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©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 1
Object-oriented Design
Designing systems using self-contained objects and objectclasses
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 2
Objectives
l To explain how a software design may berepresented as a set of interacting objects thatmanage their own state and operations
l To describe the activities in the object-orienteddesign process
l To introduce various models that describe anobject-oriented design
l To show how the UML may be used to representthese models
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 3
Topics covered
l Objects and object classes
l An object-oriented design process
l Design evolution
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 4
Characteristics of OOD
l Objects are abstractions of real-world or systementities and manage themselves
l Objects are independent and encapsulate state andrepresentation information.
l System functionality is expressed in terms ofobject services
l Shared data areas are eliminated. Objectscommunicate by message passing
l Objects may be distributed and may executesequentially or in parallel
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 5
Interacting 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 ()
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 6
Advantages of OOD
l Easier maintenance. Objects may beunderstood as stand-alone entities
l Objects are appropriate reusable components
l For some systems, there may be an obviousmapping from real world entities to systemobjects
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 7
Object-oriented development
l Object-oriented analysis, design andprogramming are related but distinct
l OOA is concerned with developing an objectmodel of the application domain
l OOD is concerned with developing an object-oriented system model to implement requirements
l OOP is concerned with realising an OOD usingan OO programming language such as Java orC++
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 8
Objects and object classes
l Objects are entities in a software system whichrepresent instances of real-world and systementities
l Object classes are templates for objects. Theymay be used to create objects
l Object classes may inherit attributes and servicesfrom other object classes
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 9
Objects
An object is an entity which has a state and a defined set ofoperations which operate on that state. The state is represented as aset of object attributes. The operations associated with the objectprovide services to other objects (clients) which request theseservices when some computation is required.
Objects are created according to some object class definition. Anobject class definition serves as a template for objects. It includesdeclarations of all the attributes and services which should beassociated with an object of that class.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 10
The Unified Modeling Language
l Several different notations for describing object-oriented designs were proposed in the 1980s and1990s
l The Unified Modeling Language is an integrationof these notations
l It describes notations for a number of differentmodels that may be produced during OO analysisand design
l It is now a de facto standard for OO modelling
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 11
Employee object class (UML)Employee
name: stringaddress: stringdateOfBirth: DateemployeeNo: integersocialSecurityNo: stringdepartment: Deptmanager: Employeesalary: integerstatus: {current, left, retired}taxCode: integer. . .
join ()leave ()retire ()changeDetails ()
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 12
Object communication
l Conceptually, objects communicate bymessage passing.
l Messages• The name of the service requested by the calling object.
• Copies of the information required to execute the serviceand the name of a holder for the result of the service.
l In practice, messages are often implementedby procedure calls• Name = procedure name.
• Information = parameter list.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 13
Message examples
// Call a method associated with a buffer// object that returns the next value// in the buffer
v = circularBuffer.Get () ;
// Call the method associated with a// thermostat object that sets the// temperature to be maintained
thermostat.setTemp (20) ;
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 14
Generalisation and inheritance
l Objects are members of classes which defineattribute types and operations
l Classes may be arranged in a class hierarchywhere one class (a super-class) is a generalisationof one or more other classes (sub-classes)
l A sub-class inherits the attributes andoperations from its super class and may addnew methods or attributes of its own
l Generalisation in the UML is implemented asinheritance in OO programming languages
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 15
A generalisation hierarchyEmployee
Programmer
projectprogLanguage
Manager
ProjectManager
budgetsControlleddateAppointed
projects
Dept.Manager
StrategicManager
dept responsibilities
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 16
Advantages of inheritance
l It is an abstraction mechanism which may be usedto classify entities
l It is a reuse mechanism at both the design and theprogramming level
l The inheritance graph is a source oforganisational knowledge about domains andsystems
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 17
Problems with inheritance
l Object classes are not self-contained. they cannotbe understood without reference to their super-classes
l Designers have a tendency to reuse theinheritance graph created during analysis. Canlead to significant inefficiency
l The inheritance graphs of analysis, design andimplementation have different functions andshould be separately maintained
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 18
Inheritance and OOD
l There are differing views as to whetherinheritance is fundamental to OOD.• View 1. Identifying the inheritance hierarchy or network is a
fundamental part of object-oriented design. Obviously this canonly be implemented using an OOPL.
