Software Design Patterns 1 Gang of Four (GoF, made up of Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides.

Software Design Patterns 1 Gang of Four (GoF, made up of Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides

What is Design Pattern?  Design patterns are “descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.” — GANG OF FOUR  Design patterns offer solutions to common application design problems.  In object-oriented programming, design patterns are normally targeted at solving the problems associated with object creation and interaction, rather than the large-scale problems faced by the overall software architecture.  They provide generalized solutions in the form of boilerplates that can be applied to real-life problems.

 Design Patterns are visualized using a class diagram They show the behaviors and relations between classes  Diagram shows the inheritance relationship between three classes.  The subclasses CheckingAccount and SavingsAccount inherit from their abstract parent class BankAccount.

public class SingleObject { //create an object of SingleObject private static SingleObject instance = new SingleObject(); //make the constructor private so that this class cannot be instantiated private SingleObject(){} //Get the only object available public static SingleObject getInstance() { return instance; } public void showMessage() { System.out.println("Hello World!"); } }

public class SingletonPatternDemo { public static void main(String[] args) { //illegal construct //Compile Time Error: The constructor SingleObject() is not visible //SingleObject object = new SingleObject(); //Get the only object available SingleObject object = SingleObject.getInstance(); //show the message object.showMessage(); } }

Why Design Patterns? object identification  Simplifies object identification system decomposition  Simplifies system decomposition Proven & tested technique  Proven & tested technique for problem solving Improves speed & quality  Improves speed & quality of design / implementation adapted / refined  Can be adapted / refined for specific system under construction 6

Design Patterns Classification 7 Two categories Class Scope: Relationship between classes & subclasses statically defined at run-time Object Scope: Object relationships (what type?) Can be manipulated at runtime (so what?)

Common Aspects of Object-Oriented Languages  Design pattern models are based and depend highly on the aspects of object-oriented languages.  Patterns-based programming doesn’t make much sense outside such languages. Aspects of object-oriented languages  Aspects of object-oriented languages like encapsulation, polymorphism, abstraction, and inheritance all extend their properties into patterns-based coding.  Patterns methodology  Patterns methodology is an extension of object- oriented methodology.

Encapsulation  Encapsulation is one of the most important aspects of object-oriented languages.  The rule of encapsulation is  one of keeping things private  masked inside the domain of an object, package, namespace, class, or interface  The rule of encapsulation allows only expected access to pieces of functionality.  The rule of encapsulation is used in almost every aspect of OOP  This rule allows us to build patterns like facades, proxies, bridges, and adapters.

Encapsulation  Encapsulation allows us to hide with an interface or class structure some functionality  we do not wish to be publicly or globally known.  Encapsulation allows us to define scope inside our programs, and helps us to define and group modules of logic.  Encapsulation provides ways to allow objects to communicate without that communication getting either too complex  Encapsulation provides rules of engagement between different code bases  It helps us decide what functionality can be known and what needs to be hidden.

Encapsulation Example class CustomerPayment { public double PostPayment (int loanId, double payment) {.....performs post of payment to customer account } public ArrayList GetAmortizedSchedule(int loanId) {...returns an amortization schedule in array }

Encapsulation Example  The two methods in the code are visible as public. CustomerPayment  We can assume that the CustomerPayment class has many other methods and code to help perform some of the functions of its two public methods  we cannot access them outside the class code  they are in effect invisible to any other classes in the code domain.  This gives us proper encapsulation of the class methods  Only the required methods to be accessed are allowed.  The method of encapsulation for this class  PostPayment()  GetAmortizedSchedule() Thet can be accessed outside the class.

Polymorphism  Polymorphism is another important aspect of object-oriented programming.  The rule of polymorphism states that classes can be altered according to their state  in effect making them a different object based on values of attributes, type, or function.  We use polymorphism in almost every coding situation, especially patterns.  This rule gives us the power to use abstraction and inheritance,  It allows inherited members to change according to how they implement their base class.

