ECE 355 Design Patterns Tutorial ECE 355 Design Patterns Tutorial Part 3Part 3

Presented by Igor Ivković

iivkovic@swen.uwaterloo.ca

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Something Fun Something Fun

CreationalPatterns

StructuralPatterns

BehaviouralPatterns

http://home.earthlink.net/~huston2/dp/patterns.html

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AgendaAgenda

Design Patterns

Factory Method

o Adapter

o Bridge

o Command

o State

o Proxy

Summary and References

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Factory Method / 1Factory Method / 1

Intent: This method allows subclasses to decide which class is to be instantiated while defining an interface for creating an object

Also Known As: Virtual Constructor

Applicability:

– When the class of objects to be created cannot be estimated by the class creating them

– The subclasses must specify the objects created by the class

– Information about the helper subclass to which classes have delegated responsibility has to be localized

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Factory Method / 2Factory Method / 2

Pattern Structure:

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Factory Method / 3Factory Method / 3

Code / 1:

public class Test{

public static void main( String arg[] ){

try{ Creator creator = new

ConcreteCreator(); creator.anOperation(); }catch( Exception e ){ e.printStackTrace(); }

}

public interface Product{}http://vico.org/pages/PatronsDisseny/Pattern%20Factory%20Method/

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Factory Method / 4Factory Method / 4

Code / 2:

// Declares the factory method, // which returns an object of type Product. // May also define a default implementation of factory // method that returns a default ConcreteProduct object, // or may call the factory method to create a Product object

public abstract class Creator{

public void anOperation(){ Product product = factoryMethod(); }

protected abstract Product factoryMethod();}

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Factory Method / 5Factory Method / 5

Code / 3:

// Overrides the factory method to return // an instance of a ConcreteProduct.

public class ConcreteCreator extends Creator{

protected Product factoryMethod(){

return new ConcreteProduct();}

}

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AgendaAgenda

Design Patterns

Factory Method

Adapter

o Bridge

o Command

o State

o Proxy

Summary and References

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Adapter / 1Adapter / 1

Intent: allow classes with incompatible interfaces to work together, by converting the interface of a given class into one expected by the clients

Also Known As: Wrapper

Applicability:

– The interface of an existing class does not match the need

– The creation of a reusable class that can work with unforeseen or unrelated classes with incompatible interfaces

– Adapting interfaces of several existing subclasses is not feasible, in which case the interface of the parent class can be adapted by the object adapter

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Adapter / 2Adapter / 2

Pattern Structure – Class Adapter:

– Uses multiple inheritance to adapt one interface to another

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Adapter / 3Adapter / 3

Pattern Structure – Object Adapter:

– Relies on object composition

adaptee

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Adapter / 4Adapter / 4

Code / 1:

public class Test{

public static void main( String arg[] ){

try{ Adapter adapter = new

Adapter();adapter.request(); }

catch( Exception e ){ e.printStackTrace(); }

}}

http://www.vico.org/pages/PatronsDisseny/Pattern%20Adapter%20Class/

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Adapter / 5Adapter / 5

Code / 2:

// Defines the domain specific interface that Client uses

public interface Target{

void request();}

// Adapts the interface of Adaptee to the Target interface

public class Adapter extends Adaptee implements Target

{public void request(){ specificRequest(); }

}

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Adapter / 6Adapter / 6

Code / 3:

// Defines an existing interface that needs adapting

public class Adaptee{

public void specificRequest(){}

}

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Adapter / 7Adapter / 7

Example / 1:

Consider a plug-in architecture:

– We have an existing plug-in PluginX for one software (e.g., FirstBrowser)

– We have similar software (e.g., SecondBrowser) with a different plug-in interface

– We want to use the PluginX as part of the SecondBrowser without re-writing it

Courtesy of: Ali Razavi

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Adapter / 8Adapter / 8

Example / 2:

FirstBrowser

<<Interface>>

FirstBrowserPlugin

StartPlugin():void

PluginX

<<Interface>>

SecondBrowserPlugin

InitPlugin():void

Start():void

SecondBrowser

XAdapter

StartPlugin():void

InitPlugin();

Start();

+

+

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AgendaAgenda

Design Patterns

Factory Method

Adapter

Bridge

o Command

o State

o Proxy

Summary and References

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Bridge / 1Bridge / 1

Intent: The decoupling of an abstraction from its implementation in order to vary both independently

