Software Design Patterns

1

What is a design pattern?

1. It describes a general usable solution to a recurring problem in an environment 2. It describes core of solution in a procedural-like structure

2

Why Design Patterns?

1. Simplifies object identification2. Simplifies system decomposition3. Proven & tested technique for problem solving4. Improves speed & quality of design / implementation5. Can be adapted / refined for specific system under construction

3

Design Patterns Classification

4

Two categoriesclass scope:relationship between classes & subclasses statically defined at run-timeobject scope:object relationships (what type?)Can be manipulated at runtime (so what?)

THE CONCEPT OF PATTERNSConstruction 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

Wallcabinet

Counter

Floorcabinet

Modern Classic Antique Arts & Crafts

menu

display area

styles

9

KitchenViewer Example

Modern Classic Antique Arts & Crafts

Wall cabinets Floor cabinetsCountertop

10

Selecting Antique Style

Modern Classic Antique Arts & Crafts

11

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 PatternrenderKitchen() method is used.

This code would have to be repeated for every styleThe code that is supposed to be duplicated becomes

different in different places.

Example Application: Applying a Design PatternrenderKitchen(myStyle) method is used

KitchenViewer design purpose is implemented by applying Abstract Factory design pattern.

KitchenViewer Without Design Patterns

Kitchen

ClientrenderKitchen()

FloorCabinet

ModernWallCabinet

ModernFloorCabinet AntiqueFloorCabinet

AntiqueWallCabinet

WallCabinet

14

Without Applying Design Patterns

renderKitchen() method have to be repeated for every styleThe method results in more prone-error and far less

maintainable code Sooner and later, 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()

AntiqueKStylegetWallCabinet()getFloorCabinet()

The Abstract Factory Idea

KitchenStylegetWallCabinet()getFloorCabinet()

ModernKStylegetWallCabinet()getFloorCabinet()

WallCabinet FloorCabinet

AntiqueWallCabinet AntiqueFloorCabinet

FloorCabinet getFloorCabinet() { return new AntiqueFloorCabinet(); }

……

FloorCabinet getFloorCabinet() { return new ModernFloorCabinet(); }

17

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

KitchenStylegetWallCabinet()getFloorCabinet()

KitchengetWallCabinet()getFloorcabinet()

ClientrenderKitchen( KitchenStyle )

ModernKStylegetWallCabinet()getFloorCabinet()

AntiqueKStylegetWallCabinet()getFloorCabinet()

WallCabinet FloorCabinet

ModernWallCabinet

ModernFloorCabinet

AntiqueWallCabinet

AntiqueFloorCabinet22

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 harmfulProblemIf 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 PatternProduct

Defines the interface for the type of objects the factory method creates

ConcreteProductImplements the Product interface

CreatorDeclares the factory method, which returns an object of type

ProductConcreteCreator

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 ConcreteProductCreator 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 byAdding 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 factorypublic 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 productpublic 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 variableis 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 PatternsBehavioral 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.Some define class instance Some hand off work from one class to another, and Some provide placeholders for other functionality.

Command PatternAn object that contains a symbol, name or key

that represents a list of commands, actions or keystrokes.

This is the definition of a macroThe Macro represents a command that is built

from the reunion of a set of other commands, in a given order.

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 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. 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 Stockspublic 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( ); }}

// Clientpublic 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 }}