Polymorphism  It also allows us to change a class’s purpose and function based on its current state.  Polymorphism helps interfaces change their implementation class type simply by allowing several different classes to use the same interface.  Polymorphic declarations of specific implementations of classes with a common base or interface are extremely common in object- oriented code

Polymorphism Example if(IsAntiqueAuto) AntiqueAuto auto = new AntiqueAuto(); int cylinders = auto.Cylinders(); int numDoors = auto.NumberDoors(); int year = auto.Year(); string make = auto.Make(); string model = auto.Model(); }

Polymorphism Example  The first thing you need to answer is why do we want to change this code?  The answer might be one of portability: You wish to have many car types and pass the logic of creating them via the class itself, rather than via a logical Boolean statement.  Or you might wish to make sure all auto classes have the same methods so your code accessing the object always knows what to expect.

Polymorphism Example  In the code example below we can see an interface, IAuto, and below it some classes that we have modified to implement this interface. 

Polymorphism Example public interface IAuto { int Cylinders(); int NumberDoors(); int Year(); string Make(); string Model(); } class AntiqueAuto : IAuto.... class SportsCar : IAuto.... class Sedan : IAuto..... class SUV : IAuto..... Each class also implements the interface’s method, Each functioning in its own way, returning values according to the logic specific to the implemented methods on each class.

class AntiqueAuto : IAuto { public int Cylinders() { return 4; } public int NumberDoors() { return 3; } public int Year() { return 1905; } public string Make() { return "Ford"; } public string Model() { return "Model T"; } }

 we instantiate the AntiqueAuto class as an instance of the IAuto interface and call each of the interface methods.  Each method returns a value from the methods implemented on the AntiqueAuto class IAuto auto = new AntiqueAuto(); int cylinders = auto.Cylinders(); int numDoors = auto.NumberDoors(); int year = auto.Year(); string make = auto.Make(); string model = auto.Model();

Polymorphism Example  If we changed the implemented class to SportsCar or another auto type, the methods would return different values.  This is how polymorphism comes into play in class relationships.  By changing the class type for a common interface or abstraction, we can change the functionality and scope of the code without having to code if...then...else statements to accomplish the same thing.

Inheritance and Abstraction  Inheritance and abstraction are also very important features of object-oriented languages.  They provide a way to make polymorphic representations of objects and object relationships that can be managed at run time or compile time..

Inheritance  Inheritance is the ability of one object to be derived by creating a new class instance from a parent or base class  overloading the constructor(s), methods, attributes of that parent object  implementing them in the instance.

Inheritance is important  Many times an object contains some base functionality that another object also needs  Instead of maintaining the same logic in two objects  They can share and even override or change this functionality by using a base or parent class.

Inheritance  The base or parent object should be defined in such a way that several common derived objects can use the same common functionality from the parent.  The parent should only contain functionality common to all its children

Abstraction  Abstraction is the actual method in which we use inheritance.  Abstraction is the ability to abstract into a base class some common functionality or design that is common to several implementation or instanced classes.  The difference between implementation and abstract classes is  Abstractions of classes cannot be instanced,  Implementations can be instanced  Abstraction and inheritance are both aspects of polymorphism  The reverse is also true

Collections of Objects  Another important aspect of object-oriented languages is how they deal with collections of objects.  The equals implementation for objects is an important aspect of dealing with objects inside a collection.  Languages like C#, VB.NET, and Java all use this method to help index and compare objects in collections.

Collections of Objects indexes and compares  When a hash table or other collection object indexes and compares an object, it uses the GetHashCode() method  This method can be overridden to capture a more accurate sampling of the intrinsic properties or state of the object.  GetHashCode() method can return an integer representation of the concatenated state of the properties within an object.

