Factory Method PatternStrategy Pattern

Proxy Pattern

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Factory Method(Creational)

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Factory MethodName: Factory MethodIntent: Define an interface for creating an

object, but let subclasses decide which class to instantiate. Defer instantiation to subclasses.

Problem: A class needs to instantiate a derivation of another class, but doesn't know which one. Factory Method allows a derived class to make this decision.

Solution: Provides an abstraction or an interface and lets subclass or implementing classes decide which class or method should be instantiated or called, based on the conditions or parameters given.

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Basic Structure

Product is the interface for the type of object that the Factory Method creates. Creator is the interface that defines the Factory Method.

Product<<<interface>>>

ConcreteProduct

Creator

FactoryMethod()

<<<interface>>>

ConcreteCreator

FactoryMethod()

<<instantiates>>

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A Case Study: Database Connections

You want to connect to a database but you want to decide at run time which type of database it is (i.e. SQL, Oracle, MySQL etc.)

Apply Factory Method

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Structure of Case study

ConnectionFactory

createConnection() : Connection

<<<interface>>>

MyConnectionFactory

createConnection() : Connection

Connection

description()open()close()

<<<interface>>>

SQLConnection MySQLConnection OracleConnection<<instantiates>>

Instantiates SQLConnection / MySQLConnection / OracleConnection

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Implementation: ConnectionFactory

public interface ConnectionFactory{public Connection createConnection();

}class MyConnectionFactory implements ConnectionFactory{

public String type;public MyConnectionFactory(String t){

type = t; }

public Connection createConnection(){if(type.equals("Oracle")){

return new OracleConnection(); }else if(type.equals("SQL")){

return new SQLConnection(); }else{ return new MySQLConnection(); }

}}

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Implementation: Connection

public interface Connection{public String description();public void open();public void close();

}class SQLConnection implements Connection{

public String description(){ return "SQL";} }class MySQLConnection implements

Connection{public String description(){ return "MySQL” ;} }

class OracleConnection implements Connection{

public String description(){ return "Oracle"; } }

These three implementing classes should have definition for Open & Close Methods of above interface as well to be concrete.

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Implementation: TestConnectionFactory

class TestConnectionFactory{public static void main(String[]args){

MyConnectionFactory con = new MyConnectionFactory("My SQL");

Connection con2 = con.createConnection();

System.out.println("Connection Created: ");

System.out.println(con2.description());}

}

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Strategy Pattern

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Strategy Pattern

Name: StrategyIntent: Define a family of algorithms,

encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Problem: How to design for varying, but related, algorithms or policies?

Solution: Define each algorithm/policy/strategy in a separate class, with a common interface.

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Basic Structure

ConcreteStrategyA ConcreteStrategyB

ContextStrategy

Algorithm()

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Participants

Strategy : declares an interface common to all supported algorithms. Context uses this interface to call the algorithm defined by a ConcreteStrategy.

ConcreteStrategy: implements the algorithm using the Strategy interface.

Context : is configured with a ConcreteStrategy object. maintains a reference to a Strategy object. may define an interface that lets Strategy

access its data.

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Case StudyWe are developing an e-commerce application

where we need to calculate Tax…there are two tax strategies…Tax rate is 10% for old citizens and 15% otherwise.

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Case Study Structure

Citizen OldCitizen

TaxStrategy

calcTax()

<<<interface>>>

ClientTaxContext

setStrategy() : TaxStrategycalculateTax()

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Implementation: TaxStrategy

public interface TaxStrategy{public void calcTax();

}class Citizen implements TaxStrategy{

public void calcTax(){System.out.println("Tax Deduction @

15%");}}

class OldCitizen implements TaxStrategy{public void calcTax(){

System.out.println("Tax Deduction @ 10%");}}

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Implementation: TaxContext

class TaxContext{

private TaxStrategy TS;

public void setStrategy(TaxStrategy ts){ TS=ts; }

public void calculateTax(){ TS.calcTax(); }}

class Client{

public static void main(String[]args){

TaxContext cont = new TaxContext();

cont.setStrategy(new Citizen());

cont.calculateTax();

}

}

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How to Create Strategies

There are different Taxing algorithms or strategies, and they change over time. Who should create the strategy?

