1

Proxy Design PatternProxy Design Pattern• Intent

– Provide a surrogate or placeholder for another object to control access to it.

• An example– Image handling in a web browser

• When a high-resolution image is embedded in an HTML file,– Only a bounding box (fake/virtual image) is drawn first, and coordinated with

surrounding text. » because most users are not patient enough to keep watching blank browser

windows until both text and image are downloaded.

– Whenever the image is downloaded, the bounding box is replaced with the real image.

– The image is gradually drawn from a lower to higher resolutions.

2

<<interface>>Image

ImageProxy ImageImpl

draw()getExtent()

ImageProxy()draw()

drawBBox()getExtent()

fetchImage()

image

1

Browser

extent

draw()getExtent()

extent

3

<<interface>>Image

ImageProxy ImageImpl

draw()getExtent()

ImageProxy()draw()

drawBBox()getExtent()

fetchImage()

image

1

Browser

extent

draw()getExtent()

extent

Obtained from a HTML file(width=“100” height=“50”)Or, the default extent used.

if( image == null){ drawBBox( getExtent() );} else { image.draw(); }

if( image == null){ return extent;} else { image.getExtent(); }

Image img = new ImageProxy(…);img.draw();

Create a thread, and using the thread, start downloading a target image from a remoteweb site. Once it is done, instantiate Image and call draw()

fetchImage(…);

4

Iterator Design PatternIterator Design Pattern

• Intent– Provides a uniform way to access/traverse the

elements of an aggregate object (e.g., collection object) sequentially without exposing its underlying representation (data structure).

• A “uniform” access/traversal way for different types of aggregate objects

– Abstract access mechanisms for collection types, and encapsulate that in the iterator.

• Separate collections from access mechanisms (i.e., how to access collections)

• Hide access mechanisms from collection users.

5

An Example in Java An Example in Java • Collection collection = ….;

java.util.Iterator iterator = collection.iterator();while ( iterator.hasNext() ) {

Object o = iterator.next();System.out.print( o );

}

– Different access mechanisms must be implemented for different collection types (e.g., list, bag, tree and table).

– Collection users can use the the same/uniform way to access different collection types.

• There are so many collection types in Java.

6

Class StructureClass StructureClient (or user)

《 interface》Iterator

《 interface》Collection

LinkedList VectorListIterator VectorIterator

<-- create use -->

Users knows thesetwo interfaces.

Users do not haveto know this level.This level is hiddenfrom users.

hasNextnext()

iterator()add()remove()…

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What’s Hidden from Users? What’s Hidden from Users? Client (or user)

《 interface》Iterator

《 interface》Collection

LinkedList VectorListIterator VectorIterator

hasNextnext()

iterator()add()remove()…

iterator()…

return new VectorIterator(this);

VectorIterator(Vector vec)hasNext()

-vector

1

this.vector = vec;this.index = 0;this.size = vec.size();

if( size == index ) return false;return true;return vec.get( index );

index: intSize: int

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Key PointsKey Points• User's point view

– java.util.Iterator iterator = collection.iterator();– Iterator instances always implement the Iterator interface.

– No need to know what specific implementation class is returned/used.

• In fact, VectorIterator does not exist in Java API. • The API documentation provides no information about

implementation classes.• Simple contract: get an interator with iterator() and call next() on that

– No need to change user code when implementation classes are changed.

– Important Principle: Program to an interface, not an implementation

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• Collection developer’s point view – No need to change existing implementation classes (collection

access mechanisms) when a new collection type is introduced.

– Iterator and its implementation classes form the Strategy pattern.

– iterator() is a factory method.

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Factory MethodFactory Method• a.k.a.

– Virtual constructor• A normal method that creates instances.

• Intent– Let a class (or interface) defer instantiation of subclasses (or

implementation classes). • At compile time, users do not need to determine and write what

subclasses (impl classes) to instantiate. The decision can be deferred to runtime.

– Separate class instantiation from users, and encapsulate that in a factory method.