Collections of Objects  The default implementation of GetHashCode() in objects that contain state variables or values is not guaranteed to be unique.  To provide a complete representation of state, the values of each variable that represents the object’s state need to be part of the hashing algorithm. public override int GetHashCode() { return _name.GetHashCode() ^ _address.GetHashCode(); }

Collections of Objects The proper operational sequence usually starts with a null check, then a class type comparison, and then a comparison of all the value types (or reference types) that influence the state of the class: public override bool Equals(object obj) { if(obj != null && obj is Component) return _name.Equals(((Component)obj).Name) && _address.Equals(((Component)obj).Address); else return false; }

Collections of Objects There are some basic rules when testing the equals implementation for proper return values:  obj1.Equals(obj1) = true — an object always equals itself.  obj1.Equals(obj2) = obj2.Equals(obj1) — equals implementations across different class instances always return true on both classes if equal.  obj1.Equals(obj2) && obj2.Equals(obj3) && obj3.Equals(obj1) = true — if object 1 is equal to object 2 and object 2 is equal to object 3, then object 3 must be equal to object 1.  All calls to Equals() return the same value unless the class’s state or internal value is modified.  Equals(null) always returns false.

THE CONCEPT OF PATTERNS Construction Architecture Patterns

 The first idea of using patterns was for building and proposed by the architect Christopher Alexander.  He found recurring themes in architecture, and captured them into descriptions He called them patterns.  The term 'pattern' appeals to the replicated similarity in a design  The similarity makes room for variability and customization in each of the elements

 Alexander defines: «Each pattern is a three part rule which express a relation between a certain context, a problem and a solution.  Each pattern is a relationship between a certain context, a certain system of forces which occurs repeatedly in that context a certain spatial configuration which allows these forces to resolve themselves.  A pattern is an instruction and shows how this configuration can be used over and over again.  The pattern is a thing that happens in the world  The rule which tell us how to create that thing and when we must create it.

KitchenViewer Interface: An architectural pattern example Wall cabinet Counter Floor cabinet  ModernClassicAntiqueArts & Crafts menu display area styles 36

KitchenViewer Example ModernClassicAntiqueArts & Crafts Wall cabinets Floor cabinets Countertop 37

Selecting Antique Style ModernClassicAntiqueArts & Crafts 38

Specific Design Purposes for KitcherViewer  The procedure of rendering the various styles is basically the same regardless of the style. The code is as follows: Counter counter =new Counter(); draw (counters);  A single block of code that executes in several possible ways, depending on the context  Polymorphism  An application must construct a family of objects at runtime.  The design must enable choice among several families of styles

An Introduction to Design Pattens  Example Application: Without applying a Design Pattern  renderKitchen() method is used. This code would have to be repeated for every style The code that is supposed to be duplicated becomes different in different places.  Example Application: Applying a Design Pattern  renderKitchen(myStyle) method is used KitchenViewer design purpose is implemented by applying Abstract Factory design pattern.

KitchenViewer Without Design Patterns Kitchen Client renderKitchen() FloorCabinet ModernWallCabinet ModernFloorCabinetAntiqueFloorCabinet AntiqueWallCabinet WallCabinet 41

Without Applying Design Patterns  renderKitchen() method have to be repeated for every style  The method results in more prone-error and far less maintainable code  The code that is supposed to be duplicated becomes different in different places.  The result is repetitive and complicated.  It is inflexible, hard to prove correct, and hard to reuse

Applying Abstract Factory Design Pattern  The object will have responsibility for creating the kitchen.  Instead of creating the object directly (for example AntiqueWallCabinet objects), a parameterized version is used for renderKitchen()  At run time, the class of myStyle determines the version of getWallCabinet()executed.  The KitchenStyle method is introduced and called This class has subclasses, and each support separate implementations of getWallCabinet() and getFloorCabinet()

AntiqueKStyle getWallCabinet() getFloorCabinet() The Abstract Factory Idea KitchenStyle getWallCabinet() getFloorCabinet() ModernKStyle getWallCabinet() getFloorCabinet() WallCabinetFloorCabinet AntiqueWallCabinetAntiqueFloorCabinet FloorCabinet getFloorCabinet() { return new AntiqueFloorCabinet(); } …… FloorCabinet getFloorCabinet() { return new ModernFloorCabinet(); } 44

 KitchenViewer design purpose is implemented by applying Abstract Factory design pattern.  AntiqueWallCabinet objects are not created directly.  A parameterized version of renderKitchen() delegates their creation such as the following: new AntiqueWallCabinet();//applies only to antique style. myStyle.getWallCabinet(); //applies to the style chosen at run time.. Processing the Abstract Factory Pattern: KitchenViewer

