TDDB84 Design Patterns Lecture 02. Factory Method, Decorator

Transcription

1 Lecture 02 Factory Method, Decorator Peter Bunus Dept of Computer and Information Science Linköping University, Sweden Factory Method Pattern Peter Bunus 2 1

2 Time for Lunch Joe, with the money earned from the duck simulator I bought a Pizza shop. I want to improve a little bit the service here. I m thinking that it would be great if one could order a pizza over the Internet No problem boss. I can fix this. I m calling you back when I m ready. Peter Bunus 3 Ordering Pizza Pizza* orderpizza(){ cout << "Ordering pizza" << endl; Pizza *pizza = new Pizza(); pizza->prepare(); pizza->bake(); pizza->cut(); pizza->box(); return pizza; But we need more than one type of pizza OO inheritance can help us here Peter Bunus 4 2

3 Ordering Pizza Pizza* orderpizza(string type){ cout << "Ordering pizza" << endl; Pizza *pizza; if (type=="cheese"){ pizza = new CheesePizza; else if (type=="greek"){ pizza = new GreekPizza; else if (type=="pepperoni"){ pizza = new PepperoniPizza; pizza->prepare(); pizza->bake(); pizza->cut(); pizza->box(); return pizza; Peter Bunus 5 Joe, We cannot sell Greek pizza anymore. It is extremely hard to find goat cheese for this pizza. We need to have clam and veggie pizza instead Typical, I need to change the code one more time Peter Bunus 6 3

4 Pizza* orderpizza(string type){ cout << "Ordering pizza" << endl; Pizza *pizza; if (type=="cheese"){ pizza = new CheesePizza; else if (type=="greek"){ pizza = new GreekPizza; else if (type=="pepperoni"){ pizza = new PepperoniPizza; else if (type== clam ){ pizza = new ClamPizza; else if (type== veggie ){ pizza = new VeggiePizza; pizza->prepare(); pizza->bake(); pizza->cut(); pizza->box(); return pizza; Peter Bunus 7 The Constitution of Software Architectcts Encapsulate that vary. Program to an interface not to an implementation. Favor Composition over Inheritance. Peter Bunus 8 4

5 Encapsulate that vary Pizza* orderpizza(string type){ cout << "Ordering pizza" << endl; Pizza *pizza; if (type=="cheese"){ pizza = new CheesePizza; else if (type=="greek"){ pizza = new GreekPizza; else if (type=="pepperoni"){ pizza = new PepperoniPizza; else if (type== clam ){ pizza = new ClamPizza; else if (type== veggie ){ pizza = new VeggiePizza; pizza->prepare(); pizza->bake(); pizza->cut(); pizza->box(); return pizza; Encapsulate that vary!!! We should place this code in an object that is only going to worry about how to create pizzas. If any other object need a pizza created this is the object to come to Peter Bunus 9 Pizza Factory Pizza* PizzaFactory::createPizza(string type){ Pizza *pizza; if (type=="cheese"){ pizza = new CheesePizza; else if (type=="pepperoni"){ pizza = new PepperoniPizza; else if (type=="clam"){ pizza = new ClamPizza; else if (type=="veggie"){ pizza = new VeggiePizza; return pizza; orderpizza() orderpizza() orderpizza() PizzaShop Supermarket PizzaDelivery Peter Bunus 10 5

6 Pizza Shop class PizzaStore{ PizzaFactory *factory; public: PizzaStore(PizzaFactory *factory){ factory = p_factory; ; Pizza* orderpizza(string type){ Pizza *pizza; pizza = factory->createpizza(type); return pizza; ; ; orderpizza() int main(){ PizzaFactory *pizzafactory = new PizzaFactory(); PizzaStore *pizzastore = new PizzaStore(pizzaFactory); Pizza *mypizza = pizzastore->orderpizza("pepperoni"); return 0; Peter Bunus 11 Pizza Shop and Factory This is the client of the Factory. PizzaStore goes through the PizzaFactory to get instances of Pizza This is the the factory where we create pizzas; it should be only part of our aplication that refers to concrete Pizza classes This is the product of the factory pizza. We ve defined Pizza as an abstract class with some helpful implementation that can be overriden These are our concrete products. Each product needs to implement the Pizza interface (which means extend the abstract Pizza class) and be concrete. Peter Bunus 12 6

