Object-oriented DesignObject-oriented Design

Designing systems using Designing systems using self-contained objects self-contained objects and object classesand object classes

ObjectivesObjectives To explain how a software design may be To explain how a software design may be

represented as a set of interacting objects represented as a set of interacting objects that manage their own state and that manage their own state and operationsoperations

To describe the activities in the object-To describe the activities in the object-oriented design processoriented design process

To introduce various models that describe To introduce various models that describe an object-oriented designan object-oriented design

To show how the UML may be used to To show how the UML may be used to represent these modelsrepresent these models

Topics coveredTopics covered Objects and object classes Objects and object classes An object-oriented design processAn object-oriented design process Design evolutionDesign evolution

Characteristics of OODCharacteristics of OOD Objects are abstractions of real-world or Objects are abstractions of real-world or

system entities and manage themselvessystem entities and manage themselves Objects are independent and encapsulate Objects are independent and encapsulate

state and representation information. state and representation information. System functionality is expressed in terms System functionality is expressed in terms

of object servicesof object services Shared data areas are eliminated. Objects Shared data areas are eliminated. Objects

communicate by message passingcommunicate by message passing Objects may be distributed and may Objects may be distributed and may

execute sequentially or in parallelexecute sequentially or in parallel

Interacting objectsInteracting objects

state o3

o3:C3

state o4

o4: C4

state o1

o1: C1

state o6

o6: C1

state o5

o5:C5

state o2

o2: C3

ops1() ops3 () ops4 ()

ops3 () ops1 () ops5 ()

Advantages of OODAdvantages of OOD Easier maintenance. Objects may be Easier maintenance. Objects may be

understood as stand-alone entitiesunderstood as stand-alone entities Objects are appropriate reusable Objects are appropriate reusable

componentscomponents For some systems, there may be an For some systems, there may be an

obvious mapping from real world obvious mapping from real world entities to system objectsentities to system objects

Object-oriented developmentObject-oriented development

Object-oriented analysis, design and Object-oriented analysis, design and programming are related but distinctprogramming are related but distinct

OOA is concerned with developing an OOA is concerned with developing an object model of the application domainobject model of the application domain

OOD is concerned with developing an OOD is concerned with developing an object-oriented system model to object-oriented system model to implement requirementsimplement requirements

OOP is concerned with realising an OOD OOP is concerned with realising an OOD using an OO programming language such using an OO programming language such as Java or C++as Java or C++

Objects and object classes Objects and object classes Objects are entities in a software Objects are entities in a software

system which represent instances of system which represent instances of real-world and system entitiesreal-world and system entities

Object classes are templates for Object classes are templates for objects. They may be used to create objects. They may be used to create objectsobjects

Object classes may inherit attributes Object classes may inherit attributes and services from other object and services from other object classesclasses

ObjectsObjects

An object is an entity which has a state and a defined set of operations which operate on that state. The state is represented as a set of object attributes. The operations associated with the object provide services to other objects (clients) which request these services when some computation is required.

Objects are created according to some object class definition. An object class definition serves as a template for objects. It includes declarations of all the attributes and services which should be associated with an object of that class.

The Unified Modeling LanguageThe Unified Modeling Language

Several different notations for describing Several different notations for describing object-oriented designs were proposed in object-oriented designs were proposed in the 1980s and 1990s the 1980s and 1990s

The Unified Modeling Language is an The Unified Modeling Language is an integration of these notationsintegration of these notations

It describes notations for a number of It describes notations for a number of different models that may be produced different models that may be produced during OO analysis and designduring OO analysis and design

It is now a It is now a de factode facto standard for OO standard for OO modellingmodelling

Employee object class (UML)Employee object class (UML)

Employee

name: stringaddress: stringdateOfBirth: DateemployeeNo: integersocialSecurityNo: stringdepartment: Deptmanager: Employeesalary: integerstatus: {current, left, retired}taxCode: integer. . .

join ()leave ()retire ()changeDetails ()

Object communicationObject communication Conceptually, objects communicate by Conceptually, objects communicate by

message passing.message passing. MessagesMessages

– The name of the service requested by the calling The name of the service requested by the calling object.object.

