Plab Tirgul 12 Design Patterns

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Plab Tirgul 12Design Patterns
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What is a Design Pattern ?

• Invented by Christopher Alexander, Architect.
• 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, Christopher Alexander.
• Originally referred to buildings and towns,
  applies to OOD as well.

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What is a Design Pattern ?

• A pattern has 4 elements
• Name Naming a pattern immediately increases our
  design vocabulary.
• Problem When to apply the pattern the problem
  and the context.
• Solution Describes the elements that make up the
  design, their relationships, responsibilities,
  and collaborations. Not a specific
  implementation.
• Consequence The results and trade-offs of
  applying the pattern. Allows evaluation between
  different patterns.

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Why Design Patterns ?

• Common terminology.
• Solves practical problem.
• Reuse of design and experience.
• Improves the documentation and maintenance of
  existing systems.

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Design Patterns The book

• Written by the Gang Of four (Go4).

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Design Patterns classification

• Creational
• Singleton
• Abstract Factory
• Factory Method
• Structural
• Composite
• Behavioral
• Strategy
• Command
• Iterator

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Singleton

• Ensures that at most a single instance exists,
  allows to access it.
• Applications
• Printer spooler.
• An accounting system will be dedicated to serving
  one company.
• Problem How do you keep track of number of
  instances ?

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Singleton Structure

• Note that the constructor is hidden (private)

Singleton static getInstance()method1()method2()
static uniqueInstance
return uniqueInstance
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Decorator

• Attaching and removing additional features to an
  object dynamically (run time) and independently.
• Avoid complicating the API of the base class.
• Examples
• Word processor Create Bold, underline, or BOTH
  characters.
• Windows system windows with scroll bar, border,
  header.

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Notation

• Part of the UML (Unified Modeling Language)

Inheritance
Association
Aggregation
Composite aggregation
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Decorator Structure
Component operation()
ConcreteComponent operation()
Decoratoroperation()
component-gtoperation()
component
ConcreteDecoratorA operation() AddedBehavior()
ConcreteDecoratorB operation() AddedBehavior()
Decoratoroperation() AddedBehavior()
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Strategy

• Encapsulates a family of algorithms, making them
  interchangeable.
• Strategy lets the algorithm vary independently
  from clients that use it.
• Used when
• many related classes differ only in their
  behavior. Strategies provide a way to configure a
  class with one of many behaviors.
• you need different variants of an algorithm
  (space/time trade-off).
• an algorithm uses data that clients shouldn't
  know about.
• a class defines many behaviors, and these appear
  as multiple conditional statements in it's
  operations. Instead of many conditionals, move
  related conditional branches into their own
  Strategy class.
• Examples
• Comparator for data structure.
• Modeling behavior of computer games characters.

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Strategy Structure
StrategyAlgorithm()
strategy
Context
StrategyAAlgorithm()
StrategyBAlgorithm()
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Abstract Factory

• Provides an interface for creating objects
  without specifying their concrete classes.
• When to use
• a system should be independent of how its
  products are created, composed, and represented.
• a system should be configured with one of
  multiple families of products.
• a family of related product objects is designed
  to be used together, and you need to enforce this
  constraint.
• you want to provide a class library of products,
  and you want to reveal just their interfaces, not
  their implementations
• Applications
• Implementation of different look and feel
  interface for windowing system.
• Levels in computer games.
• Improves portability across operating systems.

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Abstract Factory Structure
AbstractFactory createA() createB()
AbstractProductA
Client
ProdA2
ProdA1
ConcreteFactory1 createA() createB()
ConcreteFactory2 createA() createB()
AbstractProductB
ProdB2
ProdB1
Creates
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Consequences (Abstract Factory)

• It isolates concrete classes client not even
  aware of concrete class name !
• It makes exchanging product families easy.
• It promotes consistency among products.
• Supporting new kinds of products is difficult.

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Factory Method

• Define an interface for creating an object, but
  let subclasses decide which class to instantiate
• Example
• Used in Abstract Factory, but also can be used
  independently.

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Factory Method Structure
Product
Creator method1()method2()FactoryMethod()
ConcreteCreator FactoryMethod()
Creates
ConcreteProduct
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More on Design Patterns

• Links
• http//hillside.net/patterns/education.htm
• http//mindview.net/Books/TIPatterns/
• Object Oriented Design course (67615) very
  recommended !!