1. food and drink
2. cuisines
3. italian cuisine

The Simple Factory Pattern
and
Factory Method Pattern
CSCI 3132 Summer 2011
1
New is an implementa-on • Calling “new” is certainly coding to an implementa-on • In fact, it’s always related to a concrete class • (Open for Modifica-on) • That’s fine when things are simple, but . . . 2
Look Out For Change • When you have several related classes, that’s probably a good sign that they might change in the future Duck duck;!
if (picnic) {!
! duck = new MallardDuck();!
} else if (hunting) {!
! duck = new DecoyDuck();!
} else if (inBathTub) {!
! duck = new RubberDucky();!
}!
3
Instan-a-ng Concrete Classes • Using new instan-ates a concrete class. • This is programming to implementa-on instead of interface. • Concrete classes are oKen instan-ated in more than one place. • Thus, when changes or extensions are made all the instan-a-ons will have to be changed. • Such extensions can result in updates being more difficult and error-­‐prone. 4
Example Suppose that you are required to develop a system that accepts orders for pizzas. There are three types of pizzas, namely, cheese, Greek, and pepperoni. The pizzas differ according to the dough used, the sauce used and the toppings. Draw a class diagram for the system. 5
Design Principle • What should you try to do with code that changes? Pizza orderPizza() {!
! Pizza pizza = new Pizza(); !
!
!
!
!
pizza.prepare();!
pizza.bake();!
pizza.cut();!
pizza.box();!
! return pizza;!
}!
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Pizza orderPizza(String type) {!
! Pizza pizza;!
!
!
!
!
!
!
!
if (type.equals(“cheese”)) {!
!pizza = new CheesePizza();!
} else if (type.equals(“greek”)) {!
!pizza = new GreekPizza();!
} else if (type.equals(“pepperoni”)) {!
!pizza = new PepperoniPizza();!
}!
!
!
!
!
pizza.prepare();!
pizza.bake();!
pizza.cut();!
pizza.box();!
! return pizza;!
}!
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Example:Class Diagram Problems with the design 8
Pizza orderPizza(String type) {!
! Pizza pizza;!
!
!
!
!
!
!
!
!
!
!
!
if (type.equals(“cheese”)) {!
!pizza = new CheesePizza();!
} else if (type.equals(“greek”)) {!
!pizza = new GreekPizza();!
} else if (type.equals(“pepperoni”)) {!
!pizza = new PepperoniPizza();!
} else if (type.equals(“sausage”)) {!
!pizza = new SausagePizza();!
} else if (type.equals(“veggie”)) {!
!pizza = new VeggiePizza();!
}!
! pizza.prepare();!
! pizza.bake();!
! pizza.cut();!
! pizza.box();!
! return pizza;!
}!
9
Pizza orderPizza(String type) {!
! Pizza pizza;!
!
!
!
!
!
!
!
!
!
!
!
if (type.equals(“cheese”)) {!
!pizza = new CheesePizza();!
} else if (type.equals(“greek”)) {!
!pizza = new GreekPizza();!
} else if (type.equals(“pepperoni”)) {!
!pizza = new PepperoniPizza();!
} else if (type.equals(“sausage”)) {!
!pizza = new SausagePizza();!
} else if (type.equals(“veggie”)) {!
!pizza = new VeggiePizza();!
}!
! pizza.prepare();!
! pizza.bake();!
! pizza.cut();!
! pizza.box();!
! return pizza;!
}!
Encapsulate!
10
Revised System • Suppose that the Greek pizza is not popular and must be removed and two new pizzas, Clam and Veggie, must be added to the menu. • Programming to implementa-on like makes such changes difficult. • Crea-ng a SimpleFactory to encapsulate the code that changes will make the design more flexible. • Remove the code that creates a pizza – forms a factory. 11
public class SimplePizzaFactory {!
! public Pizza createPizza(String type) {!
! ! Pizza pizza;!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
if (type.equals(“cheese”)) {!
! pizza = new CheesePizza();!
} else if (type.equals(“pepperoni”)) {!
! pizza = new PepperoniPizza();!
} else if (type.equals(“sausage”)) {!
! pizza = new SausagePizza();!
} else if (type.equals(“veggie”)) {!
! pizza = new VeggiePizza();!
}!
! ! return pizza;!
! }!
}!
Now orderPizza() is tidy
12
public class PizzaStore {!
! SimplePizzaFactory factory;!
