COM 6030 Software
Analysis and Design
Lecture 9 – More on objects and classes
Dr Richard Clayton & Dr Marian Gheorghe
Module (1st part) homepage http://www.dcs.shef.ac.uk/~marian
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Outline
 Class model.
 Association, aggregation, composition,
generalisation.
 Class diagram.
 Case studies: ATM, Advertising company.
 Class attributes and operations.
Stevens & Pooley chapters 2, 3, 5; Bennett et al. chapter 7
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First part
of theand
course
Remember
objects
classes
An object is a thing you can interact with:
 It receives and reacts to messages.
 It behaves in a certain way according to its internal state which
may change.
 It interacts with other objects, normally by using their names –
identity of an object. (Booch)
A class describes a set of objects with an equivalent role.
 Every object belongs to a class (myClock object belongs to – or
is inherited from - the class Clock); classes are object factories.
 A class will have a name, optionally some attributes and may
provide specific operations.
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A simple class
Systems analysis:
•user-oriented: actors and use cases;
•object-oriented: classes and relationships between them.
Class name
Clock
Attribute
time: Time
Type
Operations
reportTime(): Time
Value returned
resetTimeTO(newTime: Time)
Formal parameter
(argument)
Class identification:
•noun identification;
•responsibility driven approach (CRC cards).
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Class model
 Consists of classes (with their attributes and operations) and
their relationships.
 Formally these models are represented by class diagrams.
Some class symbols:
Dog
{abstract}
Animal
Class symbols
Module
Module
Abstract class symbol
name: String
Class with attributes
and operations
noOfStudents:int
timetable:Timetable
getStudents()
listLectures()
Bonzo: Dog
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Object symbols
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Associations
 An association may be considered conceptually or
from an implementation point of view:
 if there is a real-world association (described by a sentence
relevant to the system: a library member borrows a book; a
module is taken by a student);
 classes A and B are associated if:
• an object of class A sends a message to an object of class B;
• an object of class A creates an object of class B;
• an object of class A has an attribute whose values are objects of
class B or collections of objects of class B;
• an object of class A receives a message with an object of class
B as an argument.
 Associations are defined by verbs and CRD cards
collaborations identify them.
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Remember CRC cards
relationships – ATM system
Card
Responsibilities
Collaborations
BankAccount
Responsibilities
Collaborations
Dispenser
Responsibilities
Collaborations
Transaction
Responsibilities
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Collaborations
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ATM system - class diagram
Card
dispenses
BankAccount
Dispenser
Transaction
Class diagrams is obtained directly from CRC cards by considering
collaborations identified; it shows business classes.
Classes are shown with no attributes and operations.
Exercise. Draw up the class diagram for the Advertising company.
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Refining associations
Student
Tutor
Association - undirected or
bidirectional structural relationship.
Association – directed structural
relationship; students associated
with university.
Student
University
Campaign
Advert
Aggregation – whole/part
structural association.
ChessBoard
Square
Composition – whole/part
exclusive association.
Composition is a stronger version of aggregation. In
composition each part lives and dies with the whole
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Refining associations
Degree
Lecturer
Book
1..3
course
1
12..32
unit
Module
Association - with end roles and
multiplicities.
*
Module
Directed named association –
with multiplicities and navigable
direction.
teaches
1..3
0..1
item
borrower
Loan
Person
Aggregation – whole/part
structural association.
Association class – stores
attributes and operations of an
association
.
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ATM – refined class diagram
Card
1..*
*
refers to
dispenses
1
BankAccount
1
registers
1
*
Dispenser
Transaction
For each Card there is one bank account, one dispenser and an
arbitrary number of transactions.
Each BankAccount should have one or more cards associated with.
The Dispenser refers to all cards.
Each Transaction has a unique card on it.
All associations are named, directed and unidirectional.
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Advertising company – class
diagrams
Client
Advert
1
*
*
1
Campaign
initiates
contains
A Client might initiates a number of campaigns.
Each Campaign should have an arbitrary number of adverts
(alternatively a range may be used, if known).
The associations are undirected (or bidirectional) relationships.
The association “contains” is an aggregation; “initiates” is
only an association.
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Generalisation
 Generalisation indicates a “is a” relationship between classes.