• Hide instantiation from users of subclasses (impl classes)

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Client (or user)

《 interface》Iterator

《 interface》Collection

LinkedList VectorListIterator VectorIterator

hasNextnext()

iterator()iterator()add()remove()…

iterator()…

return new VectorIterator(this); return new VectorIterator(this);

VectorIterator(Vector vec)hasNext()

-vector

1

this.vector = vec;this.index = 0;this.size = vec.size();

if( size == index ) return false;return vec.get( index );

index: int

An Example: IteratorAn Example: Iterator

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Another Example: ProxyAnother Example: Proxy

13

Recap: Proxy Design PatternRecap: Proxy Design Pattern

<<interface>>Image

ImageProxy ImageImpl

draw()getExtent()

ImageProxy()draw()

drawBBox()getExtent()

fetchImage()

image

1

Browser

extent

draw()getExtent()

extent

Obtained from a HTML file(width=“100” height=“50”)Or, the default extent used.

if( image == null){ drawBBox( getExtent() );} else { image.draw(); }

if( image == null){ return extent;} else { image.getExtent(); }

Image img = new ImageProxy(…);img.draw();

Create a thread, and using the thread, start downloading a target image from a remoteweb site. Once it is done, instantiate Image and call draw()

fetchImage(…);

14

<<interface>>Image

<<interface>>ImageProxy

<<interface>>AbstractImageImpl

draw()getExtent()

ImageProxy()draw()

drawBBox()getExtent()

fetchImage()

image

1

Browser

draw()getExtent()

Factory Method in ProxyFactory Method in Proxy

JpegImgProxy SvgImgProxy

JpegImage SvgImage

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List<Image> imgList = htmlFile.getImages();for each img of imgList { img.draw(); }

<<interface>>Image

<<interface>>ImageProxy

<<interface>>AbstractImageImpl

draw()getExtent()

ImageProxy()draw()

drawBBox()getExtent()

fetchImage()

image

1

Browser

draw()getExtent()

JpegImgProxy SvgImgProxy

JpegImage SvgImage

Factory method forimpl classes of AbstractImageImpl

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Other ExamplesOther Examples

• Object pool– Connection pool– Thread pool

• Singleton

17

Singleton Design PatternSingleton Design Pattern

• Guarantee that a class has only one instance.

– public class Singleton{private static Singleton instance = null;private Singleton(){}public static synchronized Singleton getInstance() {

if( instance==null){instance = new Singleton(); }

return instance; }}

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ExamplesExamples

• Object pools

• Cache

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Colleague

Colleague1 Colleague2

Mediatormediator

ConcreteMediator

Mediator Design PatternMediator Design Pattern• Intent

– Define a class that encapsulates how a set of objects interact.– Control the interaction in a centralized (single) place– Promote loose-coupling among objects

1

20

OrderProcParticipant

Faculty

Mediator mediator

OrderProcMediator

1initOrderProc(f:OrderProcParticipant)

initOrderProc(f:OrderProcParticipant)getStatus(): OrderProcStatus

Order Account

orderInitiator order account

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OrderProcParticipant

Faculty

Mediator mediator

OrderProcMediator

1initOrderProc(f:OrderProcParticipant)

initOrderProc(f:OrderProcParticipant, o: Order, a: Accout)getStatus(): OrderProcStatus

Order Account

orderInitiator order account

If( f’s type is not Faculty) throw an exception;orderInitiator = f;order = o;account = a;f is the owner of a? order.getItem().getCost() < account.balance()?status = OrderProcStatus.AUTHORIZED

returns status;

What if a more complicatedbusiness Process is implemented?Purchaser, supervisor, auditor, etc. etc.

22

IMParticipant

PremiumUser RegularUser

Mediatormediator

ConfCallMediator

1

showMsg(msg: String)register(p: IMParticipant)

sendMsg(msg: String)

showAd(ad: Ad)

pUsers

rUsers

register(p: IMParticipant)sendMsg(msg: String)

23

IMParticipant

PremiumUser RegularUser

Mediatormediator

ConfCallMediator

1

showMsg(msg: String)register(p: IMParticipant)

sendMsg(msg: String)

showAd(ad: Ad)

pUsers

rUsers

register(p: IMParticipant)sendMsg(msg: String)

if( p’s type is PremiumUser ){ pUsers.add(p); p.log();}if( p’s type is RegularUser) rUser.add(p);

for each of pUsers { each.showMsg( msg );}for each of rUsers { each.showAd( new Ad(…) ); each.showMsg( msg );}

The Strategy pattern used.What if new user types are added?

