Factory Method Explained

Intent  Define an interface for creating an object, but let subclasses decide which class to instantiate.  Factory Method lets a class defer instantiation to subclasses.  a.k.a. Virtual Constructor  The main intent of the virtual constructor idiom in C++ is to create a copy of an object or a new object without knowing its concrete type and this is exactly what the Factory Method does  Can a constructor be virtual? 2

Motivation  Frameworks use abstract classes to define and maintain relationships between objects  A framework is often responsible for creating these objects as well  They must instantiate classes but only know about abstract classes - which they cannot instantiate  Factory method encapsulates knowledge of which subclass to create - moves this knowledge out of the framework 3

Motivation (Cont’d)  Consider a framework for presenting multiple documents to the user  Key abstraction in this framework are Application and Document classes  Both classes are abstract and realized in subclasses  Application class is responsible for managing and creating the Documents as and when required. 4

Motivation (Cont’d)  The particular Document subclass to instantiate is application specific, and  The Application class can’t predict the subclass of Document to instantiate. As it only knows when a new document should be created. NOT  What kind of a Document to create.  This creates a dilemma… 5

Motivation (Cont’d)  Factory method solves this problem. It encapsulates the knowledge of which Document to create and moves this knowledge out of the framework. factory method 6

Motivation (Cont’d)  Application subclasses redefine an abstract CreateDocument operation to return Document subclasses  Once an Application subclass is instantiated, it can then instantiate application specific Documents  We call CreateDocument a factory method as it’s responsible for manufacturing a object. 7

Applicability  Use the Factory Method pattern when  A class can’t anticipate the class of objects it must create  A class wants it’s subclasses to specify the objects it creates  Classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate. 8

Structure 9

Participants  Product (Document)  defines the interface of objects the factory method creates  Concrete Product (MyDocument)  Implements the product interface  Creator (Application)  declares the factory method which return a Product type.  May also define a default implementation of the factory method that returns a default ConcreteProduct object  May call the factory method to create a Product object.  Concrete Creator (MyApplication)  Overrides the factory method to return Concrete Product 10

Collaborations  Creator relies on it’s subclasses to define the factory method so that it returns an instance of the appropriate ConcreteProduct(Subclasses of Product) 11

Consequences  Factory methods eliminate the need to bind application specific classes into your code  The code only deals with the Product interface; therefore it can work with any user defined ConcreteProduct classes.  A potential disadvantage of using Factory pattern is that client also has to subclass the Creator class in order to create a particular ConcreteProduct object. 12

Consequences (Cont’d)  Provides hooks for subclasses  Creating objects inside a class with a factory method is always more flexible than creating an object directly  Factory method gives subclasses a hook for providing an extended version of an object  Example: the Document class could define a factory method called CreateFileDialog for opening an existing document. 13

Consequences (Cont’d)  Connects Parallel Class Hierarchies  Parallel Class hierarchies result when a class delegates some of it’s responsibilities to a separate class  Clients can use factory methods to connect between parallel class hierarchies 14

Consequences (Cont’d)  The figure class provides a CreateManipulator factory method that lets clients create a Figure’s corresponding Manipulator  Figure subclasses override this method to return an instance of the required manipulator subclass  Alternatively the Figure class may implement CreateManipulator to return a default manipulator. The figure subclasses may inherit the default.  The figure classes that do so need no corresponding Manipulator subclasses. Hence partially parallel class hierarchies  This is how the factory method defines connection between these class hierarchies 15

Implementation Styles  Two Major Varieties  The case when the Creator class is an abstract class and does not provide an implementation of factory method  The case when the Creator class is a concrete class and has a default implementation of the factory method  The first case requires subclasses to define an implementation as there is no reasonable default  In the second case the subclasses or concrete Creator uses factory method primarily for flexibility. 16

Implementation Styles (Cont’d)  Parameterized factory methods  create multiple kinds of products  the factory method takes a parameter that identifies the kind of object to create  the object share the Product interface  Naming conventions  The name of the Factory method in code should clearly mark its purpose. 17

Implementation  So the situation is that you have a set of related concrete classes and there may be some conditions to call any one of them 18

Implementation (Cont’d)  When we see code like this we know that when it comes to changes and extensions, we will have to re-open this code and examine what needs to be added or deleted.  Often this type of code ends up in several parts of an application making maintenance and updates more difficult and error prone.  By coding to an interface we know we can insulate ourselves with lot of changes occurring down the road  However if you have concrete classes then you might end up in trouble if more concrete classes are added  In other words code would not be closed for modification  So what’s the solution? 19

Implementation (Cont’d) 20  Remember the design principle “Identify the aspects that vary and separate them from what stays the same”  We will now see an example implementation of Pizza shop to illustrate the application of this design principle  We will have a function of OrderPizza() to determine one type of Pizza and run different processes for making a pizza.

