Patterns go f

OOA&D - v3.2 - English

Gang-of-Four Design Patterns

Chapter 23: Gang-of-Four Design Patterns v3.2 2

Object-Oriented Analysis and Design

Architectural Patterns (example: Broker)Design Patterns (example: State)

GRASP Patterns (example: Expert)Small

Large

General

Special

About the GoF Design Patterns

Gang-of-Four: Gamma, Helm, Johnson, & Vlissides. Intermediate-level design patterns.

N



About the GoF Design Patterns (continued)

Three categories. Creational - How to build problematic objects. Structural - How to build flexible structures. Behavioral - How to build powerful behaviors.

N



CreationalPattern

Abstract Factory

Problem: Sometimes constructing an object is very complex.

The object is made of many different kinds of complexly interconnected parts.

The object consists of many parts from one of two or more sets of compatible parts, but the sets are incompatible with each other.



Abstract Factory

Example: Constructing user interface widgets in general (windows, scrollbars, radio button groups, menus, … ) when there are several widget sets to choose from (Motif, OpenLook, Presentation Manager, … )

CreationalPattern



WidgetFactory

createScrollbar()createWindow()

MotifWidgetFactory


Client

Scrollbar

MotifScrollbar PMScrollbar

PMWidgetFactory


Window

MotifWindow PMWindow

Abstract Factory

Solution: Create a Pure Fabrication to do the construction!

CreationalPattern



Prototype

Problem: Sometimes constructing an object is very complex.

The object is made of many different kinds of complexly interconnected parts.

There are so many variations and combinations that the corresponding inheritance hierarchy is extremely broad.

CreationalPattern



97FordEscortGX 97FordEscortLX

97FordTaurusGX 97FordTaurusLX

97ChevyCavalierGS 97ChevyCavalierLX

97ChevyCorsicaGS 97ChevyCorsicaLX

???

Prototype (continued)

Example:Creational

Pattern



ModelCatalog

getModel(name:String) : AutomobileSpec

AutomobileSpec

clone()

*

Engine Upholstery Doodad

Std EngineNitro Fuel

EngineSticky

UpholsteryPlush

UpholsteryBoring

DoodadReally Cool

Doodad

*

AutomobileSpec is the class of all of the Prototypes. It hasthe clone() operation. Theactual prototypes areinstances of AutomobileSpecstored by ModelCatalog.

Prototype (continued)

Solution. Rather than proliferate special-purpose classes, create

a general class and preconfigure a set of constant, baseline objects.

Clone() these baseline objects as the initial values for your objects.

CreationalPattern



Singleton

Problem: When an object is unique, it is tempting to globally access it.

But global variables are “bad.” How can we have direct visibility of the unique object

without global variables?

CreationalPattern



ModelCatalog

getModel(name:String) : AutomobileSpec

AutomobileOrder

addFeature()addPackage()

???

The ModelCatalog is a big, unique object that thousands of AutomobileOrders would like to easily access.

Singleton (continued)

Example:Creational

Pattern



<<singleton>>ModelCatalog

$instance() : ModelCataloggetModel(name:String) : AutomobileSpec

AutomobileOrder

addFeature()addPackage()

ModelCatalogProxy

???theInstance

Note the use a the UMLstereotype "<<singleton>>" toindicate that the Singletonpattern is being used.

Singleton (continued)

Solution. Define the class of the unique object to have a special method,

instance( ). This method creates the unique object if it does not already exist. Or, it finds the unique object and returns a reference to it.

CreationalPattern



StructuralPattern

Flyweight

Problem: Sometimes it seems necessary to have many copies of a few constant objects.

However, these copies take up a lot of time and space (creating and destroying them).

If they were shared, performance might improve.



Board Square

SquareState

Unmarked Flagged Questionable Uncovered

has

contains

1

1

1 1..*,1..*

Flyweight

Example: A Minesweeper game might have hundreds of Squares, each with a SquareState.

But, there are only four distinct SquareStates.

StructuralPattern



Flyweight (continued)

Solution. Preconfigure the set of constant objects and make them

available for shared use. “Shared use” typically requires:

A dispenser object, like an Abstract Factory. Parameter visibility to the using object so that Flyweight

object can remain constant.

StructuralPattern



Unmarked Questionable

Uncovered Flagged

mark mark

mark

uncover

uncover

uncover

uncovermark


StructuralPattern




:Unmarked :Squaremark(s:Square) 2: changeState(next)

1: next := getNextState()

FlyweightFactory

buildStates()getState(aStateName) : SquareState

SquareState

mark(s:Square)uncover(s:Square)

theStateSet

1 *

StructuralPattern



StructuralPattern

Proxy

Problem: Sometimes it is desirable not to directly access an object but, instead, access it through a placeholder or surrogate.