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Code Reuse in OOPCode Reuse in OOP

• Three categories of code reuse– Inherit an existing class

• SSLServerSocket extends ServerSocket

– Instantiate an existing class• new Thread(new YourOwnRunnable() )

– Combine instances at runtime• Decorator design pattern

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When is Instance Combination Useful?When is Instance Combination Useful?• class VisualString{

private String string; public void draw(){ // draw a string on the screen } }

• You want to draw a box around the string.– class BoxedString extends VisualString{

public void draw(){ // redefine how to draw a string } }

• You want to draw a string in color.– Class ColoredString extends VisualString{

public void draw(){ // redefine how to draw a string } }

• What if you want to draw a box around a colored string? • What if you want to draw a box around an icon?

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• class Customer{ public double getAnnualFee(){ … } }

• class DomesticCustomer extends Customer{ public double getAnnualFee(){ … } }

• class InternationalCustomer extends Customer{ public double getAnnualFee(){ … } }

• What if you want to set up – domestic/international cooperate, individual and government

customers – VIP discount rate employee discount rate?

• What if you want to change the status of a customer from non-VIP to VIP or domestic to international?

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Decorator Design PatternDecorator Design Pattern

• a.k.a. pluggable objects

CustomerCustomerCorporateCorporateDomesticDomestic CustomerCustomerindividualindividualIntlIntlVIPVIP

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Sample Code: Structure of ClassesSample Code: Structure of Classes<<interface>><<interface>>

EntityEntity

getAnnualFee()getAnnualFee()

CustomerCustomer

getAnnualFee()getAnnualFee()

EntityDecoratorEntityDecorator{abstract}{abstract}

getAnnualFee()getAnnualFee()

entityentity

DomesticDecoratorDomesticDecorator IntlDecoratorIntlDecorator

getAnnualFee()getAnnualFee()

IndividualDecoratorIndividualDecorator CorporateDecoratorCorporateDecorator

getAnnualFee()getAnnualFee()

VIPDecoratorVIPDecorator

getAnnualFee()getAnnualFee()

GovDecoratorGovDecorator

getAnnualFee()getAnnualFee()

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Sample Code: Instance CombinationSample Code: Instance Combination• Customer customer =

new Customer();customer.getAnnualFee(); --> 100.0

• Customer customer = new DomesticDecorator( new CorporateDecorator( new Customer() ) );customer.getAnnualFee(); --> 1000.0

– The kernel + 2 skins (decorators)

CustomerCustomerCorporateCorporateDomesticDomestic

CustomerCustomer

main()main()

getAnnualFee()getAnnualFee()

100100

100*10100*1010001000

10001000

30

• new InternationalDecorator( new GovernmentDecorator( new Customer() ) );

• new VIPDecorator( new IndividualDecorator( new Customer() ) );

CustomerCustomerGovGovIntlIntl

main()main()

getAnnualFee()getAnnualFee()

100100

100*3100*3300300

300*1.2300*1.2360360

CustomerCustomerIndividualIndividualVIPVIP

main()main()

getAnnualFee()getAnnualFee()

100100

100100

100*0100*000

31

Java I/O APIJava I/O API• InputStream and OutputStream

– Abstract classes that represent input and output byte streams.

• Reader and Writer– Abstract classes to read and write character streams

• Many filters are defined to tailor specialized input/output streams and readers/writers.

InputStreamInputStream

FiltersFilters

ObjectsObjectsAudioAudioFileFileChars, etc. Chars, etc.

FiltersFilters

ReaderReader

Byte steamByte steam

Char steamChar steamFiltersFilters

WriterWriter

OutputStreamOutputStream

FiltersFilters

Characters Characters Characters Characters ObjectsObjectsAudioAudioFileFileChars, etc. Chars, etc.

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InputStreamInputStream• InputStream objects can be decorated

with an arbitrary number of filters.

• InputStream classes– AudioInputStream– FileInputStream– ObjectInputStream– PipedInputStream– …etc.– SocketInputStream (hidden)

FileInputStreamInputStreamInputStream

FiltersFilters

Files Files

InputStreamInputStream{abstract}{abstract}

read()read()close()close()

FilterInputStreamFilterInputStream

read()read()close()close()

filtersfilters

InputStreamInputStreamclassesclasses

inin

AudioInputStream

Audio Audio

ObjectInputStream

Objects Objects

SocketInputStream

Data fromData fromsocket socket

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Initialization of InputStreamInitialization of InputStream• InputStream is = new FileInputStream(“foo.txt);

InputStream is = new FileInputStream(System.in);

fis.read();

• Socket socket = new Socket(…);InputStream is = socket.getInputStream();is.read();– System.out.println(socket.getInputStream().toString() );

• java.net.SocketInputStream@10d81b

FileInputStreamInputStreamInputStream

FiltersFilters

Files Files

SocketInputStream

Data fromData fromsocket socket

34

• Some InputStream objects can be connected with each other.

