid integer, double, string,
  date, yes/no.
• Eliminates conditional statements
• Don't need to keep asking, if this is supposed to
  be an int do this, else if it is a string do that

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Java Example of Strategy

• this.setLayout( new FlowLayout( ) )
• this.setLayout( new GridLayout( ) )
• In Java, a container contains a layout manager
• There is a default perhaps set in the
  constructor
• You can change a container's layout manager with
  a setLayout message
• Could use same approach in Ghosts
• Ghosts have common methods but could change an
  instance variable at runtime with
• setStrategy( new RandomStategy( ) )

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interface LayoutManager

• Java has a java.awt.LayoutManager interface
• Known Implementing Classes
• GridLayout, FlowLayout, ScrollPaneLayout
• Each class implements the following methods
• addLayoutComponent(String name, Component comp)
  
• layoutContainer(Container parent)
• minimumLayoutSize(Container parent)
• preferredLayoutSize(Container parent)
• removeLayoutComponent(Component comp)

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General UML Diagram of Strategy
ltltinterfacegtgt Strategy AlgorithmInterface
Context strategy Strategy ContextInterface
implements
ConcreteClassA AlgorithmInterface
ConcreteClassB AlgorithmInterface
ConcreteClassC AlgorithmInterface
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Specific UML Diagram of LayoutManager
ltltinterfacegtgt LayoutManager addLayoutComponent() l
ayoutContainer() minimumLayoutSize()
JPanel panel Panel size Dimension setLayout(lm
LayoutManager) setPreferredSize(diDimension)
implements
GridLayout addLayoutComponent() layoutContainer()
minimumLayoutSize()
FlowLayout addLayoutComponent() layoutContainer()
minimumLayoutSize()
ScrollPaneLayout addLayoutComponent() layoutContai
ner() minimumLayoutSize()
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Change the stategy at runtime

• Demonstrate LayoutControllerFrame.java
• Source code online and attached
• private class FlowListener implements
  ActionListener
• // There is another ActionListener for
  GridLayout
• public void actionPerformed( ActionEvent evt )
• // Change the layout strategy of the JPanel
• // and tell it to lay itself out
• centerPanel.setLayout( new FlowLayout( ) )
• centerPanel.validate( )

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Strategy in a familiar game

• Imagine a card game (BlackJack) that has a human
  player that requires human interaction and some
  computer players to keep you company as you loose
  money
• We want accomplices with varied skills that can
  change at runtime
• First an interface
• public interface DrawCardStrategy
• // Need dealer in most strategies
• public boolean wantAnotherCard(Dealer
  theDealer)

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One Strategy

• public class NaiveStrategy implements
  DrawCardStrategy
• private static Random generator new Random(
  )
• public boolean wantAnotherCard(Dealer
  theDealer)
• // Do not need parameter for NaivePlayers
• // However, Other strategies will use the
  dealer
• if(generator.nextInt(2) 0)
• return true
• else
• return false

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A Second Strategy

• public class IntermediateStategy implements
  DrawCardStrategy
• // Get another card when the dealer shows 7 or
  more
• public boolean wantAnotherCard(Dealer
  theDealer)
• return theDealer.showingCards().pointValue()
  gt 7

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Add a third or fourth ...

• public class ExpertStategy implements
  DrawCardStrategy
• // Draw cards based on card counting
• public boolean wantAnotherCard(Dealer
  theDealer)
• // TBA ... complex algorithm

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Used in the Player Class

• public class Player
• private String myName
• private DrawCardStrategy myStrategy
• public Player(String name)
• myName name
• // Default Strategy
• myStrategy new NaiveStrategy()
• // This is the contextInterface
• public void setDrawCardStrategy(DrawCardStrategy
  aStrategy)
• myStrategy aStrategy
• // The GameModel needs to ask this question
  often

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Specific UML Diagram of DrawCardStrategy
ltltinterfacegtgt DrawCardStrategy wantAnotherCard(d
ealerDealer) Boolean
Player currentStrategy DrawCardStrategy setStrate
gy(dsDrawCardStrategy)
implements
NaiveStrategy wantAnotherCard(dealerDealer)
Boolean
SmartStrategy wantAnotherCard(dealerDealer)
Boolean