1. Chapter 8 Structural Design Patterns

2. Facade Design Purpose Design Pattern Summary
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

3. ----------------------------
Structure of Facade
APackage 1
Client
«exposed»
Façade
cMethodOfFacade()
bMethodOfFacade() 2 B
«not exposed»
myBMethod() C
«not exposed»
myCMethod()

4. Sequence Diagram for Façade
:Client
singleton
:Facade :C
cMethodOfFacade()
myCMethod()
(return if any)
(return if any)

5. Using Façade to Access Bank Customers
framework
Customer
getCustomerName()
getNumAccounts()
getPersonalNote()
getAccount( int )
AccountException
Account
getAccountNum()
deposit( int )
getBalance()
CustomerException
1..n
bankCustomers
BankCustomer
BankAccount
Client
main()
BankCustomers: «facade»
doDeposit( int amt, Customer cust, Account acc )
getBankAccount( Customer cust, int accNum )
getBankCustomer( String custName )

6. Decorator Design Purpose Design Pattern Summary
Add responsibilities to an object at runtime.
Design Pattern Summary
Provide for a linked list of objects, each encapsulating responsibility.

7. Decorator Class Model Component 1 Client Substance Decoration
add( Component )
doAction() 1
Client
objDecorated
Substance
doAction()
Decoration
doAction()
void doAction()
{ // do actions of the decorator
objDecorated.doAction(); // pass along }

8. Linked Objects in Decorator
client:Client
decoration1:Decoration
decoration1.objectDecorated:Decoration
… :Decoration
….:Substance
Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

9. Decoration1.objDecorated
Sequence Diagram for Decorator
:Client
doAction()
decoration1
:Decoration
Decoration1.objDecorated
:Substance

10. Use of Decorator in java.io
Reader 1
InputStream
InputStreamReader
BufferedReader

11. java.io Decorator example
: BufferedStreamReader
:InputStreamReader
System.in:InputStream
Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

12. 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.

13. Composite Design Purpose Design Pattern Summary
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.

14. Basis for Composite Class Model
Objects
NonLeafNode
Component
“every object involved
is a Component object”
“non-leaf nodes have one or more components”
Classes
1..n
non-leaf node
leaf node
Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

15. Composite Class Model Client Component add( Component ) doIt() 1..n
LeafNode
doIt()
NonLeafNode
doIt()
for all elements e in comp
e.doIt()
TypeANonLeafNode
doIt()
TypeBNonLeafNode
doIt()
Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

16. Sequence Diagram for Composite
:LeafNode
nonLeaf1ChildX
:NonLeafNode
:Client
doIt()
nonLeaf1
Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

17. 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.

18. Composite in java.awt Component 1..n Container component … . . Window
Canvas
Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

19. Proxy Design Purpose Design Pattern Summary
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.

20. Proxy Design Pattern BaseActiveClass Client RealActiveClass
Instantiate with
Proxy object
BaseActiveClass
expensiveMethod()
anotherMethod()
Client
RealActiveClass
expensiveMethod()
anotherMethod()
Proxy
expensiveMethod()
anotherMethod()
realActiveObject
if ( realActiveObject == null ) // not loaded yet
{ realActiveObject = getRealActiveObject();
realActiveObject.expensiveMethod(); }

21. realExpensiveMethod()
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.

22. Proxy: An Example Graphics TexDoc Image Instantiate with Proxy object
Display()
Image
draw()
bitmap
ImageProxy
display()
fileName
image
if ( image == null ) { // not loaded yet
image = new Image(fileName); }
Image.display();

23. 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

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