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Chapter 8, Object Design:
Design Patterns II
UsingUML,Patterns,andJava
Object-OrientedSoftwareEngineering

Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 2
A Taxonomy of Design Patterns

The Proxy Pattern: 3 Types
• Caching of information (“Remote Proxy”)
• The Proxy object is a local representative for an object in a
different address space
• Good if information does not change too often
• Standin (“Virtual Proxy”)
• Object is too expensive to create or too expensive to
download.
• Good if the real object is not accessed too often
• Example: RealImage and ImageProxy
• Access control (“Protection Proxy”)
• The proxy object provides protection for the real object
• Good when different actors should have different access and
viewing rights for the same object
• Example: Grade information accessed by administrators,
teachers and students.

Command Pattern: Motivation
• You want to build a user interface
• You want to provide menus
• You want to make the menus reusable across many
applications
• The applications only know what has to be done when a
command from the menu is selected
• You don’t want to hardcode the menu commands for the
various applications
• Such a user interface can easily be implemented
with the Command Pattern.

Command Pattern
• Client (in this case a user interface builder) creates a ConcreteCommand and binds it to
an action operation in Receiver
• Client hands the ConcreteCommand over to the Invoker which stores it (for example in a
menu)
• The Invoker has the responsibility to execute or undo a command (based on a string
entered by the user)
Command
execute()
Receiver
action1()
action2()
Client
Invoker
ConcreteCommand1
execute()
«binds»
ConcreteCommand2
execute()
«binds»

Comments to the Command Pattern
• The Command abstract class declares the interface
supported by all ConcreteCommands.
• The client is a class in a user interface builder or in a
class executing during startup of the application to
build the user interface.
• The client creates concreteCommands and binds
them to specific Receivers, this can be strings like
“commit”, “execute”, “undo”.
• So all user-visible commands are sub classes of the
Command abstract class.
• The invoker - the class in the application program
offering the menu of commands or buttons - invokes
theconcreteCommand based on the string entered
and the binding between action and
ConcreteCommand.

Decouples boundary objects from control
objects
• The command pattern can be nicely used to
decouple boundary objects from control objects:
• Boundary objects such as menu items and buttons, send
messages to the command objects (I.e. the control objects)
• Only the command objects modify entity objects
• When the user interface is changed (for example, a
menu bar is replaced by a tool bar), only the
boundary objects are modified.

Command Pattern Applicability
• Parameterize clients with different requests
• Queue or log requests
• Support undoable operations
• Uses:
• Undo queues
• Database transaction buffering

Applying the Command Pattern to Command
Sets
GameBoard
«binds»
TicTacToeMove
execute()
ChessMove
execute()
Move
execute()
Match *
replay()
play()

Applying the Command design pattern to
Replay Matches in ARENA
replay()
«binds»
play()
TicTacToeMove
ChessMove
Move
execute()
Match *
GameBoard
nextMove()
ReplayedMatch
previousMove()

Observer Pattern Motivation
• Problem:
• We have an object that changes its state quite often
• Example: A Portfolio of stocks
• We want to provide multiple views of the current state
of the portfolio
• Example:Histogram view, pie chart view, time line
view, alarm
• Requirements:
• The system should maintain consistency across the
(redundant) views, whenever the state of the
observed object changes
• The system design should be highly extensible
• It should be possible to add new views without
having to recompile the observed object or the
existing views.
Portfolio
Stock
*

Observer Pattern: Decouples an Abstraction from its Views
Subject
subscribe(subscriber)
unsubscribe(subscriber)
notify()
• The Subject (“Publisher”) represents the entity object
• Observers (“Subscribers”) attach to the Subject by calling subscribe()
• Each Observer has a different view of the state of the entity object
• The state is contained in the subclass ConcreteSubject
• The state can be obtained and set by subclasses of type ConcreteObserver.
update()
Observer
*observers
ConcreteSubject
state
getState()
setState()
ConcreteObserver
observeState
update()

Observer Pattern
• Models a 1-to-many dependency between objects
• Connects the state of an observed object, the subject with
many observing objects, the observers
• Usage:
• Maintaining consistency across redundant states
• Optimizing a batch of changes to maintain consistency
• Three variants for maintaining the consistency:
• Push Notification: Every time the state of the subject changes,
all the observers are notified of the change
• Push-Update Notification: The subject also sends the state
that has been changed to the observers
• Pull Notification: An observer inquires about the state the of
the subject
• Also called Publish and Subscribe.