Processing the Abstract Factory Pattern: KitchenViewer At run time, the class of myStyle determines the version of getWallCabinet() and produces the appropriate kind of wall cabinet

 To carry out this process, a new class KitchenStyle is introduced.  KitchenStyle supports the methods getWallCabinet(), getFloorCabinet() and so on.  KitchenStyle have subclasses ModernStyle, AntiqueStyle. Processing the Abstract Factory Pattern: KitchenViewer

 Due to the polymorphism, executing myStyle.getFloorCabinet() has differently effects when myStyle is an object of ModernKStyle versus an object of AntiqueKStyle.  Client code references Kitchen, KitchenStyle,WallCabinet and FloorCabinet, but does not appear in the client code.

Abstract Factory Design Pattern Applied to KitchenViewer KitchenStyle getWallCabinet() getFloorCabinet() Kitchen getWallCabinet() getFloorcabinet() Client renderKitchen( KitchenStyle ) ModernKStyle getWallCabinet() getFloorCabinet() AntiqueKStyle getWallCabinet() getFloorCabinet() WallCabinetFloorCabinet ModernWallCabinet ModernFloorCabinet AntiqueWallCabinet AntiqueFloorCabinet 49

Abstract Factory Design Pattern Properties  Provide an interface for creating families of related or dependent objects without specifying their concrete classes.  A hierarchy that encapsulates: many possible platforms, and the construction of a suite of products.  The new operator considered harmful Problem  If an application is to be portable, it needs to encapsulate platform dependencies.  These platforms might include: windowing system, operating system, database…

 The Abstract Factory defines a Factory Method per product.  Each Factory Method encapsulates the new operator and the concrete, platform-specific, product classes.  Each platform is then modeled with a Factory derived class. General Structure: Abstract Factory Pattern

The Factory Method Pattern Product  Defines the interface for the type of objects the factory method creates ConcreteProduct  Implements the Product interface Creator  Declares the factory method, which returns an object of type Product ConcreteCreator  Overrides the factory method to return an instance of a ConcreteProduct  Creator relies on its subclasses to implement the factory method so that it returns an instance of the appropriate ConcreteProduct

The Factory Method Pattern  Code is made more flexible and reusable by the elimination of instantiation of application-specific classes  Code deals only with the interface of the Product class and can work with any ConcreteProduct class that supports this interface  Clients might have to subclass the Creator class just to instantiate a particular ConcreteProduct  Creator can be abstract or concrete Should the factory method be able to create multiple kinds of products? If so, then the factory method has a parameter (possibly used in an if-else!) to decide what object to create

Factory Design Pattern  Online bookstores that can choose different book distributors to ship the books to the customers  Both BookStoreA and BookStoreB choose which distributor (EastCoastDistributor or MidWestDistributor or WestCoastDistributor) to use based on the location of the customer.  This logic is in each bookstore's GetDistributor method.

Abstract Factory Pattern  The abstract factory design pattern is an extension of the factory method pattern,  The abstract factory pattern allows to create objects without being concerned about the actual class of the objects being produced.  The abstract factory pattern extends the factory method pattern by allowing more types of objects to be produced.

Extension of GetDistributor() Method to Abstract Factory Pattern We can extend GetDistributor() method by  Adding another product that the factories can produce.  In this example, we will add Advertisers that help the bookstores advertise their stores online.  Each bookstore can then choose their own distributors and advertisers inside their own GetDistributor and GetAdvertiser method.

public void Advertise(IBookStore s) { IAdverister a = s.GetAdvertiser(); a.Advertise(); }

This allows to have client code (calling code) such as: public void Advertise(IBookStore s) { IAdverister a = s.GetAdvertiser(); a.Advertise(); }  Regardless if you pass in BookStoreA or BookStoreB into the method, this client code does not need to be changed since it will get the correct advertiser automatically using the internal logics within the factories.  It is the factories (BookStoreA and BookStoreB) that determines which advertiser to produce.  The same goes for choosing which book distributor to produce

