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Object-oriented Design

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With a focus on OO design techniques
Software Engineering Process
 Software specification
 Software design and implementation
 Software validation
 Software evolution
Software design and implementation
 The process of converting the system
specification into an executable system.
 Software design
пѓє Design a software structure that realises the
 Implementation
пѓє Translate this structure into an executable
 The activities of design and implementation
are closely related and may be inter-leaved.
Design process activities
 Architectural design
 Abstract specification
 Interface design
 Component design
 Data structure design
 Algorithm design
The software design process
Structured methods
 Systematic approaches to developing a
software design.
 The design is usually documented as a set of
graphical models.
 Possible models
Object model;
Sequence model;
State transition model;
Structural model;
Data-flow model.
Object-oriented Design
Designing systems using selfcontained objects and object
 To explain how a software design may be
represented as a set of interacting objects
that manage their own state and operations
 To describe the activities in the objectoriented design process
 To introduce various models that describe an
object-oriented design
 To show how the UML may be used to
represent these models
Topics covered
 Objects and object classes
 An object-oriented design process
 Design evolution
Characteristics of OOD
 Objects are abstractions of real-world or
system entities and manage themselves
Objects are independent and encapsulate
state and representation information.
System functionality is expressed in terms of
object services
Shared data areas are eliminated. Objects
communicate by message passing
Objects may be distributed and may execute
sequentially or in parallel
Interacting objects
o4: C4
o1: C1
s tate o1
s tate o3
s tate o4
ops1( )
ops3 ( )
ops4 ( )
o2: C3
o6: C1
s tate o2
s tate o6
s tate o5
ops3 ( )
ops1 ( )
ops5 ( )
Advantages of OOD
 Easier maintenance. Objects may be
understood as stand-alone entities
 Objects are appropriate reusable components
 For some systems, there may be an obvious
mapping from real world entities to system
Object-oriented development
 Object-oriented analysis, design and
programming are related but distinct
 OOA is concerned with developing an object
model of the application domain
 OOD is concerned with developing an objectoriented system model to implement
 OOP is concerned with realising an OOD
using an OO programming language such as
Java or C++
Objects and object classes
 Objects are entities in a software system
which represent instances of real-world and
system entities
 Object classes are templates for objects. They
may be used to create objects
 Object classes may inherit attributes and
services from other object classes
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
 Several different notations for describing
object-oriented designs were proposed in the
1980s and 1990s
 The Unified Modeling Language is an
integration of these notations
 It describes notations for a number of
different models that may be produced
during OO analysis and design
 It is now a de facto standard for OO modelling
Employee object class (UML)
E m p loy e e
n a m e : st rin g
a d d re s s: s trin g
d a t e O fB irt h : Da t e
e m p lo ye e N o : in te g e r
s o cia lS e c u rity No : s trin g
d e p a rt me n t : De p t
ma n a g e r: E mp lo y e e
s a la ry : in te g e r
s ta t u s: {cu rre n t , le ft , re tire d }
t a xC o d e : in te g e r
. ..
jo in ()
le a v e ()
re t ire ()
c h a n g e De t a ils ()
Object communication
 Conceptually, objects communicate by
message passing.
 Messages
пѓє The name of the service requested by the calling
пѓє Copies of the information required to execute the
and the name of a holder for the result of the service.
 In practice, messages are often implemented
by procedure calls
пѓє Name = procedure name.
пѓє Information = parameter list.
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) ;
Generalisation and
 Objects are members of classes which define
attribute types and operations
 Classes may be arranged in a class hierarchy
where one class (a super-class) is a
generalisation of one or more other classes (subclasses)
 A sub-class inherits the attributes and
operations from its super class and may add
new methods or attributes of its own
 Generalisation in the UML is implemented as
inheritance in OO programming languages
A generalisation hierarchy
E m p loy e e
Ma nager
P ro g ra m me r
b u d g e t sC o n tro lle d
d a t e A p p o in te d
P ro je c t
M a n ag e r
p ro je c ts
De p t.
Ma nager
p ro je c t
p ro g L a n g u a g e
S t ra te g ic
Ma n ag e r
re s p o n sib ilit ie s
Advantages of inheritance
 It is an abstraction mechanism which may be
used to classify entities
 It is a reuse mechanism at both the design
and the programming level
 The inheritance graph is a source of
organisational knowledge about domains and
Problems with inheritance
 Object classes are not self-contained. they
cannot be understood without reference to
their super-classes
 Designers have a tendency to reuse the
inheritance graph created during analysis.