Also Known As: Handle / Body

Applicability:

– To avoid permanent binding between an abstraction and its implementation

– Both the abstraction and its implementation should be extensible through subclasses

– The clients should not be impacted by changes in the abstraction implementation

– The client is hidden from the fact that multiple objects share the same implementation

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Bridge / 2Bridge / 2

Pattern Structure:

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Bridge / 3Bridge / 3

Code / 1:

public class Test{

public static void main( String arg[] ){

try{ Abstraction abstraction = new

RefinedAbstraction();Implementor implementor = new

ConcreteImplementorB();

abstraction.setImplementor( implementor );abstraction.operation(); }

catch( Exception e ){ e.printStackTrace(); }

}}

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Bridge / 4Bridge / 4

Code / 2:

// Extends the interface defined by Abstraction

public class RefinedAbstraction extends Abstraction{

public void operation(){ super.operation(); }

}

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Bridge / 5Bridge / 5

Code / 3:

// Defines the interface for implementation classes that does not have to exactly correspond to Abstraction's interface. Usually the Implementor interface provides only primitive operations, and Abstraction defines higher-level operations based on these primitives

public interface Implementor{

void operation();}

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Bridge / 6Bridge / 6

Code / 4:

// Implements the Implementor interface and defines its conrete implementation

public class ConcreteImplementorB implements Implementor

{ public void operation(){ } }

// Implements the Implementor interface and defines its concrete implementation

public class ConcreteImplementorA implements Implementor

{ public void operation(){ } }

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Bridge / 7Bridge / 7

Code / 5:

// Defines the abstraction's interface, and maintains a reference to an object of type Implementor

public class Abstraction{

private Implementor implementor;public void setImplementor( Implementor implementor ){ this.implementor = implementor; }public Implementor getImplementor(){ return implementor; }public void operation(){ implementor.operation(); }

}

http://www.vico.org/pages/PatronsDisseny/Pattern%20Bridge/

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Bridge / 8Bridge / 8

Example:

The JDBCAdapter class adapts information in a database table to appear in Swing Jtable component.

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Bridge / 9Bridge / 9 Example Code / 1:

– Common use: Overall design of an application that uses drivers. Application development is separated from the development of the drivers that implement the application's abstract operations

– The JDBC architecture decouples an abstraction from its implementation so that the two can vary

// To use a JDBC driver, you load it, connect to a database, and create a Statement object

Class.forName(driverName); Connection c = DriverManager.getConnection(url,

user, passwd); Statement s = c.createStatement();

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Bridge / 10Bridge / 10

Example Code / 2:

// The variable s is a Statement object, capable of issuing SQL queries that return result sets.

ResultSet r = s.executeQuery( "select name, apogee from firework");

while(r.next()) {

String name = r.getString("name"); int apogee = r.getInt("apogee");System.out.println(name + ", " + apogee);

}

http://www.awprofessional.com/articles/article.asp?p=29302&rl=1

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AgendaAgenda

Design Patterns

Factory Method

Adapter

Bridge

Command

o State

o Proxy

Summary and References

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Command / 1Command / 1

Intent: A request is encapsulated as an object, allowing the parameterization of clients with different requests, queuing or logging of requests, and supporting undoable operations

Also Known As: Action, Transaction

Applicability:

– Parameterization of objects with an action to perform

– Specification, queuing, and execution of requests at different times, where a command object can have a lifetime independent of original request

– Support of logging changes that can be reapplied if the system crashes

– A system of high-level operations built on primitives operations

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Command / 2Command / 2

Pattern Structure:

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Command / 3Command / 3

Code / 1:

public class Test{

public static void main( String arg[] ){ try

{ Client client = new Client();Command command =

client.setup();command.execute(); }

catch( Exception e ){ e.printStackTrace(); }

}}

http://www.vico.org/pages/PatronsDisseny/Pattern%20Command/

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Command / 4Command / 4

Code / 2:

// Knows how to perform the operations associated with carrying out a request. Any class may serve as a Receiver

public interface Receiver{ void action();}

// Implementation of the Receiver interface

public class ConcreteReceiver implements Receiver{ public void action()

{ }}

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Command / 5Command / 5

Code / 3:

// Defines a binding between a Receiver object and an action. Implements Execute by invoking the corresponding operation on Receiver

public class ConcreteCommand implements Command{

private Receiver receiver;public void setReceiver( Receiver receiver ){ this.receiver = receiver; }public Receiver getReceiver(){ return receiver; }public void execute(){ receiver.action(); }