Abstract Factory Design Pattern

The Benefit of the Abstract Factory Pattern  The benefit of the Abstract Factory pattern is that it allows you to create a groups of products (the distributors and the advertisers) without having to know the actual class of the product being produced.  The result is that you can have client code that does not need to be changed when the internal logic of the factories changed.  We can change the types of the products (the distributors and the advertisers) by changing the code in the factories (the bookstores) without changing the client code

public enum CustomerLocation { EastCoast, WestCoast } class Program { static void Main(string[] args) { IBookStore storeA = new BookStoreA(CustomerLocation.EastCoast); Console.WriteLine("Book Store A with a customer from East Coast:"); ShipBook(storeA); Advertise(storeA); IBookStore storeB = new BookStoreB(CustomerLocation.WestCoast); Console.WriteLine("Book Store B with a customer from West Coast:"); ShipBook(storeB); Advertise(storeB); }

//**** client code that does not need to be changed *** private static void ShipBook(IBookStore s) { IDistributor d = s.GetDistributor(); d.ShipBook(); } //**** client code that does not need to be changed *** private static void Advertise(IBookStore s) { IAdvertiser a = s.GetAdvertiser(); a.Advertise(); }

//the factory public interface IBookStore { IDistributor GetDistributor(); IAdvertiser GetAdvertiser(); } //concrete factory public class BookStoreA : IBookStore { private CustomerLocation location; public BookStoreA(CustomerLocation location) { this.location = location; }

IDistributor IBookStore.GetDistributor() { //internal logic on which distributor to return / /*** logic can be changed without changing the client code **** switch (location) { case CustomerLocation.EastCoast: return new EastCoastDistributor(); case CustomerLocation.WestCoast: return new WestCoastDistributor(); } return null; }

IAdvertiser IBookStore.GetAdvertiser() { //internal logic on which distributor to return //*** logic can be changed without changing the client code **** switch (location) { case CustomerLocation.EastCoast: return new RedAdvertiser(); case CustomerLocation.WestCoast: return new BlueAdvertiser(); } return null; } } //end of factory class

public class BookStoreB : IBookStore //concrete factory { private CustomerLocation location; public BookStoreB(CustomerLocation location) { this.location = location; } IDistributor IBookStore.GetDistributor() { switch (location) { case CustomerLocation.EastCoast: return new EastCoastDistributor(); case CustomerLocation.WestCoast: return new WestCoastDistributor(); } return null; } IAdvertiser IBookStore.GetAdvertiser() {switch (location) { case CustomerLocation.EastCoast: return new BlueAdvertiser(); case CustomerLocation.WestCoast: return new RedAdvertiser(); } return null; } }

//the product public interface IDistributor { void ShipBook(); } //concrete product public class EastCoastDistributor : Idistributor { void IDistributor.ShipBook() { Console.WriteLine("Book shipped by East Coast Distributor"); } } //concrete product public class WestCoastDistributor : IDistributor { void IDistributor.ShipBook() { Console.WriteLine("Book shipped by West Coast Distributor"); } }

public interface IAdvertiser //the product { void Advertise(); } public class RedAdvertiser : IAdvertiser //concrete product { void IAdvertiser.Advertise() { Console.WriteLine("Advertised by RedAdvertiser"); } } public class BlueAdvertiser : IAdvertiser //concrete product { void IAdvertiser.Advertise() { Console.WriteLine("Advertised by BlueAdvertiser"); }

Structural Design Patterns  In software engineering, structural design patterns are design patterns that ease the design by identifying a simple way to realize relationships between entities  The adapter pattern is a design pattern that is used to allow two incompatible types to communicate.  Where one class relies upon a specific interface that is not implemented by another class, the adapter acts as a translator between the two types.

Adapter pattern  Adapter pattern is structural pattern which defines a manner for creating relationships between objects.  This pattern translates one interface for a class into another compatible interface.  Adapter pattern is newer used when creating a new system.  It is usually implemented when requirements are changed and we must implement some functionality of classes which interfaces are not compatible with ours.