Can lead to significant inefficiency
 The inheritance graphs of analysis, design
and implementation have different functions
and should be separately maintained
Inheritance and OOD
 There are differing views as to whether
inheritance is fundamental to OOD.
пѓє View 1. Identifying the inheritance hierarchy or
network is a fundamental part of object-oriented
design. Obviously this can only be implemented using
an OOPL.
пѓє View 2. Inheritance is a useful implementation
concept which allows reuse of attribute and operation
definitions. Identifying an inheritance hierarchy at the
design stage places unnecessary restrictions on the
 Inheritance introduces complexity and this is
undesirable, especially in critical systems
UML associations
 Objects and object classes participate in
relationships with other objects and object
 In the UML, a generalised relationship is
indicated by an association
 Associations may be annotated with information
that describes the association
 Associations are general but may indicate that
an attribute of an object is an associated object
or that a method relies on an associated object
An association model
Em p lo yee
i s-m em ber-of
Dep artm en t
i s-m anag ed-b y
m anag es
M anag er
Concurrent objects
 The nature of objects as self-contained
entities make them suitable for concurrent
 The message-passing model of object
communication can be implemented directly
if objects are running on separate processors
in a distributed system
Servers and active objects
 Servers.
пѓє The object is implemented as a parallel process
(server) with entry points corresponding to object
operations. If no calls are made to it, the object
suspends itself and waits for further requests for
 Active objects
пѓє Objects are implemented as parallel processes
and the internal object state may be changed by
the object itself and not simply by external calls
Active transponder object
 Active objects may have their attributes
modified by operations but may also update
them autonomously using internal operations
 Transponder object broadcasts an aircraft’s
position. The position may be updated using
a satellite positioning system. The object
periodically update the position by
triangulation from satellites
An active transponder object
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
Java threads
 Threads in Java are a simple construct for
implementing concurrent objects
 Threads must include a method called run()
and this is started up by the Java run-time
 Active objects typically include an infinite
loop so that they are always carrying out the
An object-oriented design
 Define the context and modes of use of the
Design the system architecture
Identify the principal system objects
Develop design models
Specify object interfaces
Weather system description
A 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 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 architecture
В« s u b sy st e mВ»
Da ta d isp la y
Dat a dis play layer w here obj ect s are
con cerned w it h p repari ng an d
p res ent in g th e dat a i n a hu m anreadab le form
В« s u b sy st e mВ»
Da ta a rch iv in g
Dat a archi vi ng lay er wh ere ob jects
are co ncern ed w it h st ori ng th e d ata
fo r fut ure pro cess in g
В« s u b sy st e m В»
Da ta p ro ce s sin g
Dat a process in g l ay er where o bjects
are co ncern ed w it h check in g an d
i nt eg rati ng t he co ll ected d ata
В« s u b sy st e mВ»
Da ta co lle c tio n
Dat a co llect io n layer wh ere ob jects
are co ncern ed w it h acqu iri ng dat a
from rem ot e so urces
System context and models of
 Develop an understanding of the relationships
between the software being designed and its
external environment
 System context
пѓє A static model that describes other systems in the
environment. Use a subsystem model to show other
systems. Following slide shows the systems around
the weather station system.
 Model of system use
пѓє A dynamic model that describes how the system
interacts with its environment. Employs use-cases to
show interactions
Subsystems in the weather mapping
В« s u b sy st e mВ»
Da ta co lle c tio n
В« s u b sy st e m В»
Da ta d isp la y
O b s e rve r
S a t e llite
Co mms
W e a th e r
s ta t io n
B a llo o n
Ma p
d is p la y
Ma p
p rin t e r
Ma p
В« s u b sy st e mВ»
Da ta a rch iv in g
В« s u b sy st e mВ»
Da ta p ro ce s sin g
Da ta
ch e ckin g
Us e r
in te rf a ce
Da ta
in t e g ra tio n
Da ta
s to ra g e
Ma p s to re
Da ta st o re
Use-cases for the weather
S t a rtu p
S h u t d o wn
Re p o rt
Ca lib ra te
Te s t
Use-case description
S ys tem
We a th e r s tati on
U se-c ase
R e por t
Ac tor s
We a th e r da ta co ll ec ti on sys tem, W e ather stati on
Da ta
T he w e ather sta tion send s a sum ma ry of t he w ea the r da ta t ha t h a s been
co ll ec ted f ro m t he instrum en ts in the co ll ec ti on pe riod t o the wea the r da ta
co ll ec ti on sy stem . The da ta sen t ar e the m ax im um mi nim um and ave rage
ground and air tem pe ratures , the m ax im um , m in im um and av e ra ge air
pres sur e s, the m ax im um , mi nim um a nd ave rage w ind speed s, the total
rainf a ll and th e w ind dire c tion as sam pl e d at 5 mi nu te in te rva ls .