InfoView
update()
Observer
update()
*Subject
subscribe()
unsubscribe()
notify()
getState()
setState()
File
-filename
ListView
update()
PowerpointView
update()
Applying the Observer Pattern to maintain
Consistency across Views

Strategy Pattern
• Different algorithms exists for a specific task
• We can switch between the algorithms at run time
• Examples of tasks:
• Different collision strategies for objects in video games
• Parsing a set of tokens into an abstract syntax tree (Bottom up,
top down)
• Sorting a list of customers (Bubble sort, mergesort, quicksort)
• Different algorithms will be appropriate at different
times
• First build, testing the system, delivering the final product
• If we need a new algorithm, we can add it without
disturbing the application or the other algorithms.

Strategy Pattern
Context
ContextInterface()
Strategy
AlgorithmInterface
*
ConcreteStrategyC
AlgorithmInterface()
ConcreteStrategyB
ConcreteStrategyA
Policy decides which ConcreteStrategy is best in the current Context.
Policy

Using a Strategy Pattern to Decide between
Algorithms at Runtime
Database
SelectSortAlgorithm()
Sort()
* SortInterface
Sort()
BubbleSort
Sort()
QuickSort
Sort()
MergeSort
Sort()
Policy
TimeIsImportant
SpaceIsImportant
Client

Supporting Multiple implementations of a
Network Interface
NetworkInterface
open()
close()
send()
receive()
NetworkConnection
send()
receive()
setNetworkInterface()
Application
Ethernet
open()
close()
send()
receive()
WaveLAN
open()
close()
send()
receive()
UMTS
open()
close()
send()
receive()
LocationManager
Context =
{Mobile, Home, Office}

Abstract Factory Pattern Motivation
• Consider a user interface toolkit that supports
multiple looks and feel standards for different
operating systems:
• How can you write a single user interface and make it
portable across the different look and feel standards for
these window managers?
• Consider a facility management system for an
intelligent house that supports different control
systems:
• How can you write a single control system that is
independent from the manufacturer?

Abstract Factory
Initiation Assocation:
Class ConcreteFactory2 initiates the
associated classes ProductB2 and ProductA2
AbstractProductA
ProductA1 ProductA2
AbstractProductB
ProductB1 ProductB2
AbstractFactory
CreateProductA
CreateProductB
Client
CreateProductA
CreateProductB
ConcreteFactory1
CreateProductA
CreateProductB
ConcreteFactory2

Applicability for Abstract Factory Pattern
• Independence from Initialization or Representation
• Manufacturer Independence
• Constraints on related products
• Cope with upcoming change

Example: A Facility Management System for a House
LightBulb
EIBBulb LuxmateBulb
Blind
EIBBlind LuxmateBlind
IntelligentHouse HouseFactory
createBulb()
createBlind()
LuxmateFactoryEIBFactory
createBulb()
createBlind()
createBulb()
createBlind()

Applying the Abstract Factory Pattern to
Games
Game
Match
TTTMatch ChessMatch
ChessTicTacToe
createMatch()
createStats()
Statistics
TTTStats ChessStats
Tournament
createMatch()
createStatistics()
createMatch()
createStats()

Clues in Nonfunctional Requirements for the
Use of Design Patterns
• Text: “manufacturer independent”,
“device independent”,
“must support a family of products”
=> Abstract Factory Pattern
• Text: “must interface with an existing object”
=> Adapter Pattern
• Text: “must interface to several systems, some
of them to be developed in the future”,
“ an early prototype must be demonstrated”
=>Bridge Pattern
• Text: “must interface to existing set of objects”
=> Façade Pattern

Clues in Nonfunctional Requirements for use of
Design Patterns (2)
• Text: “complex structure”,
“must have variable depth and width”
=> Composite Pattern
• Text: “must be location transparent”
=> Proxy Pattern
• Text: “must be extensible”,
“must be scalable”
=> Observer Pattern
• Text: “must provide a policy independent from
the mechanism”
=> Strategy Pattern

Summary
• Composite, Adapter, Bridge, Façade, Proxy
(Structural Patterns)
• Focus: Composing objects to form larger structures
• Realize new functionality from old functionality,
• Provide flexibility and extensibility
• Command, Observer, Strategy, Template (Behavioral
Patterns)
• Focus: Algorithms and assignment of responsibilities to
objects
• Avoid tight coupling to a particular solution
• Abstract Factory, Builder (Creational Patterns)
• Focus: Creation of complex objects
• Hide how complex objects are created and put together

Conclusion
Design patterns
• provide solutions to common problems
• lead to extensible models and code
• can be used as is or as examples of interface inheritance
and delegation
• apply the same principles to structure and to behavior
• Design patterns solve a lot of your software
development problems
• Pattern-oriented development

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