• View 2. Inheritance is a useful implementation concept whichallows reuse of attribute and operation definitions. Identifyingan inheritance hierarchy at the design stage places unnecessaryrestrictions on the implementation
l Inheritance introduces complexity and this isundesirable, especially in critical systems
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 19
UML associations
l Objects and object classes participate inrelationships with other objects and object classes
l In the UML, a generalised relationship isindicated by an association
l Associations may be annotated with informationthat describes the association
l Associations are general but may indicate that anattribute of an object is an associated object orthat a method relies on an associated object
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 20
An association model
Employee Department
Manager
is-member-of
is-managed-by
manages
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 21
Concurrent objects
l The nature of objects as self-contained entitiesmake them suitable for concurrentimplementation
l The message-passing model of objectcommunication can be implemented directly ifobjects are running on separate processors in adistributed system
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 22
Servers and active objects
l Servers.• The object is implemented as a parallel process (server)
with entry points corresponding to object operations. If nocalls are made to it, the object suspends itself and waits forfurther requests for service
l Active objects• Objects are implemented as parallel processes and the
internal object state may be changed by the object itself andnot simply by external calls
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 23
Active transponder object
l Active objects may have their attributes modifiedby operations but may also update themautonomously using internal operations
l Transponder object broadcasts an aircraft’sposition. The position may be updated using asatellite positioning system. The objectperiodically update the position by triangulationfrom satellites
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 24
An active transponder objectclass 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
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 25
Java threads
l Threads in Java are a simple construct forimplementing concurrent objects
l Threads must include a method called run() andthis is started up by the Java run-time system
l Active objects typically include an infinite loopso that they are always carrying out thecomputation
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 26
An object-oriented design process
l Define the context and modes of use of thesystem
l Design the system architecture
l Identify the principal system objects
l Develop design models
l Specify object interfaces
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 27
Weather system description
A weather data collection system is required to generate weather maps on aregular basis using data collected from remote, unattended weather stationsand other data sources such as weather observers, balloons and satellites.Weather stations transmit their data to the area computer in response to arequest from that machine.
The area computer validates the collected data and integrates it with the datafrom different sources. The integrated data is archived and, using data fromthis archive and a digitised map database a set of local weather maps iscreated. Maps may be printed for distribution on a special-purpose mapprinter or may be displayed in a number of different formats.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 28
Weather station description
A weather station is a package of software controlled instrumentswhich collects data, performs some data processing and transmitsthis data for further processing. The instruments include air andground thermometers, an anemometer, a wind vane, a barometerand a rain gauge. Data is collected every five minutes.
When a command is issued to transmit the weather data, theweather station processes and summarises the collected data. Thesummarised data is transmitted to the mapping computer when arequest is received.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 29
Layered 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
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 30
System context and models of use
l Develop an understanding of the relationshipsbetween the software being designed and itsexternal environment
l System context• A static model that describes other systems in the environment.
Use a subsystem model to show other systems. Following slideshows the systems around the weather station system.
l Model of system use• A dynamic model that describes how the system interacts with
its environment. Use use-cases to show interactions
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 31
Subsystems 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
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 32
Use-cases for the weather station
Startup
Shutdown
Report
Calibrate
Test
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 33
Use-case descriptionSystem 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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 34
Architectural design
l Once interactions between the system and itsenvironment have been understood, you use thisinformation for designing the system architecture
l Layered architecture is appropriate for theweather station• Interface layer for handling communications
• Data collection layer for managing instruments
• Instruments layer for collecting data
l There should be no more than 7 entities in anarchitectural model
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 35
Weather station architecture
«subsystem»Data collection
«subsystem»Instruments
«subsystem»Interface
Weather station
Manages allexternal
communications
Collects andsummarisesweather data
Package ofinstruments for raw
data collections
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 36
Object identification
l Identifying objects (or object classes) is the mostdifficult part ofobject oriented design
l There is no 'magic formula' for objectidentification. It relies on the skill, experienceand domain knowledge of system designers
l Object identification is an iterative process. Youare unlikely to get it right first time
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 37
Approaches to identification
l Use a grammatical approach based on a naturallanguage description of the system (used in Hoodmethod)