– Copies of the information required to execute the Copies of the information required to execute the service service and the name of a holder for the result of the service.and the name of a holder for the result of the service.

In practice, messages are often implemented In practice, messages are often implemented by procedure callsby procedure calls– Name = procedure name.Name = procedure name.– Information = parameter list.Information = parameter list.

Message examplesMessage examples

// Call a method associated with a buffer // Call a method associated with a buffer // object that returns the next value // object that returns the next value // in the buffer// in the buffer

v = circularBuffer.Get () ;v = circularBuffer.Get () ;

// Call the method associated with a// Call the method associated with a// thermostat object that sets the // thermostat object that sets the // temperature to be maintained// temperature to be maintained

thermostat.setTemp (20) ;thermostat.setTemp (20) ;

Generalisation and inheritanceGeneralisation and inheritance

Objects are members of classes which define Objects are members of classes which define attribute types and operationsattribute types and operations

Classes may be arranged in a class hierarchy Classes may be arranged in a class hierarchy where one class (a super-class) is a where one class (a super-class) is a generalisation of one or more other classes (sub-generalisation of one or more other classes (sub-classes)classes)

A sub-class inherits the attributes and operations A sub-class inherits the attributes and operations from its super class and may add new methods or from its super class and may add new methods or attributes of its ownattributes of its own

Generalisation in the UML is implemented as Generalisation in the UML is implemented as inheritance in OO programming languagesinheritance in OO programming languages

A generalisation hierarchyA generalisation hierarchyEmployee

Programmer

projectprogLanguage

Manager

ProjectManager

budgetsControlled

dateAppointed

projects

Dept.Manager

StrategicManager

dept responsibilities

Advantages of inheritanceAdvantages of inheritance It is an abstraction mechanism which It is an abstraction mechanism which

may be used to classify entitiesmay be used to classify entities It is a reuse mechanism at both the It is a reuse mechanism at both the

design and the programming leveldesign and the programming level The inheritance graph is a source of The inheritance graph is a source of

organisational knowledge about organisational knowledge about domains and systemsdomains and systems

Problems with inheritanceProblems with inheritance Object classes are not self-contained. Object classes are not self-contained.

they cannot be understood without they cannot be understood without reference to their super-classesreference to their super-classes

Designers have a tendency to reuse Designers have a tendency to reuse the inheritance graph created during the inheritance graph created during analysis. Can lead to significant analysis. Can lead to significant inefficiencyinefficiency

The inheritance graphs of analysis, The inheritance graphs of analysis, design and implementation have design and implementation have different functions and should be different functions and should be separately maintainedseparately maintained

Inheritance and OODInheritance and OOD There are differing views as to whether There are differing views as to whether

inheritance is fundamental to OOD.inheritance is fundamental to OOD.– View 1. Identifying the inheritance hierarchy or View 1. Identifying the inheritance hierarchy or

network is a fundamental part of object-oriented network is a fundamental part of object-oriented design. Obviously this can only be implemented design. Obviously this can only be implemented using an OOPL.using an OOPL.

– View 2. Inheritance is a useful implementation View 2. Inheritance is a useful implementation concept which allows reuse of attribute and concept which allows reuse of attribute and operation definitions. Identifying an inheritance operation definitions. Identifying an inheritance hierarchy at the design stage places unnecessary hierarchy at the design stage places unnecessary restrictions on the implementationrestrictions on the implementation

Inheritance introduces complexity and this is Inheritance introduces complexity and this is undesirable, especially in critical systemsundesirable, especially in critical systems

UML associationsUML associations

Objects and object classes participate in Objects and object classes participate in relationships with other objects and object relationships with other objects and object classesclasses

In the UML, a generalised relationship is In the UML, a generalised relationship is indicated by an associationindicated by an association