A straightforward approach is to apply the Factory pattern

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Strategy and Factory Method

Product<<<interface>>>

ConcreteProduct

Creator

FactoryMethod()

<<<interface>>>

ConcreteCreator

FactoryMethod()

<<instantiates>>

ConcreteStrategyA ConcreteStrategyB

Strategy

Algorithm()

Context

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Case Study Structure

StrategyFactory

getStrategy() : TaxStrategy

<<<interface>>>

MyStrategyCitizen OldCitizen

TaxStrategy

calcTax()

<<<interface>>>TaxContext

setStrategy() : TaxStrategycalculateTax()

Client

<<instantiates>>

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Implementation: StrategyFactorypublic interface StrategyFactory{

public TaxStrategy getStrategy();

}

class MyStrategy implements StrategyFactory{

String type;

public MyStrategy(String t){ type=t; }

public TaxStrategy getStrategy(){

if(type.equals("Citizen"))

return new Citizen();

else

return new OldCitizen();} }

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Implementation: Clientclass Client{

public static void main(String[]args){MyStrategy m = new

MyStrategy("Citizen");TaxContext cont = new TaxContext();cont.setStrategy(m.getStrategy());cont.calculateTax();

}}

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Proxy Pattern

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Proxy Pattern

Name: ProxyIntent: Provide a surrogate or placeholder for

another object to control access to it.Problem: You want to control the access to an

object for different reasons. You may want to delay the creation / initialization of expensive objects or you may want to provide a local representation of a remote object.

Solution: Provide a Stub / placeholder for actual object.

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Basic Structure

Client Subject

someOperation()

<<<interface>>>

Proxy

someOperation()

RealSubject

someOperation()

RealSubject someOperation()

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ParticipantsSubject - Interface implemented by the

RealSubject and representing its services. The interface must be implemented by the proxy as well so that the proxy can be used in any location where the RealSubject can be used.

Proxy- Maintains a reference that allows the Proxy to access the RealSubject. Implements the same interface implemented by the RealSubject so that the Proxy can be substituted for the RealSubject. Controls access to the RealSubject and may be responsible for its creation and deletion.

RealSubject- the real object that the proxy represents.

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Case Study

Consider an image viewer program that lists and displays high resolution photos. The program has to show a list of all photos however it does not need to display the actual photo until the user selects an image item from a list.

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Case Study Structure

ProxyImage

showImage() : void

HRImage

showImage() : void

ImageViewer Image

showImage() : void

<<<interface>>>

showImage() displays the Image only when needed

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Implementation: Imagepublic interface Image{

public void showImage();

}

class HRImage implements Image{

public HRImage(){

System.out.println("loading a High Resolution image");

}

public void showImage(){

System.out.println("Showing a High Resolution Image");

}

}

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Implementation: ProxyImageclass ProxyImage implements Image{

private Image proxyImage;public ProxyImage(){ System.out.println("Loading a proxy image");}public void showImage(){

proxyImage = (HRImage)new HRImage();proxyImage.showImage();} }

class ImageViewer{public static void main(String[]args){

Image HRImage1 = new ProxyImage();Image HRImage2 = new ProxyImage();Image HRImage3 = new ProxyImage();

HRImage1.showImage();} }

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Applicability

Virtual Proxies: delaying the creation and initialization of expensive objects until needed, where the objects are created on demand

Remote Proxies: providing a local representation for an object that is in a different address space. A common example is Java RMI stub objects. The stub object acts as a proxy where invoking methods on the stub would cause the stub to communicate and invoke methods on a remote object (called skeleton) found on a different machine.

Protection Proxies: where a proxy controls access to RealSubject methods, by giving access to some objects while denying access to others.

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The End