Object-oriented Design Designing systems using self-contained objects and object classes Designing...
-
Upload
darlene-burns -
Category
Documents
-
view
218 -
download
1
Transcript of Object-oriented Design Designing systems using self-contained objects and object classes Designing...
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