}

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Command / 6Command / 6

Code / 4:

// Declares an interface for executing an operation

public interface Command{ void setReceiver( Receiver receiver );

Receiver getReceiver(); void execute(); }

// Creates a ConcreteCommand object & specifies its receiver

public class Client{ public Command setup()

{ Command command = new ConcreteCommand();Receiver receiver = new ConcreteReceiver();command.setReceiver( receiver );

// Return the command so that the Invoker may use itreturn command; } }

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AgendaAgenda

Design Patterns

Factory Method

Adapter

Bridge

Command

State

o Proxy

Summary and References

37

State / 1State / 1

Intent: When the internal state of an object changes it is allowed to alter its behaviour. The object appears to change its class

Also Known As: Objects for States

Applicability:

– An object must undergo change in its behaviour at run-time depending on its state

– Operations that have large, multipart conditional statements that depend on the object’s state

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State / 2State / 2

Pattern Structure:

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State / 3State / 3 Code / 1:

public class Test{ public static void main( String arg[] )

{ try { State state = new ConcreteStateA();

Context context = new Context();context.setState( state );context.request(); }

catch( Exception e ){ e.printStackTrace(); } } }

// Defines an interface for encapsulating the behavior associated with a particular state of the Context

public interface State{ void handle(); }

http://www.vico.org/pages/PatronsDisseny/Pattern%20State/

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State / 4State / 4

Code / 2:

// Defines the interface of interest to clients. Maintains an instance of a ConcreteState subclass that defines the current state

public class Context{ private State state;

public void setState( State state ){ this.state = state; }public State getState(){ return state; }public void request(){ state.handle(); }}

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State / 5State / 5

Code / 3:

// Each subclass implements a behavior associated with a state of the Context

public class ConcreteStateB implements State{ public void handle()

{ }}

// Each subclass implements a behavior associated with a state of the Context

public class ConcreteStateA implements State{ public void handle()

{ } }

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AgendaAgenda

Design Patterns

Factory Method

Adapter

Bridge

Command

State

Proxy

Summary and References

43

Proxy / 1Proxy / 1

Intent: “Provide a surrogate or placeholder for another object to control access to it”

Also Known As: Surrogate

Applicability:

– Need for a more versatile or sophisticated reference to an object than a simple pointer

• Remote, virtual, protection proxies

• Smart proxy

Design Patterns – Gamma et. al.

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Proxy / 2Proxy / 2

Pattern Structure:

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Proxy / 3Proxy / 3 Code / 1:

public class Test{ public static void main( String arg[] )

{ try { Subject real = new RealSubject();Proxy proxy = new Proxy();proxy.setRealSubject( real );proxy.request(); }

catch( Exception e ){ e.printStackTrace(); } } }

// Defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected

public interface Subject{ void request(); }

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Proxy / 4Proxy / 4

Code / 2:

// Defines the real object that the proxy represents

public class RealSubject implements Subject{ public void request()

{ // Do something based on the interface }}

// Maintains a reference that lets the proxy access the real subject. Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same. Provides an interface identical to Subject's so that a proxy can by substituted for the real subject

http://www.vico.org/pages/PatronsDisseny/Pattern%20Proxy/

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Proxy / 5Proxy / 5

Code / 3:

public class Proxy implements Subject{ private Subject realSubject;

public void setRealSubject( Subject subject ){ realSubject = subject; }public Subject getRealSubject(){ // This may not be possible if the proxy is communicating over a network

return realSubject; }public void request(){ // This is very simplified. This could actually be a call to a different run-time environment locally, a machine over a TCP socket, or something completely different.

realSubject.request(); }}

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AgendaAgenda

Design Patterns

Factory Method

Adapter

Bridge

Command

State

Proxy

Summary and References

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Tutorial SummaryTutorial Summary

In this tutorial, we have presented the following design patterns:

– Factory Method, Adapter, Bridge, Command, State, and Proxy

We have demonstrated intent, applicability, and illustrative examples for each pattern

We have also discussed pointers for further study

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ReferencesReferences

E. Gamma, R. Helm, R. Johnson, H. Vlissides, Design Patterns, Addison-Wesley, 1994.

B. Bruegge and A. H. Dutoit, Object-Oriented Software Engineering, Prentice Hall, 2004.