Client: represents the class which need to use an incompatible interface. This incompatible interface is implemented by Adaptee. ITarget: defines a domain-specific interface that client uses. In this case it is an simple interface, but in some situations it could be an abstract class which adapter inherits. In this case methods of this abstract class must be overriden by concrete adapter. Adaptee: represents a class provides a functionality that is required by client. Adapter: is concrete implementation of adapter. This class translates incompatible interface of Adaptee into interface of Client. Adapter Design Pattern

static class Program { static void Main() { var client = new Client(new Adapter()); client.Request(); } public interface ITarget { void MethodA(); } public class Client { private readonly ITarget _target; public Client(ITarget target) { _target = target; } public void Request() { _target.MethodA(); } }

public class Adaptee { public void MethodB() { Console.WriteLine("Adaptee's MethodB called"); } public class Adapter : ITarget { readonly Adaptee _adaptee = new Adaptee(); public void MethodA() { _adaptee.MethodB(); }

Bridge Pattern Simple inheritance cannot meet the immediate needs where abstraction is not desired. When abstraction or inheritance is used, you are tied to the exact definition of that abstraction. Some cases would require classes not to be inherited  instead we would like to adapt other classes to act as the desired type without modifying either class.

 The adapter houses an instance variable of the desired type to adapt as a private instance variable.  This instance variable is not changeable in the class. This means it is not set as an abstract or base variable but as a concrete type. We hide this instance variable’s methods, properties, and events behind overridden methods, properties

Behavioral Patterns  Behavioral patterns are patterns whose purpose is to facilitate the work of algorithmic calculations and communication between classes.  They use inheritance to control code flow.  They define and produce process and run-time flow and identify hierarchies of classes and when and where they become instantiated in code. class instance  Some define class instance work from one class to another  Some hand off work from one class to another, and placeholdersother functionality.  Some provide placeholders for other functionality.

Command Pattern  An object that contains a symbol, name or key that represents a list of commands, actions or keystrokes.  This is the definition of a macro  The Macro represents a command that is built from the reunion of a set of other commands, in a given order. without knowing requested operation requesting object  Just as a macro, the Command design pattern encapsulates commands (method calls) in objects allowing us to issue requests without knowing the requested operation or the requesting object.

Command Pattern  The Command pattern has three main components: the commands and the receiver The invoker component acts as a link between the commands and the receiver houses the receiver and the individual commands as they are sent. The command is an object that encapsulates a request to the receiver. each request. The receiver is the component that is acted upon by each request.

Command Pattern  Command design pattern provides the options to queue commands, undo/redo actions other manipulations. Intent  encapsulate a request in an object  allows the parameterization of clients with different requests  allows saving the requests in a queue

Placing Orders for Buying and Selling Stocks public interface Order { public abstract void execute ( ); } // Receiver class. class StockTrade { public void buy() { System.out.println("You want to buy stocks"); } public void sell() { System.out.println("You want to sell stocks "); }

// Invoker. class Agent { private m_ordersQueue = new ArrayList(); public Agent() { } void placeOrder(Order order) { ordersQueue.addLast(order); order.execute(ordersQueue.getFirstAndRemove()); }

//ConcreteCommand Class. class BuyStockOrder implements Order { private StockTrade stock; public BuyStockOrder ( StockTrade st) { stock = st; } public void execute( ) { stock. buy( ); }

//ConcreteCommand Class. class SellStockOrder implements Order { private StockTrade stock; public SellStockOrder ( StockTrade st) { stock = st; } public void execute( ) { stock. sell( ); }

// Client public class Client { public static void main(String[] args) { StockTrade stock = new StockTrade(); BuyStockOrder bsc = new BuyStockOrder (stock); SellStockOrder ssc = new SellStockOrder (stock); Agent agent = new Agent(); agent.placeOrder(bsc); // Buy Shares agent.placeOrder(ssc); // Sell Shares }

Software Design Patterns 1 Gang of Four (GoF, made up of Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides.

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Software Design Patterns 1 Gang of Four (GoF, made up of Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides.

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