Modelling classes

Drawing a Class Diagram

Class diagram

• First pick the classes– Choose relevant nouns, which have attributes and operations.

• Find the attributes– Any relevant information about the class objects.

• Next find the operations– Any behaviour of the class – any behaviour that changes the

attribute values of the class.

• Next find the associations– Classes are associated if they

• Invoke each other’s operations• Have protected access to each other’s attributes.

Use Case to Class

• The actors are not always persistent classes

• The primary tasks in the use case diagram must be the responsibility of a class.

• Is it a persistent class?

• Which class holds the attributes that the operation adds / amends/ updates / deletes?

Class operations

• Is it public or private?– Public, if other classes use this operation– Private, if it only called by the owning class.

• Does it imply an association?– Yes, if other classes invoke it, or it invokes

operations in other classes.

Association

ClaimantDateOfBirthNameAddressContactNo

GetAge()CalculateAge()

ClaimClaim DateClaim status

Make a claim()Settle a claim()Close a claim()

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• When the operation ‘close a claim is invoked– It invokes the ‘getage’ operation in claimant– Which invokes the operation ‘CalculateAge’ in

claimant

Sub and Super Classes

• A super-class is an abstraction of several classes that have mostly common attributes and operations.

• A sub-class instance IS A super-class instance.

• Some super-classes have no instances themselves - they are merely abstract classes.

Generalisation• A super-class is a GENERALISATION of a

sub-class.

• A sub-class is a SPECIALISATION of a super-class.

• Operations and attributes in the super-class are automatically INHERITED by objects in the sub-class.

• Operations and attributes in the sub-class can OVERRIDE those in the super-class.

Generalisation

• Inheritance • Information hiding• Mono and polymorphism• Overriding needed for

– extension– restriction– convenience– Optimisation

Generalisation examples

• A generic ‘person’ class will have – attributes of name, address, e-mail address, phone no,

etc.– Operations of

• Phone• Mail• E-mail

• An employee is a person• A driver isan employee• An assessor isan employee

And more…

• A radio alarm clock isa clock

• A watch isa clock

• Is a timer a clock?

General rules

• An object of a specialised class can be substituted for an object of a more general class in any context which expects a member of the more general class, but not the other way around.

• There must be no conceptual gulf between what objects of the two classes do on receipt of the same message.

Generalisations

• Are denoted as solid paths with a large hollow triangle pointing at the more general element.

• Must not be circular -i.e. an element cannot have a generalisation relationship to itself.

Sample Generalisations• A School of Computing student is a student.

• A school of computing student is an account-holder.

• A lecture is a member of academic staff is a member of staff of the school of Computing.

• A school of computing lecturer is an account holder.

• A frog is a water-based animal.

• A frog is a land-based animal.

• A fish is a water-based animal.

• A rabbit is a land-based animal.

• An elephant is a land-based animal.

Specialisations

• More specific elements.– Specialize more general elements.

– Receive all the characteristics (attributes, operations, methods and associations) of the more general elements via the mechanism of inheritance.

– May add their own characteristics.

– May override inherited methods.

– May be substituted for their more general elements.

Starting a Class Diagram

• Choose your candidate objects from the system description.

• For each candidate:– Is there more than one object in this class?– Is this object a system user / location?– Can you describe the type of information you

need to know about each member of this group of objects?

Starting Class Diagrams

• Rather than trying to define the entire class– concentrate on the data that is required for

business classes.– set up a data model, that can be converted into a

relational database, only without the keys.– later, loosen the structure, to give object-

oriented advantages.– add operations on the data.

What Is an Attribute?

• An attribute is the type of information you need to know about each object in a class.

• An attribute is an attribute when:-– it has a finite length– it has a single value for each object

• An attribute is not an attribute when:-– it in turn has several attributes– it has multiple values

Operations

The operations in a class include:– Constructor operations create new objects in the class.– Selector operations get information about and from an

object in a class.– Mutator operations set information about and to an

object in a class.– Destructor operations destroy objects of the class.