Associations may be annotated with Associations may be annotated with information that describes the associationinformation that describes the association

Associations are general but may indicate that Associations are general but may indicate that an attribute of an object is an associated an attribute of an object is an associated object or that a method relies on an object or that a method relies on an associated objectassociated object

An association modelAn association model

EmployeeDepartment

Manager

is-member-of

is-managed-by

manages

Concurrent objectsConcurrent objects

The nature of objects as self-The nature of objects as self-contained entities make them contained entities make them suitable for concurrent suitable for concurrent implementationimplementation

The message-passing model of The message-passing model of object communication can be object communication can be implemented directly if objects are implemented directly if objects are running on separate processors in a running on separate processors in a distributed systemdistributed system

Servers and active objectsServers and active objects Servers. Servers.

– The object is implemented as a parallel process The object is implemented as a parallel process (server) (server) with entry points corresponding to object with entry points corresponding to object operations. If no operations. If no calls are made to it, the object suspends itself and calls are made to it, the object suspends itself and waits for further requests for servicewaits for further requests for service

Active objectsActive objects– Objects are implemented as parallel processes Objects are implemented as parallel processes

and the and the internal object state may be changed by the internal object state may be changed by the object itself and object itself and not simply by external callsnot simply by external calls

Active transponder objectActive transponder object

Active objects may have their attributes Active objects may have their attributes modified by operations but may also modified by operations but may also update them autonomously using update them autonomously using internal operationsinternal operations

Transponder object broadcasts an Transponder object broadcasts an aircraft’s position. The position may be aircraft’s position. The position may be updated using a satellite positioning updated using a satellite positioning system. The object periodically update system. The object periodically update the position by triangulation from the position by triangulation from satellitessatellites

class Transponder extends Thread {

Position currentPosition ;Coords c1, c2 ;Satellite sat1, sat2 ;Navigator theNavigator ;

public Position givePosition (){

return currentPosition ;}

public void run (){

while (true){

c1 = sat1.position () ;c2 = sat2.position () ;currentPosition = theNavigator.compute (c1, c2) ;

}

}

} //Transponder

An active transponder objectAn active transponder object

Java threadsJava threads

Threads in Java are a simple Threads in Java are a simple construct for implementing construct for implementing concurrent objectsconcurrent objects

Threads must include a method Threads must include a method called run() and this is started up by called run() and this is started up by the Java run-time systemthe Java run-time system

Active objects typically include an Active objects typically include an infinite loop so that they are always infinite loop so that they are always carrying out the computationcarrying out the computation

An object-oriented design processAn object-oriented design process

Define the context and modes of use Define the context and modes of use of the systemof the system

Design the system architectureDesign the system architecture Identify the principal system objectsIdentify the principal system objects Develop design modelsDevelop design models Specify object interfacesSpecify object interfaces

Weather system descriptionWeather system descriptionA weather data collection system is required to generate weather maps on a regular basis using data collected from remote, unattended weather stations and other data sources such as weather observers, balloons and satellites. Weather stations transmit their data to the area computer in response to a request from that machine.

The area computer validates the collected data and integrates it with the data from different sources. The integrated data is archived and, using data from this archive and a digitised map database a set of local weather maps is created. Maps may be printed for distribution on a special-purpose map printer or may be displayed in a number of different formats.

Weather station descriptionWeather station description

A weather station is a package of software controlled instruments which collects data, performs some data processing and transmits this data for further processing. The instruments include air and ground thermometers, an anemometer, a wind vane, a barometer and a rain gauge. Data is collected every five minutes.

When a command is issued to transmit the weather data, the weather station processes and summarises the collected data. The summarised data is transmitted to the mapping computer when a request is received.