Typically, this need arises from reliability and/or complexity concerns.



StructuralPattern

Proxy (continued)

Examples. Remote Proxy - A local presentation of a remote

object. Virtual Proxy - A lightweight object that creates its

heavyweight object on demand. Protection Proxy - A sentry that guards a secure object. Device Proxy - A logical device that manages a physical

device.



Proxy (continued)

Solution. Create an abstract class to specify the logical

interface to the object. Next, create two subclasses.

One is the Proxy and one is the actual object. Give the Proxy a reference to the actual object and

make it pass messages through after doing its own work.

See next page.

StructuralPattern



Proxy (continued)

Printer

status : { online ,outofpaper , jammed }

print(psFile)

Document

print()

RealPrinter

status ??? : { online ,outofpaper , jammed }

print(psFile)

PrinterProxy

status : { online ,outofpaper , jammed }

print(psFile)

StructuralPattern



StructuralPattern

Adapter

Problem. Sometimes the interface that you want a client to

use is not the one provided by the supplier. How can you convert the actual interface of the

supplier to the logical interface of the client?



USRoboticsSportster

init(initString)takeOffhook()dial(phoneNum)send(octetString)checkConnection()placeOnhook()

AuthorizationRequest

authorize()

???

An AuthorizationRequest is a transaction step, which really should not be an expert at using a particular kind of modem.

StructuralPattern

Adapter (continued)

Example:



StructuralPattern

Adapter (continued)

Solution. Create an abstract class for a logical interface to

the (category of) device. Inherit from this interface to create several

adapters, which are experts at using their corresponding physical devices.

See next page.



StructuralPattern

Adapter (continued)

SerialCommPort

send(msg,destination)

AuthorizationRequest

authorize()

3ComEthernet10/100

setParms(...)send(buffer,address)broadcast(buffer)

3ComEthernetAdapter

send(msg,address)adaptee

USRoboticsSportster

init(initString)takeOffhook()dial(phoneNum)send(octetString)checkConnection()placeOnhook()

USRModemAdapter

send(msg,phoneNum)adaptee



StructuralPattern

Bridge

Problem. Sometimes, logical concepts can get mixed up with

physical concepts in the same inheritance hierarchy. This is bad.

Often, this situation arises because a family of concepts have a related family of implementations.

These family trees can cause problems if they are mixed.



Square

CoveredSquare UncoveredSquare

Flag QueryMarkPlain Number MineBlank

StructuralPattern

Bridge (continued)

Example:



Square

Number MineBlankFlag QueryMarkPlain

Cover

Transparent

Covers represent the user’s guesses about the state of the board. Squares contain the actual state of the board.

StructuralPattern

Bridge (continued)

Solution. Separate the two inheritance hierarchies and link them

by a Bridge. For example, a reference from the logical abstract

class to the physical abstract class.



StructuralPattern

Facade

Problem. Often groups of classes (and their instances) are

strongly interdependent on each other. If a client of a cluster of classes (or their instances)

misuses one element of the cluster, then the whole cluster might become inconsistent.



Arm

Manipulator

Board

Scheduler

Bin

ChipSpec PressureSensor

Schedule

Inventory

ImageRecognizerStatistical

Process Control

StructuralPattern

Façade (continued)

Example:



StructuralPattern

Facade

Solution. Create a Pure Fabrication to be the interface for the

cluster. The Façade is a composite object that encapsulates

all of the classes/objects in the cluster. All clients must now send their requests to the

Façade. See next page.



StructuralPattern

Façade (continued)

Arm

Manipulator

Board

Scheduler

Bin

ChipSpec

PressureSensor

Schedule

Inventory

ImageRecognizerStatistical

Process Control

<<Facade>>ManipularArm

Assembler Subsystem

<<Facade>>Assembler

Robot

ManipulatorArm Subsystem



BehavioralPattern

Observer

Problem. It is often extremely useful for one object to monitor

the state of another object. However, the observed object must actually do all the

work. Also, with more objects monitoring it, the observed

object gets very highly coupled.



LIS - ResourceInformation

Design Patterns, Gamma et. al. 1994 Addison-Wesley callNumber: 94-34264 status: checked out

LIS - Patron Information

name: CLarman borrowerID: 63910002 resources checked out: Design Patterns:94-34264 Java in a Nutshell:94-7463or

Book

namecallNumberstatus (Subject)

(Observer)(Observer)???