7 Pizza Shop and Factory Do we have a pattern here? NOT YET!!!! Peter Bunus 13 Pizza everywhere Joe, the Pizza store idea is really great. We are making a lot of money. I m out of the duck business. I would like to have pizza shops everywhere: Paris, Rome, Stockholm.. Una pepperoni pizza semplice, ma saporita con il delicato prosciutto, per favore. Je voudrais commander une pizza s'il vous plaît avec boeuf haché et sauce tomate. Je n'aime pas le bacon et le ananas. Jag skulle vilja ha en pepperoni pizza med kötbullar Peter Bunus 14 7

8 Regional Pizza Factories int main(){ ParisPizzaFactory *parispizzafactory = new ParisPizzaFactory(); PizzaStore *parispizzastore = new PizzaStore(parisPizzaFactory); Pizza *mypizza = parispizzastore->orderpizza("pepperoni"); return 0; int main(){ RomePizzaFactory *romepizzafactory = new RomePizzaFactory(); PizzaStore *romepizzastore = new PizzaStore(romePizzaFactory); Pizza *mypizza = romepizzastore->orderpizza("pepperoni"); return 0; Peter Bunus 15 class PizzaStore{ PizzaFactory *factory; public: PizzaStore(PizzaFactory *factory){ factory = p_factory; ; Pizza* orderpizza(string type){ Pizza *pizza; pizza = factory->createpizza(type); createpizza(type); return pizza; ; virtual createpizza(type); ; Pizza* PizzaFactory::createPizza(string type){ Pizza *pizza; if (type=="cheese"){ pizza = new CheesePizza; else if (type=="pepperoni"){ pizza = new PepperoniPizza; else if (type=="clam"){ pizza = new ClamPizza; else if (type=="veggie"){ pizza = new VeggiePizza; return pizza; Now we ve got a store waiting for subclasse Peter Bunus 16 8

9 RomePizzaStore +createpizza() virtual +createpizza() createpizza(type); +orderpizza() PizzaStore ParisPizzaStore +createpizza() createpizza() is abstract, so all pizza store subtypes MUST implement the method Pizza* RomePizzaStore::createPizza(string type){ if (type=="cheese"){ pizza = new RomeCheesePizza; else if (type=="pepperoni"){ pizza = new RomePepperoniPizza; else if (type=="clam"){ pizza = new RomeClamPizza; else if (type=="veggie"){ pizza = new RomeVeggiePizza; return pizza; StockholmPizzaStore Pizza* ParisPizzaStore::createPizza(string type){ if (type=="cheese"){ +createpizza() pizza = new ParisCheesePizza; else if (type=="pepperoni"){ pizza = new ParisPepperoniPizza; else if (type=="clam"){ pizza = new ParisClamPizza; else if (type=="veggie"){ pizza = new ParisVeggiePizza; return pizza; Peter Bunus 17 Do we have a pattern here? PizzaStore +createpizza() +orderpizza() RomePizzaStore ParisPizzaStore StockholmPizzaStore +createpizza() +createpizza() +createpizza() Pizza +prepare() +bake() +cut() +box() RomeCheesePizza RomePepperoniPizza RomeClamPizza RomeVeggiePizza... Peter Bunus 18 9

10 The Factory Method Pattern Provide an interface for creating families of related or dependent objects without specifying their concrete classes. Peter Bunus 19 The Factory Method Pattern The Creator is a class that contains the implementation for all of the methods to manipulate products except for the factory method All products must implement the same interface so that the classes which use the products can refer to the interface not to the concrete class The abstract FactoryMethod() is what all Creator subclasses must implement. The ConcreteCreator is responsible for creating one or more concrete products. It is the only class that has the knowledge of how to create these products The ConcreteCreator implements the Factory Method() which is the method that actually produce products. Peter Bunus 20 10

11 The Factory Method Advantages/Disadvantages Eliminates the need to bind application-specific classes into your code Provides hooks for subclassing. Creating objects inside a class with a factory method is always more flexible than creating an object directly. This method gives subclasses a hook for providing an extended version of an object Connects parallel heirarchies. Factory method localises knowledge of which classes belong together. Parallel class heirarchies result when a class delegates some of its responsibilities to a separate class. Clients might have to subclass the Creator class just to create a particular Concreate object. Peter Bunus 21 class PizzaStore{ public: Pizza* orderpizza(string type){ Pizza *pizza; pizza = createpizza(type); return pizza; ; protected virtual createpizza(type); ; Provides hooks for subclassing. Creating objects inside a class with a factory method is always more flexible than creating an object directly. This method gives subclasses a hook for providing an extended version of an object. Peter Bunus 22 11