Layered architectureLayered architecture

«subsystem»Data collection

«subsystem»Data processing

«subsystem»Data archiving

«subsystem»Data display

Data collection layer where objectsare concerned with acquiring datafrom remote sources

Data processing layer where objectsare concerned with checking andintegrating the collected data

Data archiving layer where objectsare concerned with storing the data for future processing

Data display layer where objects areconcerned with preparing andpresenting the data in a human-readable form

System context and models of useSystem context and models of use

Develop an understanding of the relationships Develop an understanding of the relationships between the software being designed and its between the software being designed and its external environmentexternal environment

System contextSystem context– A static model that describes other systems in A static model that describes other systems in

the environment. Use a subsystem model to the environment. Use a subsystem model to show other systems. Following slide shows the show other systems. Following slide shows the systems around the weather station system.systems around the weather station system.

Model of system useModel of system use– A dynamic model that describes how the system A dynamic model that describes how the system

interacts with its environment. Use use-cases to interacts with its environment. Use use-cases to show interactionsshow interactions

Subsystems in the weather mapping systemSubsystems in the weather mapping system

«subsystem»Data collection

«subsystem»Data processing

«subsystem»Data archiving

«subsystem»Data display

Weatherstation

Satellite

Comms

Balloon

Observer

Datachecking

Dataintegration

Map store Data store

Datastorage

Map

Userinterface

Mapdisplay

Mapprinter

Use-cases for the weather stationUse-cases for the weather station

Startup

Shutdown

Report

Calibrate

Test

Use-case descriptionUse-case description

System Weather stationUse-case ReportActors Weather data collection system, Weather stationData The weather station sends a summary of the weather data that has been

collected from the instruments in the collection period to the weather datacollection system. The data sent are the maximum minimum and averageground and air temperatures, the maximum, minimum and average airpressures, the maximum, minimum and average wind speeds, the totalrainfall and the wind direction as sampled at 5 minute intervals.

Stimulus The weather data collection system establishes a modem link with theweather station and requests transmission of the data.

Response The summarised data is sent to the weather data collection systemComments Weather stations are usually asked to report once per hour but this

frequency may differ from one station to the other and may be modified infuture.

Architectural designArchitectural design Once interactions between the system and its Once interactions between the system and its

environment have been understood, you use this environment have been understood, you use this information for designing the system architectureinformation for designing the system architecture

Layered architecture is appropriate for the Layered architecture is appropriate for the weather stationweather station– Interface layer for handling communicationsInterface layer for handling communications– Data collection layer for managing instrumentsData collection layer for managing instruments– Instruments layer for collecting dataInstruments layer for collecting data

There should be no more than 7 entities in an There should be no more than 7 entities in an architectural modelarchitectural model

Weather station architectureWeather station architecture

«subsystem»Data collection

«subsystem»Instruments

«subsystem»Interface

Weather station

Manages allexternal

communications

Collects andsummarisesweather data

Package ofinstruments for raw

data collections

Object identificationObject identification Identifying objects (or object classes) is the Identifying objects (or object classes) is the

most difficult part of most difficult part of object oriented designobject oriented design

There is no 'magic formula' for object There is no 'magic formula' for object identification. It relies on the skill, identification. It relies on the skill, experience experience and domain knowledge of system designersand domain knowledge of system designers

Object identification is an iterative process. Object identification is an iterative process. You are unlikely to get it right first timeYou are unlikely to get it right first time

Approaches to identificationApproaches to identification Use a grammatical approach based on a Use a grammatical approach based on a

natural language description of the system natural language description of the system (used in Hood method)(used in Hood method)

Base the identification on tangible things in Base the identification on tangible things in the application domainthe application domain

Use a behavioural approach and identify Use a behavioural approach and identify objects based on what participates in what objects based on what participates in what behaviourbehaviour

Use a scenario-based analysis. The objects, Use a scenario-based analysis. The objects, attributes and methods in each scenario are attributes and methods in each scenario are identifiedidentified

Weather station object classesWeather station object classes

Ground thermometer, Anemometer, BarometerGround thermometer, Anemometer, Barometer– Application domain objects that are ‘hardware’ Application domain objects that are ‘hardware’

objects related to the instruments in the objects related to the instruments in the systemsystem