S timulu s
T he w e ather da ta co ll ec ti on sys tem es ta bl is hes a mod em li nk w it h t he
wea the r s tati on and reque st s tran smi ss ion of the da ta.
Re sp on se
T he sum m a ris ed d a ta i s se nt to the wea the r da ta co lle cti on sys tem
Co m m e nts
We a th e r s tati on s a re usua ll y asked t o repo rt onc e pe r hou r but t his
frequ e ncy m ay dif fe r fr om one stati on t o the other and m ay be m od ified i n
future .
Architectural design
 Once interactions between the system and its
environment have been understood, you use this
information for designing the system
 Layered architecture is appropriate for the
weather station
пѓє Interface layer for handling communications
пѓє Data collection layer for managing instruments
пѓє Instruments layer for collecting data
 Rule of thumb: there should be no more than 7
entities in an architectural model
Weather station architecture
W e a t h e r s ta t io n
В« s u b sy st e m В»
I n te rf a ce
Manages all
ex ter nal
c ommunic ations
В« s u b sy st e m В»
Da ta co lle c tio n
Collec ts and
s ummar is es
w eather data
В« s u b sy st e m В»
I n st ru m e n t s
Package of
ins truments f or raw
data c ollec tions
Object identification
 Identifying objects (or object classes) is the
most difficult part of object oriented design
 There is no 'magic formula' for object
identification. It relies on the skill, experience
and domain knowledge of system designers
 Object identification is an iterative process.
You are unlikely to get it right first time
Approaches to identification
 Use a grammatical approach based on a natural
language description of the system (used in
Hood method)
 Base the identification on tangible things in the
application domain
 Use a behavioural approach and identify objects
based on what participates in what behaviour
 Use a scenario-based analysis. The objects,
attributes and methods in each scenario are
Weather station object
 Ground thermometer, Anemometer, Barometer
 Application domain objects that are �hardware’
objects related to the instruments in the system
 Weather station
пѓє The basic interface of the weather station to its
environment. It therefore reflects the interactions
identified in the use-case model
 Weather data
пѓє Encapsulates the summarised data from the
Weather station object
W e a th e rD a ta
W e a th e rS ta ti on
id e n t ifie r
a irTe m p e ra t u re s
g ro u n d Te m p e ra t u re s
win d S p e e d s
win d Dire c tio n s
p re s su re s
ra in fa ll
re p o rt W e a t h e r ()
c a lib ra te (in st ru me n t s)
t e st ()
s ta rt u p (in s tru m e n ts )
s h u td o w n (in s tru m e n ts )
c o lle ct ()
su m ma ris e ()
G rou nd
the rm o m e te r
An e m o m e te r
Ba rom e te r
te m p e ra t u re
win d S p e e d
win d Dire c tio n
p re s su re
h e ig h t
te s t ()
c a lib ra t e ()
te s t ()
t e st ()
c a lib ra t e ()
Further objects and object
 Use domain knowledge to identify more objects
and operations
пѓє Weather stations should have a unique identifier
пѓє Weather stations are remotely situated so instrument
failures have to be reported automatically. Therefore
attributes and operations for self-checking are
 Active or passive objects
пѓє In this case, objects are passive and collect data on
request rather than autonomously. This introduces
flexibility at the expense of controller processing time
Design models
 Design models show the objects and object
classes and relationships between these
 Static models describe the static structure of
the system in terms of object classes and
 Dynamic models describe the dynamic
interactions between objects.
