design pattern intro+ factory method pattern

Design Patterns

What are they?

• Repeatable solution to commonly occurring problems

• Language-independent strategies for solving common object oriented design problems

• They provide a lingo

Is-A Relationship(Inheritance)

Animal

Has-A Relationship(Composition)

Animal a = new Animal()

Dog d = new Dog()

Animal aDog = new Dog()

Pizza Store

Prepare

Bake

Cut

Box

Create

Bake

Cut

Box

Menu

Cheese Pizza

Veggie Pizza

Spicy Chicken Pizza

Barbeque Chicken Pizza

Cheese Pizza

Veggie Pizza

Spicy Chicken Pizza

Barbeque Chicken Pizza

Pizza

Is-A Relationship

MumbaiThin crustFresh VeggiesLess sauce

PuneThick crustFrozen VeggiesLots of sauce and cheese

What kinda pizza’s do people in Mumbai and Pune like?

Store Locations

Mumbai Pune

How would the customer’s order for a pizza be?

Choose store location

Choose pizza

Mumbai

Cheese Pizza

public static void main(String[] a){ PizzaStore p = new PizzaStore(); p.orderPizza(“Pune”,”Cheese”);}

class Main {

}

class PizzaStore{

Pizza createPizza(store, pizzaType){ if(store==“Mumbai”){ if(type==“Cheese”) return new MumbaiStyleCheesePizza(); if(type==“SpicyChickenPizza”) return new MumbaiStyleSpicyChickenPizza(); …. }else if(store==“Pune”){ if(type==“Cheese”) ….}

void orderPizza(store, pizzaType){ pizza = createPizza(store,pizzaType); pizza.bake(); pizza.cut(); pizza.box();}

}

Methods in the Pizza class or its sub-classes

if(store==“Mumbai”){ if(type==“Cheese”) return new MumbaiStyleCheesePizza(); if(type==“SpicyChickenPizza”) return new MumbaiStyleSpicyChickenPizza(); ….}else if(store==“Pune”){ if(type==“Cheese”) ….}

Is this good code?

What if we open a new store in Gurgaon?What if we add a new type of Pizza?What if we stop making a pizza?

if (store==“Gurgaon”){…}

If(type==“Margherita” new margheritaPizza()

If(type==“Veggie”) new veggiePizza()

Design Principle

Classes should be open for extension but closed for modification

class PizzaStore{

}

….

Design Principle

Separate out what changes from what remains the same

void createPizza(store, pizzaType){..}

void orderPizza(store,pizzaType){..}

How do we do that?

MumbaiStore PuneStore

PizzaStore

GurgaonStore ChennaiStore

abstract class PizzaStore{

abstract Pizza createPizza(type);

void orderPizza(pizzaType){ pizza = createPizza(pizzaType); pizza.prepare(); pizza.bake(); pizza.cut(); pizza.box();}

}

Pizza pizza;

class MumbaiStore extends PizzaStore{

public Pizza createPizza(String pizzaType){ if(pizzaType.equals(“Cheese”)) return new MumbaiStyleCheesePizza(); if(pizzaType.equals(“Veggie”)) return new MumbaiStyleVeggiePizza();

….}

}

MumbaiStore PuneStore

PizzaStore

Code

PizzaStore.java

//Has reference to pizza (Pizza pizza;)

// Has orderPizza(String type) method which internally calls:// pizza = createPizza(type)// pizza.bake()// pizza.cut()// pizza.box()

//Has an// abstract createPizza(String type) method that returns Pizza

PuneStore.java

//extends PizzaStore

// Pizza createPizza(String type) method//returns pizza object based on the type

MumbaiStore.java

//extends PizzaStore

// Pizza createPizza(String type) method//returns pizza object based on the type

Factory Method

abstract Pizza createPizza(type);

abstract Product factoryMethod(String type);

A factory method is abstract so that the subclasses handle the object creation.

A factory method returns a product that is typically used within methods defined in the super class.

A factory method isolates the client code in the superclass from knowing what kind of concrete product is actually created.

A factory method may be parameterised to select among several variations of a product.

Customers!!!

Joel’s order

I need Pune style Pizza.PizzaStore puneStore = new PuneStore();

I want Cheese PizzapuneStore.orderPizza(“Cheese”);

Internally within the orderPizza methodPizza pizza = createPizza(“Cheese”);pizza.bake();pizza.cut();pizza.box();

MainClass.java

//public static void main(String[] a)

PizzaStore puneStore = new PuneStore();puneStore.orderPizza(“Cheese”);

MumbaiStyleCheesePizza PuneStyleCheesePizza

Pizza

Pizza.java

// Has bake() method// Has cut() method// Has box() method

MumbaiStyleCheesePizza.java//extends Pizza class

// Overrides bake() method

PuneStyleCheesePizza.java//extends Pizza class

// Overrides bake() method

Factory Method Pattern

Factory Pattern encapsulates the object creation by letting subclasses decide what objects to create.

Creator Classes

MumbaiStore PuneStore

PizzaStore

GurgaonStore ChennaiStore

Product Classes

Cheese Pizza

Veggie Pizza

Spicy Chicken Pizza

Barbeque Chicken Pizza

Pizza

Product

Concrete Product

Creator

Concrete Creator

factoryMethod()otherMethods()

factoryMethod()

Thank you!