Weather stationWeather station– The basic interface of the weather station to its The basic interface of the weather station to its

environment. It therefore reflects the environment. It therefore reflects the interactions identified in the use-case modelinteractions identified in the use-case model

Weather dataWeather data– Encapsulates the summarised data from the Encapsulates the summarised data from the

instrumentsinstruments

Weather station object classesWeather station object classes

identifier

reportWeather ()calibrate (instruments)test ()startup (instruments)shutdown (instruments)

WeatherStation

test ()calibrate ()

Groundthermometer

temperature

Anemometer

windSpeedwindDirection

test ()

Barometer

pressureheight

test ()calibrate ()

WeatherData

airTemperaturesgroundTemperatureswindSpeedswindDirectionspressuresrainfall

collect ()summarise ()

Further objects and object refinementFurther objects and object refinement Use domain knowledge to identify more objects and Use domain knowledge to identify more objects and

operationsoperations– Weather stations should have a unique identifierWeather stations should have a unique identifier– Weather stations are remotely situated so Weather stations are remotely situated so

instrument failures have to be reported instrument failures have to be reported automatically. Therefore attributes and operations automatically. Therefore attributes and operations for self-checking are requiredfor self-checking are required

Active or passive objectsActive or passive objects– In this case, objects are passive and collect data In this case, objects are passive and collect data

on request rather than autonomously. This on request rather than autonomously. This introduces flexibility at the expense of controller introduces flexibility at the expense of controller processing timeprocessing time

Design modelsDesign models

Design models show the objects and Design models show the objects and object classes and relationships object classes and relationships between these entitiesbetween these entities

Static models describe the static Static models describe the static structure of the system in terms of structure of the system in terms of object classes and relationshipsobject classes and relationships

Dynamic models describe the Dynamic models describe the dynamic interactions between dynamic interactions between objects.objects.

Examples of design modelsExamples of design models Sub-system models that show logical Sub-system models that show logical

groupings of objects into coherent subsystemsgroupings of objects into coherent subsystems Sequence models that show the sequence of Sequence models that show the sequence of

object interactionsobject interactions State machine models that show how State machine models that show how

individual objects change their state in individual objects change their state in response to eventsresponse to events

Other models include use-case models, Other models include use-case models, aggregation models, generalisation aggregation models, generalisation models,etc.models,etc.

Subsystem modelsSubsystem models

Shows how the design is organised Shows how the design is organised into logically related groups of into logically related groups of objectsobjects

In the UML, these are shown using In the UML, these are shown using packages - an encapsulation packages - an encapsulation construct. This is a logical model. The construct. This is a logical model. The actual organisation of objects in the actual organisation of objects in the system may be different.system may be different.

Weather station subsystemsWeather station subsystems

«subsystem»Interface

CommsController

WeatherStation

«subsystem»Data collection

«subsystem»Instruments

Air thermometer

WeatherData

Ground thermometer

Anemometer

WindVane

RainGauge

InstrumentStatus

Barometer

Sequence modelsSequence models Sequence models show the sequence Sequence models show the sequence

of object interactions that take placeof object interactions that take place– Objects are arranged horizontally across Objects are arranged horizontally across

the topthe top– Time is represented vertically so models Time is represented vertically so models

are read top to bottomare read top to bottom– Interactions are represented by labelled Interactions are represented by labelled

arrows, Different styles of arrow represent arrows, Different styles of arrow represent different types of interactiondifferent types of interaction

– A thin rectangle in an object lifeline A thin rectangle in an object lifeline represents the time when the object is represents the time when the object is the controlling object in the systemthe controlling object in the system

Data collection sequenceData collection sequence:CommsController

request (report)

acknowledge ()report ()

summarise ()

reply (report)

acknowledge ()

send (report)