Examples of design models
 Sub-system models that show logical groupings
of objects into coherent subsystems
 Sequence models that show the sequence of
object interactions
 State machine models that show how individual
objects change their state in response to events
 Other models include use-case models,
aggregation models, generalisation models,etc.
Subsystem models
 Shows how the design is organised into
logically related groups of objects
 In UML, these are shown using packages - an
encapsulation construct. This is a logical
model. The actual organisation of objects in
the system may be different.
Weather station subsystems
В« s u b sy st e mВ»
I n te rf a ce
В« s u b sy st e m В»
Da ta co lle c tio n
Co mms Co n tro lle r
W e a t h e rS ta t io n
W e a t h e rDa t a
In s tru m e n t
S t a tu s
В« s u b sy st e mВ»
I n st ru me n t s
A ir
th e rmo me te r
Ra in G a u g e
A n e m o me t e r
G ro u n d
th e rmo me te r
B a ro m e te r
W in d Va n e
Sequence models
 Sequence models show the sequence of
object interactions that take place
пѓє Objects are arranged horizontally across the top
пѓє Time is represented vertically so models are read
top to bottom
пѓє Interactions are represented by labelled arrows,
Different styles of arrow represent different types
of interaction
пѓє A thin rectangle in an object lifeline represents the
time when the object is the controlling object in
the system
Data collection sequence
 Show how objects respond to different service
requests and the state transitions triggered by
these requests
пѓє If object state is Shutdown then it responds to a
Startup() message
In the waiting state the object is waiting for further
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
Weather station state
O pe rat ion
cali brat e ()
C ali brat in g
cali brat io n OK
S h ut do wn
s tartu p ()
t es t ()
W ait in g
Test in g
t ran sm i ss i on do ne
s hu td ow n ()
t es t com p let e
T rans m it ti ng
clo ck
col lect io n
d on e
repo rtW eath er ()
S u m m ari s in g
C o ll ect in g
weat her su m m ary
com p let e
Object interface specification
 Object interfaces have to be specified so that
the objects and other components can be
designed in parallel
 Designers should avoid designing the
interface representation but should hide this
in the object itself
 Objects may have several interfaces which
are viewpoints on the methods provided
 UML uses class diagrams for interface
specification but Java may also be used
Weather station interface
in te rfa ce We athe rS ta tio n {
p ub lic v o id We athe rS ta tio n () ;
p ub lic v o id s ta rtu p () ;
p ub lic v o id s ta rtu p (I n strume nt i) ;
p ub lic v o id s hu td ow n () ;
p ub lic v o id s hu td ow n (In stru m en t i ) ;
p ub lic v o id r ep or tW ea th er ( ) ;
p ub lic v o id te st () ;
p ub lic v o id te st ( In s trume nt i ) ;
p ub lic v o id c alibrate ( In strume nt i) ;
p ub lic in t g etID ( ) ;
} //We athe rS ta tio n
Design evolution
 Hiding information inside objects means that
changes made to an object do not affect other
objects in an unpredictable way
 Assume pollution monitoring facilities are to be
added to weather stations. These sample the
air and compute the amount of different
pollutants in the atmosphere
 Pollution readings are transmitted with weather
Changes required
 Add an object class called �Air quality’ as part
of WeatherStation
 Add an operation reportAirQuality to
WeatherStation. Modify the control software
to collect pollution readings
 Add objects representing pollution
monitoring instruments
Pollution monitoring
W e a th e rS ta ti on
Ai r q ua l ity
id e n t ifie r
re p o rt W e a t h e r ()
re p o rt A irQ u a lity ()
c a lib ra te (in st ru me n t s)
t e st ()
s ta rt u p (in s tru m e n ts )
sh u t d o wn (in s tru me n t s)
NO Da t a
s m o k e D a ta
b e n z e n e D a ta
c o lle ct ()
su m ma ris e ()
P o ll utio n m on ito ri ng i ns tr um e n ts
NO me t e r
S m o ke M e te r
B enz eneMeter
Key points
 OOD is an approach to design so that design
components have their own private state and
 Objects should have constructor and inspection
operations. They provide services to other
 Objects may be implemented sequentially or
 The Unified Modeling Language provides
different notations for defining different object
Key points
 A range of different models may be produced
during an object-oriented design process.
These include static and dynamic system
 Object interfaces should be defined precisely
using e.g. a programming language like Java
 Object-oriented design simplifies system
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