l Base the identification on tangible things in theapplication domain
l Use a behavioural approach and identify objectsbased on what participates in what behaviour
l Use a scenario-based analysis. The objects,attributes and methods in each scenario areidentified
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 38
Weather station object classes
l Ground thermometer, Anemometer, Barometer• Application domain objects that are ‘hardware’ objects related
to the instruments in the system
l Weather station• The basic interface of the weather station to its environment. It
therefore reflects the interactions identified in the use-casemodel
l Weather data• Encapsulates the summarised data from the instruments
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 39
Weather 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 ()
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 40
Further objects and object refinement
l Use domain knowledge to identify more objectsand operations• Weather stations should have a unique identifier
• Weather stations are remotely situated so instrument failureshave to be reported automatically. Therefore attributes andoperations for self-checking are required
l Active or passive objects• In this case, objects are passive and collect data on request
rather than autonomously. This introduces flexibility at theexpense of controller processing time
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 41
Design models
l Design models show the objects and objectclasses and relationships between these entities
l Static models describe the static structure of thesystem in terms of object classes and relationships
l Dynamic models describe the dynamicinteractions between objects.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 42
Examples of design models
l Sub-system models that show logical groupingsof objects into coherent subsystems
l Sequence models that show the sequence ofobject interactions
l State machine models that show how individualobjects change their state in response to events
l Other models include use-case models,aggregation models, generalisation models,etc.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 43
Subsystem models
l Shows how the design is organised into logicallyrelated groups of objects
l In the UML, these are shown using packages - anencapsulation construct. This is a logical model.The actual organisation of objects in the systemmay be different.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 44
Weather station subsystems«subsystem»
Interface
CommsController
WeatherStation
«subsystem»Data collection
«subsystem»Instruments
Air thermometer
WeatherData
Ground thermometer
Anemometer
WindVane
RainGauge
InstrumentStatus
Barometer
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 45
Sequence models
l Sequence models show the sequence of objectinteractions that take place• Objects are arranged horizontally across the top
• Time is represented vertically so models are read top to bottom
• Interactions are represented by labelled arrows, Different stylesof arrow represent different types of interaction
• A thin rectangle in an object lifeline represents the time whenthe object is the controlling object in the system
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 46
Data collection sequence:CommsController
request (report)
acknowledge ()report ()
summarise ()
reply (report)
acknowledge ()
send (report)
:WeatherStation :WeatherData
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 47
Statecharts
l Show how objects respond to different servicerequests and the state transitions triggered bythese requests• If object state is Shutdown then it responds to a Startup()
message
• In the waiting state the object is waiting for further messages
• If reportWeather () then system moves to summarising state
• If calibrate () the system moves to a calibrating state
• A collecting state is entered when a clock signal is received
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 48
Weather 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 ()
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 49
Object interface specification
l Object interfaces have to be specified so that theobjects and other components can be designed inparallel
l Designers should avoid designing the interfacerepresentation but should hide this in the objectitself
l Objects may have several interfaces which areviewpoints on the methods provided
l The UML uses class diagrams for interfacespecification but Java may also be used
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 50
Weather 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
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 51
Design evolution
l Hiding information inside objects means thatchanges made to an object do not affect otherobjects in an unpredictable way
l Assume pollution monitoring facilities are to beadded to weather stations. These sample theair and compute the amount of differentpollutants in the atmosphere
l Pollution readings are transmitted with weatherdata
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 52
Changes required
l Add an object class called ‘Air quality’ as part ofWeatherStation
l Add an operation reportAirQuality toWeatherStation. Modify the control software tocollect pollution readings
l Add objects representing pollution monitoringinstruments
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 53
Pollution monitoring
NODatasmokeDatabenzeneData
collect ()summarise ()
Air qualityidentifier
reportWeather ()reportAirQuality ()calibrate (instruments)test ()startup (instruments)shutdown (instruments)
WeatherStation
Pollution monitoring instruments
NOmeter SmokeMeter
BenzeneMeter
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 54
l OOD is an approach to design so that designcomponents have their own private state andoperations
l Objects should have constructor and inspectionoperations. They provide services to other objects
l Objects may be implemented sequentially orconcurrently
l The Unified Modeling Language providesdifferent notations for defining different objectmodels
Key points
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12 Slide 55
Key points
l A range of different models may be producedduring an object-oriented design process. Theseinclude static and dynamic system models
l Object interfaces should be defined preciselyusing e.g. a programming language like Java
l Object-oriented design simplifies systemevolution
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