Chapter 8Structural Design Patterns

Facade

Design Purpose– Provide an interface to a package of classes

Design Pattern Summary– Define a singleton which is the sole means for

obtaining functionality from the package

Notes: the classes need not be organized as a package; more than one class may be used for the façade

APackage

Structure of Facade

C«not exposed»myCMethod()

«exposed»

----------------------------cMethodOfFacade()bMethodOfFacade()

Client1

2

B«not exposed»myBMethod()

Sequence Diagram for Façade

:Client

cMethodOfFacade()

singleton:Facade

:C

myCMethod()

(return if any)

(return if any)

framework

Using Façade to Access Bank Customers

bankCustomers

Clientmain()

BankCustomers: «facade»doDeposit( int amt, Customer cust, Account acc )

getBankAccount( Customer cust, int accNum )getBankCustomer( String custName )

BankCustomer BankAccount

CustomergetCustomerName()getNumAccounts()getPersonalNote()getAccount( int )

AccountgetAccountNum()

deposit( int )getBalance()

1..n

AccountException

CustomerException

Decorator

Design Purpose– Add responsibilities to an object at runtime.

Design Pattern Summary– Provide for a linked list of objects, each

encapsulating responsibility.

Decorator Class Model

Client

objDecoratedDecoration

doAction()

1

SubstancedoAction()

Componentadd( Component )

doAction()

void doAction(){ // do actions of the decorator objDecorated.doAction(); // pass along}

Linked Objects in Decorator

decoration1:Decoration

decoration1.objectDecorated:Decoration

… :Decoration

….:Substance

client:Client

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Sequence Diagram for Decorator

:Client

doAction()

decoration1:Decoration

doAction()

Decoration1.objDecorated:Decoration

:Substance

doAction()

Use of Decorator in java.io

InputStreamReaderInputStream BufferedReader

Reader1

java.io Decorator example

:InputStreamReader

System.in:InputStream

: BufferedStreamReader

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Key Concept: Decorator

-- allows addition to and removal from objects at runtime

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Composite

Design Purpose– Represent a Tree of Objects

Design Pattern Summary– Use a Recursive Form in which the tree class

aggregates and inherits from the base class for the objects.

Basis for Composite Class Model

leaf node

non-leaf node

Objects

NonLeafNode

Component

“every object involvedis a Component object”

“non-leaf nodes have one or more

components”

Classes1..n

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

NonLeafNodedoIt()

Composite Class Model

comp

Componentadd( Component )

doIt()

LeafNodedoIt()

TypeANonLeafNodedoIt()

TypeBNonLeafNodedoIt()

1..nClient

for all elements e in comp e.doIt()

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Sequence Diagram for Composite

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

:LeafNodenonLeaf1ChildX:NonLeafNode

nonLeaf1ChildX:NonLeafNode

:Client

doIt()

nonLeaf1:NonLeafNode

doIt()

doIt()

nonLeaf1ChildX:NonLeafNode

:LeafNode:LeafNode

Design Goal At Work: Flexibility, Correctness

We need to add and remove employees at runtime and execute operations on all of them.

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Component

Composite in java.awt

Container

Window … . .

component

1..n

Canvas

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Proxy

Design Purpose– Avoid the unnecessary execution of

expensive functionality in a manner transparent to clients.

Design Pattern Summary– Interpose a substitute class which accesses

the expensive functionality only when required.

Proxy Design Pattern

BaseActiveClassexpensiveMethod()anotherMethod()

RealActiveClassexpensiveMethod()anotherMethod()

ProxyexpensiveMethod()

anotherMethod()

Client

realActiveObject

if ( realActiveObject == null ) // not loaded yet{ realActiveObject = getRealActiveObject(); realActiveObject.expensiveMethod(); }realActiveObject.expensiveMethod();

Instantiate withProxy object

Sequence Diagram for Proxy

realExpensiveMethod()

:Client

expensiveMethod()

:Proxy :RealActiveClass

( if needed: )

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Proxy: An Example

GraphicsDisplay()

Imagedraw()

bitmap

ImageProxydisplay()fileName

TexDocgraphics

image

if ( image == null ) { // not loaded yet image = new Image(fileName); }Image.display();

Instantiate withProxy object

Sample code

Class TextDoc {graphics g;TextDoc(ImageProxy ip) {

g = ip;}void display() {

g.display();}

}Class ImageProxy implements Graphics {

FileName fileName;Image image;ImageProxy(FileName fn) {

fileName = fn;}display() { if (image == null) image = new Image(fileName); image.display();}

}

Interface Graphics {display();

}Class Image Implements Graphics {

Bitmap bitmap;Image(FileName fn) {

bitmap = readImage(fn);}display() {

// draw the bitmap}readImage(FileName fn) {

// read from the file(fn)// create a bitmap

}}

Facade provides an interface to collections of objects

Decorator adds to objects at runtime Composite represents trees of objects Proxy avoids calling expensive operations

unnecessarily

Summary of Structural Patterns

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.