Slide 1

Chapter 14: Object-Oriented Design

Object-oriented design

based on abstractions of real-world or system entities

objects manage a private state

communication by message passing

advantages

• obvious map to real-world

• easier to debug due to reduced system coupling

• easier reuse

• easier maintenance (objects are stand-alone entities)

Functional design

based on interacting functional components of a solution

functions share a global state

communication via shared variables

advantages

• functional data flow may be more intuitive

• more obvious control model

Slide 2

Object-oriented developmentThree related but distinct OO stages

• analysis (OOA) : develop an object model of the application domain

• design (OOD): develop an object-oriented system model to implement requirements

• programming (OOP) : realize an OOD using an OO language such as C++

Commonalities:• identifying objects, attributes and services

• organizing objects into an aggregation hierarchy

• constructing object-use descriptions to show how services are used

• specifying object interfaces

Slide 3

Passive and active objectsPassive objects

• all state changes effected through operations defined in the object interface

• e.g. aStack.Push(item)

Active objects• can change their own state

• e.g. an object might have a (private) timer to update its state every 100 ms (similar to JavaScript setTimeout)

Slide 4

14.2 Object identificationThe most difficult part of object oriented design

No 'magic formula' for object identification

Relies on skill, experience, and domain knowledge

Iterative--you are unlikely to get it right first time

How is this like anything else you do well?

Approaches• grammatical based on a description of the system: nouns are objects,

adjectives are attributes, verbs are operations• identify tangible things in the application domain• behavioral, identifying objects based on what participates in what behavior• scenario analysis

Slide 5

An office information system

The Office Information Retrieval System (OIRS) is an automatic file clerk which can file documents under some name in one or more indexes, retrieve documents, display and maintain document indexes, archive documents and destroy documents. The system is activated by a request from the user and always returns a message to the user indicating the success or failure of the request.

Slide 6

An office information systemThe Office Information Retrieval System (OIRS) is an automatic file clerk which can file documents under some name in one or more indexes, retrieve documents, display and maintain document indexes, archive documents and destroy documents. The system is activated by a request from the user and always returns a message to the user indicating the success or failure of the request.

FileRetrieveArchiveDestroy

Document

NameNameDisplayDelete entryAdd entry

Index

Put message

User Retrievalsystem

Name Name User commandGet command

Slide 7

Weather mapping system A weather mapping 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 system 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.

Will look at this two ways 1. non-layered 2. layered

Slide 8

Top-level architecturesNon-layered Layered

«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 and

integrating 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

Weatherdata processor

Dataarchive

Mapdatabase

Telecommssystem

Satellitereceiver

Mapprinter

Mapdisplay

Manual datacollection

Weatherstations

Slide 9

Subsystems derived from layered view

Datastorage

Userinterface

«subsystem»Data collection

«subsystem»Data processing

«subsystem»Data archiving

«subsystem»Data display

Weatherstation

Satellite

Comms

Balloon

Observer

Map store Data store

Datastorage

Map

Userinterface

Mapdisplay

Mapprinter

Datachecking

Dataintegration

Slide 10

Comment on notationThe design is organized into logically related groups of objects

In the UML, these are encapsulated using packages

This is a logical model

The actual organization of objects in the system may be different

Next step: decompose weather station subsystem

Slide 11

Weather station objects Use domain knowledge to identify more objects and operations

• weather stations should have a unique identifier• weather stations are remotely situated so instrument failures have to be

reported automatically, therefore attributes and operations for self-checking are required

Active or passive objects?• in this case, objects are passive and collect

data on request rather than autonomously• this introduces flexibility at the expense

of controller processing time

What are the objects inside this?

IdentifierWeather dataInstrument status

InitializeTransmit dataTransmit statusSelf testShut down

Weather station

Slide 12

Hardware control objects

Test

Wind vane

Direction

ResetTest

Rain gauge

Rainf all

TestCalibr ate

Barometer

PressureHeight

TestCalibr ate

Airthermometer

Temperature

TestCalibr ate

Groundthermometer

Temperature

Test

Anemometer

Wind speed

Slide 13

Weather data objects

Air temperature dataGround temperature data

Wind speed dataWind direction dataPressureRainf all

Mak e readingsProcess data

Weather data

Readings

MaximumMinimumAver ageRead

Temperature data

Readings

ReadAver age

Pressure

Readings

Aver ageMax. gust

Read

Wind speed data

Readings

Read

Wind directiondata

Cumulativ e

Read

Rainfall

Slide 14

Weather station subsystems (layered)

«subsystem»Interface

«subsystem»Data collection

CommsController

WeatherStation

WeatherData

InstrumentStatus

«subsystem»Instruments

Air thermometer

Ground thermometer

RainGauge

Barometer

Anemometer

WindVane

Slide 15

Object interface designSpecify the detail of the object

interfaces so that objects can be developed in parallel

This means defining• attribute types

• the signatures and semantics of object operations

Use a programming language to achieve precision

Avoid representation information

interface 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

Slide 16

Design evolutionHow would you add pollution monitoring facilities to the weather stations?

(to sample the air and compute the amounts of different pollutants)

(pollution readings are transmitted with weather data)

Changes required:• add an object class called AirQuality as part of WeatherStation.

• add an operation reportAirQuality to WeatherStation. Modify the control software to collect pollution readings.

• add objects representing pollution monitoring instruments.

identifier

reportWeather ()reportAirQuality ()

calibrate (instruments)test ()startup (instruments)

shutdown (instruments)

WeatherStation

NODatasmokeDatabenzeneData

collect ()summarise ()

Air quality Pollution monitoring instruments

NOmeter SmokeMeter

BenzeneMeter

Slide 17

Weather station subsystems (layered)

«subsystem»Interface

«subsystem»Data collection

CommsController

WeatherStation

WeatherData

InstrumentStatus

«subsystem»Instruments

Air thermometer

Ground thermometer

RainGauge

Barometer

Anemometer

WindVane

NO meter Smoke meter Benzine meter

Slide 18

Other pieces mentioned in text

State machines show responses

to requests for services

Sequence diagrams show

interactions among objects

:CommsController

request (report)

acknowledge ()report ()

summarise ()

reply (report)

acknowledge ()

send (report)

:WeatherStation :WeatherData

transmission done

calibrate ()

test ()startup ()

shutdown ()

calibration OK

test complete

weather summarycomplete

clock collectiondone

Operation

reportWeather ()

Shutdown Waiting Testing

Transmitting

Collecting

Summarising

Calibrating