1. Chapter 14: 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

2. Object-oriented development
Three 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

3. Passive and active objects
Passive 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)

4. 14.2 Object identification
The 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

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.

6. 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.
File
Retr ie v e
Archiv
Destro y
Document
Name
Displa
Delete entr
Add entr
Inde x
Put message
User
Retrie al
system
Name Name
Diff names for the same object
obvious knowledge left out (index has a name, documents have owners
GO to behavoiral approach
Go to use case diagrams
User command
Get command

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

8. Top-level architectures Non-layered Layered
«subsystem»
Data collection
Data processing
Data archiving
Data display
Data collection layer where objects
are concerned with acquiring data
from remote sources
Data processing layer where objects
are concerned with checking and
integ
rating the collected data
Data archiving layer where objects
are concerned with storing the data
for future processing
Data display layer where objects are
concerned with preparing and
presenting the data in a human-
readable form

9. Subsystems derived from layered view

10. Comment on notation
The 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

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?
Identifier W
eather data
Instr
ument status
Initializ e T r
ansmit data
ansmit status
Self test
Shut do wn
eather station

12. Hardware control objects
est W
ind v
ane
Direction
Reset
Rain gaug e
Rainf
all
Calibr
ate
Bar
ometer
Pressure
Height
Air
thermometer
emper
ature Gr
ound
Anemometer
thermometer T
emper
ature
Wind speed T
est T
est
Calibr
ate
Hardware objects correspond directly to sensors or actuators connected to the system
They conceal the details of the hardware control e.g. buffer address, masking bit pattern etc.
Hardware changes can often be introduced by hardware object re-implementation

13. Weather data objects Air temper ature data Ground temper
Wind speed data
Wind direction data
Pressure
Rainf
all
Mak
e readings
Process data W
eather data
Readings
Maxim um
Minim A v er
age
Read T
emperature data
Max.
gust
ind speed data W
ind direction
Rainfall
data
Readings
Cum
ulativ e
Read
Read

14. Weather station subsystems (layered)

15. Object interface design
Specify 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 i n t e rf a c
e We h r S ti o { p u b li v d We at he ta ( ) ; s () (I st me i) w m en
t i ep or tW In nt al
e ( I g D } //

16. Design evolution
How 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. W
eatherStation
Air quality P
ollution monitoring instruments
identifier
NOData
smok
eData
repor tW
eather ()
NOmeter
SmokeMeter
benz
eneData
repor
tAirQuality ()
calibrate (instruments)
collect ()
test ()
BenzeneMeter
summarise ()
star
tup (instruments)
shutdown (instruments)

17. Weather station subsystems (layered)
Inter
face
Data collection
CommsController W
eatherData
Instrument W
eatherStation
Status
«subsystem»
Instruments
Air
thermometer
RainGauge
Anemometer
Ground
Barometer W
indV
ane
thermometer
NO meter
Smoke meter
Benzine meter

18. Other pieces mentioned in text
State machines show responses
to requests for services
Sequence diagrams show
interactions among objects