1. Design by Abstraction (Continuation) CS 3331 Spring 2005

2. Outline Design Patterns
Template method
Strategy pattern
Factory pattern
CS 3331

3. Template Methods Intent Participants
To define the skeleton of an algorithm by deferring some steps to subclasses
To allow subclasses to redefine certain steps
Participants
Generic Class defines abstract hook methods and implements template methods
Concrete Class implements hook methods
GenericClass
templateMethod()
hookMethod1()
hookMethod2()
ConcreteClass …
CS 3331

4. Template Methods (Cont.)
Terminology
Template methods: methods containing hook methods
Hook methods: placeholders for the behaviour to be implemented differently by subclasses
Frozen spots: fixed behaviours of generic classes represented by template methods
Hot spots: changeable behaviours of generic classes represented by hook methods
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5. Example Complex numbers Rectangular Polar imaginary (x, y) (r, a) r y
real
imaginary
Polar
(r, a) r a
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6. Example Template Methods Hook Methods ComplexNumber add() mul()
realPart()
imaginaryPart()
magnitude()
angle()
RectangularComplex
PolarComplex
Template Methods
Hook Methods
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7. Strategy Design Pattern
Intent
To define a family of algorithms, encapsulate each one, and make them interchangeable
Participants
Context maintains references to Strategy objects
Strategy declares a common interface to all supported algorithms
ConcreteStrategy implements the algorithm using the Strategy interface
Context
contextMethod()
Strategy
ConcreteStrategyA
algorithm()
alogorithm()
ConcreteStrategyB
CS 3331

8. Factory Design Pattern
Intent
To decouple object creation from its use and to support different way of creating objects
To define an interface for creating objects but let subclasses decide which class to instantiate and how
Participants
Abstract Factory defines a factory method that returns a Product object
ConcreteFactory overrides the factory method to return an instance of ConcreteProduct
Product defines an interface of the objects that the factory will create
ConcreteProduct implements the product interface
Product
AbstractFactory
makeProduct()
creates
ConcreteFactory
makeProduct()
ConcreteProuct
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9. Example Complex numbers Complex ComplexFactory makeComplext()
PolarFactory
makeComplex()
PolarComplex
creates
RectangularFactory
makeComplex()
RectangularComplex
creates
CS 3331