12 Connecting Parallel Class hierarchies Pizza +prepare() +bake() +cut() +box() RomeCheesePizza ParisCheesePizza SthmCheesePizza RomePepperoniPizza ParisPepperoniPizza SthmPepperoniPizza RomeClamPizza ParisClamPizza SthmClamPizza RomeVeggiePizza ParisVeggiePizza SthmVeggiePizza Connects parallel heirarchies. Factory method localises knowledge of which classes belong together. Parallel class heirarchies result when a class delegates some of its responsibilities to a separate class. Peter Bunus 23 Descriptive names class Complex{ public: static Complex fromcartesian(double real,double imag) { return new Complex(real, imag); static Complex frompolar(double rho, double theta) { return new Complex(rho * cos(theta), rho * sin(theta)); private: Complex(double a, double b) { //... ; In many object-oriented languages, constructors must have the same name as the class they are in, which can lead to ambiguity if there is more than one way to create an object. Factory methods have no such constraint and can have descriptive names. As an example, when complex numbers are created from two real numbers the real numbers can be interpreted as cartesian or polar coordinates, but using factory methods, the meaning is clear Complex c = Complex.fromPolar(1, pi); Peter Bunus 24 12

13 Factory Method Non Software Example Peter Bunus 25 Decorator Pattern Peter Bunus 26 13

14 Extending the Business Joe, people are not coming to our pizza places in the morning. They need coffee in the morning. I decided to open a coffee shop next to each pizzeria. Could you please implement an application for ordering coffee? Peter Bunus 27 The First Design of the Coffee Shop No problem boss. I can fix this. I have now experience with the pizza store so this will be a piece of cake The cost() method is abstract; subclasses need to define their implementation Each subclass implements cost() the cost of the beverage Peter Bunus 28 14

15 Class Explosion Beverage -description +getdescription() What I m doing wrong here? Which of the design principles we are violating here? HouseBlend DarkRoast Decaf Expresso Peter Bunus 29 The Constitution of Software Architectcts Encapsulate that vary. Program to an interface not to an implementation. Favor Composition over Inheritance. Peter Bunus 30 15

16 Why do we need so many classes??? We add instance variables to represent whether or not each beverage has milk, soy, mocha and whip... Now we ll implement cost() in Beverage (instead of keeping it abstract), so that it can calculate the costs associated with the condiments for a particular beverage instance. Subclasses will still override cost(), but they will also invoke the super version so that they can calculate the total cost of the basic beverage plus the costs of the added condiments. Peter Bunus 31 Excellent Joe, good job. Five classes. This will decrease the complexity of our ordering system I m not so sure about this. My experience with high management is not so good. They change the requirements all the time. An the customers they want new things all the time Peter Bunus 32 16

17 What can happend? New condiments will appear and will force us to add new methods and change the cost method each time Price changes for condiments so we need to change the cost method. New beverages like iced tea. The iced tee class will still inherit the methods like haswhip(). How about double espresso. Peter Bunus 33 Decorating Coffee Inheritance doesn t worked very well for us. What we should do? Hi Jamie. One of my guys have problem with coffee classes. Could you please help him out. 1. Take the DarkRoast object 2. Decorate it with a Mocha object 3. Decorate it with the Whip object 4. Call the cost() method and relay on delegation to add to the condiment cost. Peter Bunus 34 17