! public PizzaStore(SimplePizzaFactory factory) {!
! ! this.factory = factory;!
! }!
! public Pizza orderPizza(String type) {!
! ! Pizza pizza;!
! ! pizza = factory.createPizza(type);!
!
!
!
!
!
!
!
!
pizza.prepare();!
pizza.bake();!
pizza.cute();!
pizza.box();!
No new operator
No new() operator
! ! return pizza;!
! }!
}!
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Example: Revised Class Diagram 14
Simple Factory • Pull the code that builds the instances out and put it into a separate class – Iden-fy the aspects of your applica-on that vary and separate them from what stays the same 15
Simple Factory Pa]ern Client
orderProduct()
SimpleFactory
createProduct()
<<abstract>>
Product
productMethod()
ConcreteProductA
ConcreteProductB
ConcreteProductC
16
Why would we do this? • Mul-ple clients needing same types of object • Ensure consistent object ini-aliza-on 17
Example: System Revision Again Franchises in different parts of the country are now adding their own special touches to the pizza. For example, customers at the franchise in New York like a thin base, with tasty sauce and li]le cheese. However, customers in Chicago prefer a thick base, rich sauce and a lot of cheese. Some franchises also cut the pizza slices differently (e.g. square) You need to extend the system to cater for this. 18
The Factory Method Defini-on: • The factory Method Pa]ern defines an interface for crea-ng an object, but lets the subclasses decide which class to instan-ate. • Factory Method lets a class defer instan-a-on to subclasses. 19
Creator Class 20
Product Class 21
public abstract class PizzaStore {!
! !
! public Pizza orderPizza(String type) {!
! ! Pizza pizza;!
! ! pizza = createPizza(type);!
!
!
!
!
!
!
!
!
pizza.prepare();!
pizza.bake();!
pizza.cute();!
pizza.box();!
No new operator
! ! return pizza;!
! }!
abstract Pizza createPizza(String type);!
}!
22
public class NYPizzaStore extends PizzaStore{!
Pizza createPizza(String item) {!
! !! if (type.equals(“cheese”)) {!
! !! ! return new NYStyleCheesePizza();!
! !! } else if (type.equals(“pepperoni”)) {!
! !! ! return new NYStylePepperoniPizza();!
! !! } else if (type.equals(“veggie”)) {!
! !! ! return new NYStyleVeggiePizza();!
! !! } else return null;!
}!
}!
23
Structure 24
Factory Method Pa]ern <<abstract>>
Product
abstract!
IS-A
ConcreteProduct
<<abstract>>
Creator
factoryMethod()
operation()
IS-A
HAS-A
ConcreteCreator
factoryMethod()
No more SimpleFactory class
Object creation is back in our class, but …
delegated to concrete classes
25
Applicability • Use Factory Method when: – A class can’t an-cipate the class of objects it must create – A class wants its subclasses to specify the objects it creates – Classes delegate to one of several helper subclasses, and you want to localize knowledge about which is the delegate 26
Par-cipants • Product – Defines interface of objects to be created • ConcreteProduct – Implements Product interface • Creator – Declares factory method • Returns object of type Product • May define default implementa-on – May call factory method to create Products 27
Consequences • Eliminates need to bind crea-on code to specific subclasses • May need to subclass Creator for each ConcreteProduct • Provides hooks for subclasses • Connects parallel class hierarchies 28
An Example • Applica-on framework – App subclasses “Applica-on,” “Document” – Framework knows when to create Document, but Doesn’t know what type of Document to create – Factory Method moves knowledge about specific Document subclass out of framework 29
Guidelines • No variable should hold a reference to a concrete class • No class should derive from a concrete class • No method should override an implemented method of an of its base classes. 30
Summary • The creator gives you an interface for wri-ng objects. • The creator specifies the “factory method”, e.g. the createPizza method. • Other methods generally included in the creator are methods that operate on the product produced by the creator, e.g. orderPizza. • Only subclasses implement the factory method. 31
Summary • Simple Factory – Use when you have only one family of objects • Factory Method Pa]ern – Use when you have mul-ple families of objects • Abstract Factory (next -me) – Use when you have mul-ple families of object components 32
Thoughts… • When you have code that instan-ates concrete classes that may change, encapsulate the code that changes. • The factory pa]ern allows you to encapsulate the behaviour of the instan-a-on. • Duplica-on of code is prevented. • Client code is decoupled from actual implementa-ons. • Programming to interface not implementa-on. 33