design pattern (abstract factory & singleton)

Design Pattern

Introduction Singleton & Abstract Factory Design

Pattern

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Agenda• Introduction to Design Patterns

o What is a Design Patterno Why Study Design Patternso History of Design Patternso The Gang of Four

• The Singleton Patterno Introductiono Logger Exampleo Lazy Instantiationo Limitations

Abstract Factory Patterno Abstract Factory in real lifeo Exampleo Real life vs Java Objecto Limitations

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What is a Design Pattern?• A problem that someone has already solved.• A model or design to use as a guide

• More formally: “A proven solution to a common problem in a specified context."

Real World Examples• Blueprint for a house• Manufacturing

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Why Study Design Patterns?• Provides software developers a toolkit for handling

problems that have already been solved.

• Provides a vocabulary that can be used amongst software developers.o The Pattern Name itself helps establish a vocabulary

• Helps you think about how to solve a software problem.

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History of Design Patterns

• Christopher Alexander (Civil Engineer) in 1977 wroteo “Each pattern describes a problem which occurs over and over again in our

environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice.”

• Each pattern has the same elementso Pattern Name – helps develop a catalog of common problemso Problem – describes when to apply the pattern. Describes problem and its

context.o Solution – Elements that make up the design, their relationships,

responsibilities, and collaborations.o Consequences – Results and trade-offs of applying the pattern

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The Gang of Four

• Defines a Catalog of different design patterns.

• Three different typeso Creational – “creating objects in a manner suitable for the situation”o Structural – “ease the design by identifying a simple way to realize relationships

between entities”o behavioural– “common communication patterns between objects”

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The Gang of Four: Pattern CatalogCreational

Abstract FactoryBuilderFactory MethodPrototypeSingleton

StructuralAdapterBridgeCompositeDecoratorFaçadeFlyweightProxy

BehavioralChain of ResponsibilityCommandInterpreterIteratorMediatorMementoObserverStateStrategyTemplate MethodVisitor

Patterns in red we will discuss in this presentation

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Singleton Pattern

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Singleton• Definition: “The Singleton Pattern ensures a class has

only one instance, and provides a global point of access to it.”

• Best Useso Loggingo Cacheso Registry Settingso Access External Resources

• Printer• Device Driver• Database

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Example: LoggerWhat is wrong with this code?

public class Logger { public Logger() { }

public void LogMessage() { //Open File "log.txt" //Write Message //Close File } }

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Example: Logger• Since there is an external Shared Resource (“log.txt”),

we want to closely control how we communicate with it.

• We shouldn’t have to create the Logger class every time we want to access this Shared Resource. Is there any reason to?

• We need ONE.

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Logger – as a Singletonpublic class Logger{ private Logger{}

private static Logger uniqueInstance;

public static Logger getInstance() {

if (uniqueInstance == null) uniqueInstance = new Logger();

return uniqueInstance;

}}

Note the parameterless constructor

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Lazy Instantiation

• Objects are only created when it is needed

• Helps control that we’ve created the Singleton just once.

• If it is resource intensive to set up, we want to do it once.

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Singleton Consequences• Controlled access to sole instance facilitates strict

control over when and how the clients access it

• The singleton patter is improvement over global variables.

• It is easy to configure an instance of the application that extends the functionality of singleton at run-time

• More flexible than class operations

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Singleton Limitations

• The main limitation of the singleton pattern is that is permits the creation of only one instance of the class, while most practical applications require multiple instances to be initialized.

• Furthermore, in case of singleton, the system threads fight to access the single instance thereby degrading the performance of the applications.

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Abstract Factory Pattern

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Abstract Factory Pattern is similar to Sub Contracting in real world.Basically delegating the creation of Objects to expert Factories-----------------------------------Orders in a restaurant are received by a Kitchen.Then are assigned to Special Chefs like Chinese, Indian, Continental.

Abstract Factory Pattern is a Creational Pattern.Similar to Factory Pattern it is Object Creation without exposing “HOW” ?

Abstract Factory Pattern in Real Life

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Abstract Factory Pattern in Java

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Factory

Kitchen

Real Life vs Java Object

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1 Orders a Dish from Menu

2Receives the OrderCreates the Dish

4 Delivers the Dish3 Outsources to Chef

How Factory Pattern works in

Real Life ?

KitchenFactory factory = new KitchenFactory();Food dosa = factory.getFood("Dosa");dosa.print();

Food noodles = factory.getFood("Noodles");noodles.print();

public Food getFood(String name) { if (name.equals("Dosa")) { IndianFactory factory = new IndianFactory(); return factory.getFood(name);} else if (name.equals("Noodles")) { ChineseFactory factory = new ChineseFactory(); return factory.getFood(name);}Return null;}

1

4

Food

Dosa Noodles

2

3

Create food from Respective Factory Class

How Factory Pattern works in

Java?

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Abstract Factory Consequences• Isolate concrete classes.

• Make exchanging product families easy.

• Promote consistency among products

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Abstract Factory Limitation

• Supporting new kinds of products is difficult.

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References

Linkso http://en.wikipedia.org/wiki/Design_patterno http://sourcemaking.com/design_patterns

Linkso Design Patterns in Ruby – Russ Olseno Head First Design Patterns – Elisabeth Freeman, Eric Freeman, Bert Bates and

Kathy Sierra

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If(Questions){

Ask; }else {

Thank you; }

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