18 Jamie s recipe 1. Take the DarkRoast object cost() Whip cost() Mocha cost() DarkRoast 2. Decorate it with a Mocha object 3. Decorate it with the Whip object 4. Call the cost() method class DarkRoast : public Beverage{ public: DarkRoast(); double cost(); ; class Whip : public CondimentDecorator{ Beverage *beverage; public: Whip(Beverage *p_beverage) ; string getdescription(); double cost(); ; class Mocha : public CondimentDecorator{ Beverage *beverage; public: Mocha(Beverage *p_beverage) ; string getdescription(); double cost(); ; Peter Bunus 35 Barista Training for Sofware Engineers Beverage acts like an abstract component class HouseBlend Decaf Beverage -description +getdescription() class DarkRoast : public Beverage{ public: DarkRoast(); double cost(); ; Expresso DarkRoast * class Beverage{ public: string description; Beverage(); virtual string getdescription(); virtual double cost()=0; ; CondimentDecorator class CondimentDecorator 1 : public Beverage{ +getdescription()() public: CondimentDecorator(){; virtual string getdescription()=0; ; Milk Soy -beverage : Beverage -beverage : Beverage +getdescription() +getdescription() class Mocha : public CondimentDecorator{ Whip Mocha Beverage *beverage; -beverage : Beverage -beverage : Beverage public: Mocha(Beverage *p_beverage){ +getdescription() +getdescription() beverage = p_beverage; ; string getdescription(){ return beverage->getdescription()+ " Whip"; ; double cost(){ return beverage->cost() ; ; Peter ; Bunus 36 18

19 Running the Coffe Shop A Whipped Dark Roast with double Mocha expresso House blend void main(){ cout << "Testing the Coffe Shop application" << endl; Beverage *beverage1 = new Expresso(); cout << beverage1->getdescription() << endl; cout << "Cost: " << beverage1->cost() << endl << endl; Beverage *beverage2 = new DarkRoast(); beverage2 = new Mocha(beverage2); beverage2 = new Mocha(beverage2); beverage2 = new Whip(beverage2); cout << beverage2->getdescription() << endl; cout << "Cost: " << beverage2->cost() << endl << endl; Beverage *beverage3 = new HouseBlend(); cout << beverage3->getdescription() << endl; cout << "Cost: " << beverage3->cost() << endl << endl; Peter Bunus 37 Running the Coffe Shop void main(){ cout << "Testing the Coffe Shop application" << endl; Beverage *beverage1 = new Expresso(); cout << beverage1->getdescription() << endl; cout << "Cost: " << beverage1->cost() << endl << endl; Beverage *beverage2 = new DarkRoast(); beverage2 = new Mocha(beverage2); beverage2 = new Mocha(beverage2); beverage2 = new Whip(beverage2); cout << beverage2->getdescription() << endl; cout << "Cost: " << beverage2->cost() << endl << endl; Beverage *beverage3 = new HouseBlend(); cout << beverage3->getdescription() << endl; cout << "Cost: " << beverage3->cost() << endl << endl; Peter Bunus 38 19

20 How is the Cost Computed?... Beverage *beverage2 = new DarkRoast(); beverage2 = new Mocha(beverage2); beverage2 = new Mocha(beverage2); beverage2 = new Whip(beverage2); cout << beverage2->getdescription() << endl; cout << "Cost: " << beverage2->cost() << endl;... cost() Whip cost() Mocha cost() Mocha cost() DarkRoast double Whip::cost(){ return beverage->cost() ; = 3.55 double Mocha::cost(){ return beverage->cost() + 0.9; double DarkRoast::cost(){ return 0.99; Peter Bunus 39 The Decorator Pattern Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality Peter Bunus 40 20

21 The Constitution of Software Architectcts Encapsulate that vary. Program to an interface not to an implementation. Favor Composition over Inheritance. Classes should be open for extension but closed for modification Peter Bunus 41 Decorating Text Peter Bunus 42 21

22 Decorator Non Software Example Peter Bunus 43 The Decorator Advantages/Disadvantages Provides a more flexible way to add responsibilities to a class than by using inheritance, since it can add these responsibilities to selected instances of the class Allows to customize a class without creating subclasses high in the inheritance hierarchy. A Decorator and its enclosed component are not identical. Thus, tests for object types will fail. Decorators can lead to a system with lots of little objects that all look alike to the programmer trying to maintain the code Peter Bunus 44 22

23 What we have learned? Inheritance is one form of extension, but not necessarily he best way to achieve flexibility in our design In our design we should allow behavior to extended without the need to modify the existing code Composition and delegation can often be used to add new behaviors at runtime The Decorator Pattern involves a set of decorator classes that are used to wrap concrete components Decorators change the behavior of their components by adding new functionality before and/or after (or even in place of) method calls to the component Decorators can result in many small objects in our design, and overuse can be complex Peter Bunus 45 23