• InputStream is = new AudioInputStream(new FileInputStream( “audiofile”) );

• InputStream is = new AudioInputStream(socket.getInputSteam() );

• InputStream is = new ObjectInputStream(new FileInputStream( “objectfile”) );

AudioInputStream

Audio Audio

FileInputStream

AudioInputStream

Audio Audio

SocketInputStream

ObjectInputStream

Object Object

FileInputStream

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Filters of InputStreamFilters of InputStream• Filters

– BufferedInputStream– CheckedInputStream (java.util.zip)– ZipInputStream (java.util.zip)– GZIPInputStream (java.util.zip)– JarInputStream (java.util.zip)– CipherInputStream (javax.crypto)– DifestInputStream (java.security)– …etc.

• A single InputStream object can be decorated with multiple filters.

InputStreamInputStream{abstract}{abstract}

read()read()close()close()

FilterInputStreamFilterInputStream

read()read()close()close()

filtersfilters

InputStreamInputStreamclassesclasses

inin

FileInputStreamInputStreamInputStream

FiltersFilters

Files Files

AudioInputStream

Audio Audio

ObjectInputStream

Objects Objects

SocketInputStream

Data fromData fromsocket socket

36

• InputStream is = new Buffered(socket.getInputStream() );

is.read();

• InputStream in =new ZipInputStream (

new FileInputStream(…) );in.read();

• InputStream in = new BufferredInputStream( new ZipInputStream( socket.getInputStream() ));in.read();

37

OutputStreamOutputStream• OutputStream objects can be decorated

with an arbitrary number of filters.

• OutputStream classes– FileOutputStream– ObjectOutputStream– PipedOutputStream– …etc.– SocketOutputStream (hidden)

OutputStreamOutputStream{abstract}{abstract}

write()write()close()close()

FilterOutputStreamFilterOutputStream

write()write()close()close()

filtersfilters

OutputStreamOutputStreamclassesclasses

FileOutputStreamOutputStreamOutputStream

FiltersFilters

Files Files

ObjectOutputStream

Objects Objects

SocketOutputStream

Data fromData fromsocket socket

outout

38

Initialization of OutputStreamInitialization of OutputStream• OutputStream os = new FileOutputStream(“foo.txt);

OutputStream os = new FileOutputStream(System.in);

fis.read();

• Socket socket = new Socket(…);OutputStream os = socket.getOutputStream();is.read();– System.out.println(socket.getOutputStream().toString() );

• java.net.SocketOutputStream@11d32c

FileOutputStreamInputStreamInputStream

FiltersFilters

Files Files

SocketOutputStream

Data fromData fromsocket socket

39

• Some OutputStream objects can be connected with each other.

• OutputStream os = new ObjectOutputStream(new FileOutputStream( “objectfile”) );

ObjectInputStream

Object Object

FileInputStream

40

Filters of OutputStreamFilters of OutputStream• Filters

– BufferedOutputStream

– PrintStream

– CheckedOutputStream (java.util.zip)

– ZipOutputStream (java.util.zip)

– GZIPOutputStream (java.util.zip)

– JarOutputStream (java.util.zip)

– CipherOutputStream (javax.crypto)

– DifestOutputStream (java.security)

– …etc.

• A single OutputStream object can be decorated with multiple filters.

OutputStreamOutputStream{abstract}{abstract}

write()write()close()close()

FilterOutputStreamFilterOutputStream

write()write()close()close()

filtersfilters

OutputStreamOutputStreamclassesclasses

outout

41

• OutputStream os =new ZipOutputStream(

new FileOutputStream(…) );os.write();

• OutputStream os =new BufferredOutputStream(

new ZipOutputStream(socket.getOutputStream() ));

os.write();

• OutputStream os =new PrintStream(

socket.getOutputStream() );os.write();

42

Readers, Writers and Utility ClassesReaders, Writers and Utility Classes

• PrintWriter

• Scanner– A text scanner that can parse primitive types and

strings using regular expressions.

• FileReader– Convenience class for reading character files.

• FileWriter – Convenience class for writing character files.