:WeatherStation :WeatherData

Show how objects respond to different service Show how objects respond to different service requests and the state transitions triggered by requests and the state transitions triggered by these requeststhese requests– If object state is Shutdown then it responds If object state is Shutdown then it responds

to a Startup() messageto a Startup() message– In the waiting state the object is waiting for In the waiting state the object is waiting for

further messagesfurther messages– If reportWeather () then system moves to If reportWeather () then system moves to

summarising statesummarising state– If calibrate () the system moves to a If calibrate () the system moves to a

calibrating statecalibrating state– A collecting state is entered when a clock A collecting state is entered when a clock

signal is receivedsignal is received

Weather station state diagramWeather station state diagram

Shutdown Waiting Testing

Transmitting

Collecting

Summarising

Calibrating

transmission done

calibrate ()

test ()startup ()

shutdown ()

calibration OK

test complete

weather summarycomplete

clock collectiondone

Operation

reportWeather ()

Object interface specificationObject interface specification

Object interfaces have to be specified so Object interfaces have to be specified so that the objects and other components that the objects and other components can be designed in parallelcan be designed in parallel

Designers should avoid designing the Designers should avoid designing the interface representation but should hide interface representation but should hide this in the object itselfthis in the object itself

Objects may have several interfaces which Objects may have several interfaces which are viewpoints on the methods providedare viewpoints on the methods provided

The UML uses class diagrams for interface The UML uses class diagrams for interface specification but Java may also be usedspecification but Java may also be used

Weather station interfaceWeather station interfaceinterface WeatherStation {

public void WeatherStation () ;

public void startup () ;public void startup (Instrument i) ;

public void shutdown () ;public void shutdown (Instrument i) ;

public void reportWeather ( ) ;

public void test () ;public void test ( Instrument i ) ;

public void calibrate ( Instrument i) ;

public int getID () ;

} //WeatherStation

Design evolutionDesign evolution Hiding information inside objects means that Hiding information inside objects means that

changes made to an object do not affect changes made to an object do not affect other other objects in an unpredictable wayobjects in an unpredictable way

Assume pollution monitoring facilities are to Assume pollution monitoring facilities are to be be added to weather stations. These sample the added to weather stations. These sample the

air and compute the amount of different air and compute the amount of different pollutants in the atmospherepollutants in the atmosphere

Pollution readings are transmitted with Pollution readings are transmitted with weather dataweather data

Changes requiredChanges required Add an object class called ‘Air Add an object class called ‘Air

quality’ as part of WeatherStationquality’ as part of WeatherStation Add an operation reportAirQuality to Add an operation reportAirQuality to

WeatherStation. Modify the control WeatherStation. Modify the control software to collect pollution readingssoftware to collect pollution readings

Add objects representing pollution Add objects representing pollution monitoring instrumentsmonitoring instruments

Pollution monitoringPollution monitoring

NODatasmokeDatabenzeneData

collect ()summarise ()

Air qualityidentifier

reportWeather ()reportAirQuality ()calibrate (instruments)test ()startup (instruments)shutdown (instruments)

WeatherStation

Pollution monitoring instruments

NOmeter SmokeMeter

BenzeneMeter

OOD is an approach to design so that design OOD is an approach to design so that design components have their own private state and components have their own private state and operationsoperations

Objects should have constructor and Objects should have constructor and inspection operations. They provide services to inspection operations. They provide services to other objectsother objects

Objects may be implemented sequentially or Objects may be implemented sequentially or concurrentlyconcurrently

The Unified Modeling Language provides The Unified Modeling Language provides different notations for defining different object different notations for defining different object modelsmodels

Key pointsKey points

Key pointsKey points A range of different models may be A range of different models may be

produced during an object-oriented produced during an object-oriented design process. These include static design process. These include static and dynamic system modelsand dynamic system models

Object interfaces should be defined Object interfaces should be defined precisely using e.g. a programming precisely using e.g. a programming language like Javalanguage like Java

Object-oriented design simplifies Object-oriented design simplifies system evolutionsystem evolution