BehavioralPattern

Observer (continued)

Example:



BehavioralPattern

Observer (continued)

Warning. The classic GoF Observer solution is outdated and

weak. It should probably not be used in its classic form, as

described in “Design Patterns.” The idea of publish-subscribe is useful, but the early

1980’s-inspired GoF solution is limited. Better to replace it with a modern event-based

publish-subscribe form. Java delegation event model.



Subject

addObserver(anObserver)removeObserver(anObserver)notifyObservers( )

Book

namecallNumberstatus

setStatus(newStatus)

Observer

update( )

ResourceInformationW indow

update( )myResource

myObservers

1 *

11

Observer is an abstractclass. update( ) is definedin the Concrete Observersubclass

calls update( ) on all ofthe Observers in itscollection

BehavioralPattern

Classic Observer

Solution. Generalize the behavior of an Observer as an

abstract class. Give the observed object a collection of concrete observers. Let the observed object send a generic notification message to the

observers whenever its state changes.



BehavioralPattern

Classic Observer (continued)

What are the weaknesses in Classic Observer?



Java Interface and Event-Based Observers

java.awt.Button

addActionListener( ActionListener)removeActionListener( ActionListener )

processActionEvent( ActionEvent )

*

MyActionListener

actionPerformed(ActionEvent)

adds/removeslisteners froma collection.

walks throughcollection and sendsactionPerformed()message to eachlistener

«interface»ActionListener


«interface»EventListener

Observer/Listener/Subscriber

Subject/Source/Publisher

1



java.awt.Button

addActionListener( ActionListener)removeActionListener( ActionListener )

addMouseListener( MouseListener)removeMouseListener( MouseListener )

*

MyActionListener


«interface»ActionListener



1*

DavesMouseListener

mouseReleased(MouseEvent)

«interface»MouseListener

mouseReleased(MouseEvent)


1

Java Interface and Event-Based Observers (continued)

The delegation event model approach to Observer providesfine-grained notification and low coupling via interfaces.



BehavioralPattern

Command

Problem. Sometimes requests need to be treated in a more

complex way than as simple messages between objects.

Client objects might not know exactly which action should be performed or who should carry out the task.

Requests might need to be queued, performed at a later time, or sorted based on priority.

New kinds of requests might be added (even at run time.)



BehavioralPattern

Command (continued)

Example.



Command

execute()

ConcreteCommand1

execute()

ConcreteCommand2

execute()

Menu

addMenuItem()

MenuItem

clicked()

while(moreCommands){ commands.execute(); }

**

BehavioralPattern

Command (continued)

Solution. Create an object with a method, execute(). This object is essentially a single method as an object,

except it can carry its own private state.



BehavioralPattern

State

Problem. Sometimes it is necessary for an object’s responses

to messages to vary depending on its state. Therefore, the meaning of a method depends on the history of

methods previously called.



WaitingForNewLoan

BorrowingResources

enterBorrower(ID)

printBorrowReport()

borrowResource(callNum)

Library InformationSystem

State (continued)

Example.



BehavioralPattern

State (continued)

Solution. Make the State an abstract class. Let the specific states inherit from this abstract state. Give the client object a state object.

See next page.



BehavioralPattern

State (continued)

LIS

enterBorrower(ID)borrowResource(callNum)printBorrowReport()setState(newState)reallyEnterBorrower(ID)startNewLoan(ID)reallyBorrowResource(callNum)reallyPrintResource()

LISState

enterBorrower(ID, LIS)borrowResource(CallNum, LIS)printBorrowReport(LIS)

W aitingForNewLoanLISState

enterBorrower(ID)

BorrowingResourcesLISState

borrowResource(CallNum)printBorrowReport()

1 1

currentState



:LISenterBorrower(ID) 1:enterBorrower(ID) :WaitingForNewLoanLISState

newState:BorrowingResourcesLISState

4:create()

2:idOK:= reallyEnterBorrower(ID)3[idOK]: startNewLoan(ID)5:setState(newState)

BehavioralPattern

State (continued)

The client passes messages through to the state. The state chooses its own successor.



BehavioralPattern

Strategy

Problem. Sometimes it can be desirable to have a method

with implementation that changes depending on the current circumstances.

In other words, it is desirable to use a different algorithm to accomplish the same task.

The decision must be made at run time.



BehavioralPattern

Strategy (continued)

Example:



Inbox

sort()setStrategy()

Sorter

sort()

SubjectQuickSort

sort()

DateInsertionSort

sort()

BehavioralPattern

Strategy (continued)

Solution. Create an object for the specific method. Have the client object pass through to the Strategy.

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