• An operation may also send a message to a class that is associated directly with the class to which the operation belongs. It can only send a message – it cannot operate on the other class!

Operations

• Are named services that may be requested of instances of a classifier.

• Are implemented by methods.• May have parentheses containing a comma-

separated parameter list that indicates the formal parameters passed to a method.

• May have a return list consisting of a comma-separated list of formal parameters passed back from a method.

Parameters

– May have a kind specified. This value may be ‘in’, ‘out’ or ‘inout’.

– May have a colon followed by a type expression that indicates the types of values a parameter may have.

– May have an equal sign followed by a default value that is used to set the value for unspecified parameters when the method is invoked.

Return parameter

• Must have a name or identifier string that represents the name of the parameter.

• May have a colon followed by a type expression that indicates the type of values a parameter may have.

What is an association?

• An association is a relationship between object classes.

• An association is used to implement a link between objects – to send messages or instructions from one class to another.– An object from one class can invoke a method

on an object from another class, thereby accessing it through its public interface.

Associations

• Associations correspond to verbs• Express the relationship between classes

– Class A and class B are associated if an object of class A

• Sends a message to an object of class B• Creates an object of class B• Has an attribute whose values are objects of class B

or collections of objects of class B• Receives a message with an object of class B as an

argument

Multiplicity

• Specifies how many objects from the class are involved in each association.

• Each class in the association has multiplicity.

• It can be.– An exact number e.g. 1.– A range of numbers e.g. 1..10.– An arbitrary, unspecified number, using *.

Associations

• Associate objects from two classes.• The multiplicity of the association is decided at

both ends of the association.• To decide on multiplicity,

– Put yourself in the position of a single object from the origin class.

– Ask yourself how many objects you are related to in the destination class.

– This decides the multiplicity at the destination end of the association.

Types of Association

• Simple binary associations.– Involve two classes, where each may get

information from the other.

• Uni-directional associations.– Involve two classes, where one can get

information from the other, but not vice-versa.– E.G. Customer and order. Depending on the

implementation, this could be uni- or bi-directional.

Associations

• May be reflexive – i.e. associate a class with itself.• May have a name that represents the name of the

association.• May have a name-direction arrow. (Small solid

triangle attached to the name, pointing in the direction of application of the name). If omitted, names are read right – left and top-bottom.

• May be association classes.• May have an aggregation indicator.

Association Classes

• Are denoted as class symbols attached by dashed lines to associations.

• Are associations with class properties or classes with association properties.

• Define a set of characteristics that belong to the relationship. The characteristics are not owned by any of the classes they relate.

Aggregation

• Indicated by a diamond.• A hollow diamond indicates weak or shared

aggregation.• A solid diamond indicates composition –

associated class objects must belong to only one component and are deleted if the composite is deleted.

• Must not be associated to both ends of an association.

Multiple Class Associations

• Associations among three or more classes are shown as a diamond with paths from each corner / side. Such an association.– Must not involve aggregation or qualifiers.– May have a single class appear more than once

or on multiple paths.

Aggregation

• Indicated by a diamond.• A hollow diamond indicates weak or shared

aggregation.• A solid diamond indicates composition –

associated class objects must belong to only one component and are deleted if the composite is deleted.

• Must not be associated to both ends of an association.

Driver

Staff numberPositionHome DepotReport outstanding

Notify of report()Verify report() Expert

Expert nameExpertiseContact details

Register expert()ExpertByExpertise()

Accident

Accident date/timeAccident locationAccident descriptionBus registration

Report Accident()Verify Accident()Getstatus()

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Payment

PayeePayment amountPayment statusPayment reasonAuthoriserIssuer

Request payment()Authorise payment()Issue payment()Reconcile payment()Cancel payment()

Expert Report

Report dateReport contentDate requested

Request report()Submit report()ReportSubmittedYN()

Claimant

Claimant nameClaimant addressDateOfBirth

Register a claim()GetAge()CalculateAge()

Claim

claim reported dateclaim stateclaim type

Report claim()Assess claim()Get claim details()Review claim()Get unassessed claims()Refer to expert()Refer to underwriter()

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