Implementation (Cont’d) 21 public class Pizza { Pizza pizza; //Declarations public Pizza() { } //Constructor of Pizza //Declarations of Pizza Functions public virtual void Prepare() {} public virtual void Bake() { } public virtual void Cut() { } public virtual void Box() { } //Method of Ordering a Pizza public void OrderPizza() { pizza = new Pizza(); pizza.Prepare(); pizza.Bake(); pizza.Cut(); pizza.Box(); } //Return only one type of Pizza

Implementation (Cont’d) 22  The concrete classes of Pizza like cheese pizza and pepperoni pizza public class CheesePizza : Pizza { public CheesePizza() { Console.WriteLine("Constructing CHEESE PIZZA"); } public override void Prepare() { Console.WriteLine("Preparing CHEESE PIZZA"); } public override void Bake() { Console.WriteLine("Baking CHEESE PIZZA"); } public override void Cut() { Console.WriteLine("Cutting CHEESE PIZZA"); } public override void Box() { Console.WriteLine("Boxing CHEESE PIZZA"); } }

Implementation (Cont’d) 23 public class PepperoniPizza : Pizza { public PepperoniPizza() { Console.WriteLine("Constructing Pepperoni Pizza"); } public override void Prepare() { Console.WriteLine("Preparing Pepperoni Pizza"); } public override void Bake() { Console.WriteLine("Baking Pepperoni Pizza"); } public override void Cut() { Console.WriteLine("Cutting Pepperoni Pizza"); } public override void Box() { Console.WriteLine("Boxing Pepperoni Pizza"); } }

Implementation (Cont’d) 24  Now we will add code that determines different types of pizza and then goes about making it. The order pizza function will change public void OrderPizza(String type){ if (type.Equals("Cheese")){ pizza = new CheesePizza(); //Calling object of Subclass for Cheese Pizza } else if (type.Equals("Pepperoni")) { pizza = new PepperoniPizza();//Calling object of Subclass for Pepperoni Pizza } pizza.Prepare(); pizza.Bake(); pizza.Cut(); pizza.Box(); }

Implementation (Cont’d) 25  We have now passed the type of pizza (string type) to OrderPizza function  Based on the type of pizza we instantiate the correct concrete class and assign it to the pizza instance variable.  Note that each pizza here would have to implement the pizza interface.  So each pizza type knows how to prepare itself.  But the pressure in on to add or subtract pizza types

Implementation (Cont’d) 26 public void OrderPizza(String type){ if (type.Equals("Cheese")){ pizza = new CheesePizza(); } else if (type.Equals("Pepperoni")){ pizza = new PepperoniPizza(); } // “here we can add more pizza types” pizza.Prepare(); pizza.Bake(); pizza.Cut(); pizza.Box(); } This part of code is changing This part of code is supposed to remain unchanged

Implementation (Cont’d) 27  Clearly we can now make out what’s variable in our code and we need to separate it  What we can do is that place the object creation code into another object (class) whose sole purpose is to create different types of pizza  This object can be called a SimplePizzaFactory  Once we have a factory the OrderPizza() becomes a client of that factory. Anytime it needs a pizza it asks the factory to make one  OrderPizza will get a correct pizza and would call prepare(),bake(),cut() and box()

Implementation (Cont’d) 28  Here is the new class SimplePizzaFactory public class SimplePizzaFactory { public SimplePizzaFactory() { } public Pizza CreatePizza(String type)//This method would be used by all to create new objects { Pizza pizza = null; if (type.Equals("Cheese")) { pizza = new CheesePizza(); //Calling object of Subclass for Cheese Pizza } else if (type.Equals("Pepperoni")) { pizza = new PepperoniPizza();//Calling object of Subclass for Pepperoni Pizza } return pizza; }

Implementation (Cont’d) 29  The orderpizza method also be moved out of the Pizza class and would be changed as under public class PizzaStore { SimplePizzaFactory factory = new SimplePizzaFactory(); //Creates the factory public PizzaStore() { } public void OrderPizza(String type) { Pizza pizza; pizza = factory.CreatePizza(type); //Get the correct pizza from Factory pizza.Prepare(); pizza.Bake(); pizza.Cut(); pizza.Box(); }//Method of Ordering a Pizza }

Implementation (Cont’d) 30

Implementation (Cont’d) 31  Now the Pizza class would be changed as under public abstract class Pizza //Superclass Pizza { public Pizza() { } //Contructor of Pizza //Declarations of Pizza Funtions public virtual void Prepare() { } public virtual void Bake() { } public virtual void Cut() { } public virtual void Box() { } }

Implementation (Cont’d) 32  The calling program for our code class Program { static void Main(string[] args) { PizzaStore pizzastore = new PizzaStore();// The client just knows the pizza store pizzastore.OrderPizza("Pepperoni"); Console.ReadLine(); }

Quiz # 1 33  Elements of a Design Pattern with an example  Describe Scope Based Design Patterns Classification?