Saloon
Car
Vehicle
Hatchback
 Remember generalisation relationships between uses cases/actors.
Transaction
Get
balance
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Card manager
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Bank
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Generalisation
Subclass
Superclass
 Arrow shows direction of association between subclass and its
superclass.
 Useful for showing abstraction of common behaviours.
 Abstract super-classes are not instantiated in the final system;
they can be re-used elsewhere.
 Sub-classes inherit all the properties (attributes and operations)
of the superclass as well as all of the associations.
 Avoid over use of generalisation. Large differences between
subclasses and forced creation of superclass may lead to
confusion.
 For example Helicopter and Bicycle are both vehicles, but
there is little to be gained from generalising both.
 Are Book and Journal related by generalisation?
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Examples of generalisations
SuccessfulTrans
Transaction
ATM system generalisation diagram
FailedTrans
NewspaperAdv
Advert
Advertising company generalisation diagram
Module
University system generalisation diagram
TelevisionAdv
UGModule
MScModule
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Object’s state and behaviour
The system that we build will consist of a collection of
objects which interact. Interactions are shown through
class diagrams.
So far we have identified:
 classes;
 relationships (associations) between classes.
Remember, an object is a thing which has behaviour,
state and identity (Booch).
The state is given by the attributes’ values and the
behaviour is shown by operations.
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Operations and attributes
 Operations define the ways in which objects interact:
 an object sends a message to another, asking to perform an
operation;
 the receiver will invoke a method to perform the operation; there
may be more than one method implementing an operation.
 Attributes describe the data contained in an object of a class.
 According to the conceptual approach on defining classes and
their associations, only data and messages expected to be
understood by an object are considered. Attributes
implementing associations are not considered in this stage.
 When is necessary/possible types associated with operations
and attributes are identified – remember slide 4.
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ATM – Card class
Card
cardNo
Card
cardHolder
Responsibilities
Collaborations
pin
Manage (validate, change) PIN.
Bank account provides bank details.
dayLimit
validatePIN()
changePIN()
Bank account provides balance and
Initiate withdrawal.
overdraft.
Dispenser provides amount requested.
startWidraw()
Card class’ attributes and operations;
validatePIN() validates the PIN introduced against the value in pin;
changePIN() changes the current pin value with what this operation provides;
startWithdraw() initiates the withdrawal by i) checking that the amount requested is within dayLimit
range; ii) checks with BankAccount that there is enough in the current bank account or the overdraft
limit is sufficient for this transaction; iii) it also checks that there is enough cash in dispenser; if all
these are fulfilled it asks BankAccount to update the balance and Dispenser to release the cash.
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ATM – BankAccount class
BankAccount
accountNo
BankAccount class:
balance
getBalance() provides the current balance;
overdraft
checkWithdrawal() checks the amount requested is within
balance range and does not exceed the overdraft limit;
getBalance()
checkWithdrawal()
updateBalance()
updateBalance() changes the current balance (and maybe
the overdraft limit) value(s) according to the amount taken
out from the account.
BankAccount
Responsibilities
Collaborations
Provide balance.
Check if amount available in the current
balance and overdraft.
Update balance.
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ATM – Dispenser class
Dispenser
cash
checkCash()
widrawCash()
Dispenser has an attribute cash with the current available
cash;
it checks that a given request coming through checkCash()
may be withdrawn (is less than cash value);
withdrawCash() updates the current cash value by taking
out the amount to be withdrawn.
Dispenser
Responsibilities
Collaborations
Check if cash available.
Update cash available.
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ATM – Transaction class
Transaction
date
cardNo
accountNo
status
Transaction keeps for every transaction the date the card
number and the account details as well as the status, either
failed or successful;
It uses recordTransaction() to store suitable values
according to the attributes described.
recordTransaction()
No CRC card has been produced so far for Transaction, but there
is the following description from the last lecture: let’s consider that
transactions involving cash withdrawal, either failed or successful,
are recorded. In this case, session (retained as vague) will be
reconsidered and Transaction is the class that it will be identified.
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Summary
 Objects, classes and relationships.
 Class/object diagrams and their relationships to CRC
cards.
 Class attributes and operations.
 Class operations obtained from CRC card
responsibilities.
 Class diagram associations and CRC card
collaborations.
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