Class diagram

Purpose of Structural Models

• Class diagrams represent

– Generic Categories (classes) of things (objects)

– Define relationships among Generic Categories (classes) of things (objects)

Classes

• Generic type of thing• Objects are the individual things• Classes are templates for creating instances or

objects– Concrete– Abstract

• Typical examples:– Application domain, user interface, data structure, file

structure, operating environment, document, and multimedia classes

Attributes

• Facts related to a type of thing– Attributes of a class– Class – Car Attribute may include: Make,

Model, Engine size, etc

• Only attributes important to the task should be included

Operations

• Behavior types of things do– Class – Car, Operation – accelerate(),

break(), etc

• Action that instances/objects can take – defined by the class

• Focus on relevant problem-specific operations (at this point)

• Quick Exercise – in 1 minute

Exercise 1 - Classes

• A Cinema shows films. Each film has a title, a producer, a director, a length, certificate and a genre. Patrons watch films at the cinema. Patrons are either adults or children and have a name and an age. To see a film a patron has to buy a ticket. A ticket has a price, a time and is for a film.

Exercise 1 – Solution Template

Class Attributes

Class Diagram

• Mechanism for visualising classes• Became truly central within all object oriented

methods• A Class Diagram shows

– Classes• Attributes• Operations

– Relationships• Which are consistent over time• Shows the structure of things and ideas in a

system

Basic Diagram

• Identify Classes

• Draw a rectangle for each class

• Name it

Add attributes and operations

• For each class identified

• Identify attributes– Facts associated with classes

• Identify operations/methods– Actions objects of the classes do

• Re-factor– Are the attributes in the right class?– Are the operations in the right class?

Simple Class Diagram

Attributes

Operations

Class

Studentnameage

takesNotes()doHomework()sitExam()gotoLibrary()gotoStudentUnion()

Relationships

• Some Classes can be related

• Relationships have difference types• Generalization / Specialization• Aggregation• Composition• Association

Relationships

• Classes do not exist in isolation

• It is important to consider how they – Interact– Relate

• Tend to be VERBS

• Five principal types of relationships

Type One - Dependency

• Where one class must know about another• Example:

– Prospective student knows of a course

• Not commonly used• Used more commonly in design / implementation

– Java – import javax.swing.*;

Car Wheel

Type Two - Association

• Classes associated with / related to one another

• Example– Student studies a course

• Very Commonly used

• “Default” type of relationship

Student Coursestudies►

Type Three - Aggregation

• Classes made up of other classes

• “Part of” relationship

• Objects of classes that can exist in their own right

• Example– Student is a member of a seminar group

SeminarStudent memberOf ►

Type Four - Composition

• Class is an integral part of another

• “Integral part of” relationship

• Object of classes that cannot exist without the other

• Example:– Seminar is delivered for a Module

Seminar ModuledeliveredFor ►

Type Five Inheritance

• Specialisation / generalisation between two classes

• Example– Part Time Student is a type of Student

StudentPartTimeStudent

Visibility

Visibility Description Symbol

Public Is directly accessible by an instance of any class

+

Private May only be used by an instance of the class that includes it

-

Protected

May be used either by an instance of the class that includes it or by a subclass of that class

#

Package Is directly accessible only by instances of a class in the same package

~

Association Classes

• Sometimes the way in which objects of a class are associated is just as important as the objects of a class themselves

• Consider the association between Student and Module. Where should our system record the student’s marks on for each student on each module?

Student Module1..* 6is taking

is takingmark : Integer

associationclass

Constraints

• a constraint is a condition that has to be satisfied • Use with care

– Too much text on a diagram– Can be over used

RentalCar Customerage

0..*1..* 0..*

rents

1..* {customer.age > 25}

Recursive Association - is allowed, and can be useful !

• an employee, as a manager, may manage other employees; each employee may be managed by another employee

employee

manages

0..*

0..1

Class v. Object DiagramStaff

staffNamestaffIDstaffstartdatestaffdept

AkhtarAli:Staff

staffName=Akhtar Ali staffID=313staffstartdate=140800staffdept=SDE

full symbol for an object is a box with 3 compartments• class name• attributes• operations

an instance symbol shows bothinstance and class name underlinedwith a colon to separate

in an instance symbol, both attributenames and values are given

ที่��มา• IS0514, Business Systems Development

Tools and Techniques, Northumbria University : Dr. M. Akhtar Ali