Object-oriented Design - University Of St Andrews

Object-oriented Design Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 1

Objectives To explain how a software design may berepresented as a set of interacting objectsthat manage their own state and operationsTo describe the activities in the objectoriented design processTo introduce various models that can beused to describe an object-oriented designTo show how the UML may be used torepresent these models Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 2

Topics covered Objects and object classesAn object-oriented design processDesign evolution Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 3

Object-oriented development Object-oriented analysis, design and programmingare related but distinct.OOA is concerned with developing an object modelof the application domain.OOD is concerned with developing an objectoriented system model to implement requirements.OOP is concerned with realising an OOD using anOO programming language such as Java or C . Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 4

Characteristics of OOD Objects are abstractions of real-world or systementities and manage themselves.Objects are independent and encapsulate state andrepresentation information.System functionality is expressed in terms of objectservices.Shared data areas are eliminated. Objectscommunicate by message passing.Objects may be distributed and may executesequentially or in parallel. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 5

Interacting objects Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 6

Advantages of OOD Easier maintenance. Objects may beunderstood as stand-alone entities.Objects are potentially reusable components.For some systems, there may be an obviousmapping from real world entities to systemobjects. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 7

Objects and object classes Objects are entities in a software systemwhich represent instances of real-world andsystem entities.Object classes are templates for objects.They may be used to create objects.Object classes may inherit attributes andservices from other object classes. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 8

Objects and object classesAn object is an entity that 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 2004Software Engineering, 7th edition. Chapter 14Slide 9

The Unified Modeling Language Several different notations for describing objectoriented designs were proposed in the 1980s and1990s.The Unified Modeling Language is an integration ofthese notations.It describes notations for a number of differentmodels that may be produced during OO analysisand design.It is now a de facto standard for OO modelling. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 10

Employee object class (UML) Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 11

Object communication Conceptually, objects communicate bymessage passing.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.In practice, messages are often implementedby procedure calls Name procedure name;Information parameter list. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 12

Message examples// Call a method associated with a buffer// object that returns the next value// in the bufferv circularBuffer.Get () ;// Call the method associated with a// thermostat object that sets the// temperature to be maintainedthermostat.setTemp (20) ; Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 13

Generalisation and inheritance Objects are members of classes that defineattribute types and operations.Classes may be arranged in a class hierarchywhere one class (a super-class) is a generalisationof one or more other classes (sub-classes).A sub-class inherits the attributes andoperations from its super class and may addnew methods or attributes of its own.Generalisation in the UML is implemented asinheritance in OO programming languages. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 14

A generalisation hierarchy Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 15

Advantages of inheritance It is an abstraction mechanism which may beused to classify entities.It is a reuse mechanism at both the designand the programming level.The inheritance graph is a source oforganisational knowledge about domainsand systems. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 16

Problems with inheritance Object classes are not self-contained. theycannot be understood without reference totheir super-classes.Designers have a tendency to reuse theinheritance graph created during analysis.Can lead to significant inefficiency.The inheritance graphs of analysis, designand implementation have different functionsand should be separately maintained. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 17

UML associations Objects and object classes participate inrelationships with other objects and object classes.In the UML, a generalised relationship is indicatedby an association.Associations may be annotated with information thatdescribes the association.Associations are general but may indicate that anattribute of an object is an associated object or thata method relies on an associated object. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 18

An association model Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 19

Concurrent objects The nature of objects as self-containedentities make them suitable for concurrentimplementation.The message-passing model of objectcommunication can be implemented directlyif objects are running on separate processorsin a distributed system. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 20

Servers and active objects 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 waitsfor further requests for service.Active objects Objects are implemented as parallel processes and theinternal object state may be changed by the object itselfand not simply by external calls. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 21

Active transponder object Active objects may have their attributesmodified by operations but may also updatethem autonomously using internaloperations.A Transponder object broadcasts anaircraft’s position. The position may beupdated using a satellite positioning system.The object periodically update the position bytriangulation from satellites. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 22

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 2004Software Engineering, 7th edition. Chapter 14Slide 23

Java threads Threads in Java are a simple construct forimplementing concurrent objects.Threads must include a method called run()and this is started up by the Java run-timesystem.Active objects typically include an infiniteloop so that they are always carrying out thecomputation. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 24

An object-oriented design process Structured design processes involvedeveloping a number of different systemmodels.They require a lot of effort for developmentand maintenance of these models and, forsmall systems, this may not be costeffective.However, for large systems developed bydifferent groups design models are anessential communication mechanism. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 25

Process stages Highlights key activities without being tied toany proprietary process such as the RUP. Define the context and modes of use of thesystem;Design the system architecture;Identify the principal system objects;Develop design models;Specify object interfaces. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 26

Weather system descriptionA weather mapping 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 system validates the collected data and integrates it withthe data from different sources. The integrated data is archived and, usingdata from this archive and a digitised map database a set of local weathermaps is created. Maps may be printed for distribution on a special-purposemap printer or may be displayed in a number of different formats. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 27

System context and models of use Develop an understanding of the relationshipsbetween the software being designed and itsexternal environmentSystem context A static model that describes other systems in theenvironment. Use a subsystem model to show othersystems. Following slide shows the systems around theweather station system.Model of system use A dynamic model that describes how the system interactswith its environment. Use use-cases to show interactions Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 28

Layered architecture Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 29

Subsystems in the weather mapping system Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 30

Use-case models Use-case models are used to represent eachinteraction with the system.A use-case model shows the systemfeatures as ellipses and the interacting entityas a stick figure. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 31

Use-cases for the weather station Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 32

Use-case eCommentsWeather stationReportWeather data collection system, Weather stationThe weather station sends a summary of the weather data that has beencollected 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.The weather data collection system establishes a modem link with theweather station and requests transmission of the data.The summarised data is sent to the weather data collection systemWeather stations are usually asked to report once per hour but thisfrequency may differ from one station to the other and may be modified infuture. Ian Sommerville 2004Software Engineering, 7th edition. Chapter 14Slide 33