object orientation in design

Object-Oriented Software Engineering

Chapter 4: Object-orientation in Design

Chapter 4: Object-orientation in Design 2

Table of Contents

• The OO Design Pyramid• What is UML?• What Constitutes A Good Model?• Essentials of UML Class Diagrams• Associations and Multiplicity• Analyzing and Validating Associations


Chapter’s Directory


The OO Design Pyramid…

The subsystem layer• Contains a representation of each of the

subsystems that enable the software to achieve its customer-defined requirements

The class and object layer• Contains the class hierarchies that enable the

system to be created using generalizations• Contains representations of each object


The OO Design Pyramid…

The message layer• Contains the design details that enable each object

to communicate with its collaborators• Establishes the external and internal interfaces for

the system

The responsibilities layer• Contains the data structure and algorithmic design

for all attributes and operations for each object


The OO Design Pyramid

responsibilitiesdesign

messagedesign

class and objectdesign

subsystemdesign


What is UML?…

The Unified Modelling Language is a standardgraphical language for modelling object oriented software

• At the end of the 1980s and the beginning of 1990s, the first object-oriented development processes appeared

• The proliferation of methods and notations tended to cause considerable confusion

• Two important methodologists Rumbaugh and Booch decided to merge their approaches in 1994

—They worked together at the Rational Software Corporation


What is UML?

• In 1995, another methodologist, Jacobson, joined the team

—His work focused on use cases • In 1997 the Object Management Group (OMG)

started the process of UML standardizationThe objective of UML is to assist in software development


What Constitutes A Good Model?

A model should• Use a standard notation • Be understandable by clients and users • Lead software engineers to have insights about the

system• Provide abstraction (eliminate complexity)

Models are used:• To help create designs• To permit analysis and review of those designs • As the core documentation describing the system


Essentials of UML Class Diagrams

The main symbols shown on class diagrams are:• Classes

- represent the types of data themselves• Associations

- represent linkages between instances of classes• Attributes

- are simple data (characteristics) found in classes and their instances

• Operations- represent the functions performed by the classes and

their instances• Generalisations

- group classes into inheritance hierarchies


Classes

A class is simply represented as a box with the name of the class inside

• The diagram may also show the attributes and operations

Rectangle

height: intwidth: int

getArea(): intresize(int,int)

Rectangle

heightwidth

getArearesize

Rectangle

heightwidth

Rectangle

getArearesize

Rectangle


Associations and Multiplicity…Multiplicity Multiplicity

Works for

0..1Has

0..*Makes

UML Notation Association with Multiplicity Association Meaning

Exactly 11or

leave blank

DepartmentEmployee 1Works for

DepartmentEmployee

An employee works for one and only one department.

Zero or one 0..1 SpouseEmployeeAn employee has either one or no

spouse.

Zero or more

0..*or

*

PaymentCustomer

PaymentCustomerMakes *

A customer can make no payment

up to many payments.

One or more 1..* CourseUniversityOffers 1..* A university

offers at least 1 course up to

many courses.

Specific range 7..9 GameTeam 7..9

Hasscheduled A team has either

7, 8, or 9 games scheduled


Associations and Multiplicity

An association is used to show how two classes are related to each other

• Symbols indicating multiplicity are shown at each end of the association

0,3..8 ******

Employee

*

* *****1..*

*0..1

Secretary

Office

Person

Company

Employee Company

Manager

BoardOfDirectors

BoardOfDirectors

If the interval has no upper bound, then * is used. Hence, 0..* and * mean the same thing


Labelling Associations…

• Each association can be labelled, to make explicit the nature of the association

• 2 types of labels:—Association name

- “An employee works for a company”

—Role name

- “A board of directors has either zero or 3 to 8 persons as board members”

* worksForEmployee Company

boardMember

0,3..8 ******Person BoardOfDirectors



• You could omit the association name and role name, then consider that an association’s name is simply has by default

• Provided the meaning might be clear by simply looking at the two classes

- “A company has a board of directors”

Company BoardOfDirectors



• A good rule of thumb is:—Add sufficient names to make the

association clear and unambiguous —Not necessary to add both role names and

association names—If “employer” is added next to the company,

is it sufficient?—Or instead, “workFor” would sound better?

* worksForEmployee Company


Labelling Associations

Each association can be labelled, to make explicit the nature of the association

*supervisor

*****1..*

* worksFor

*allocatedTo0..1

boardMember

0,3..8 ******

Employee

Secretary

Office

Person

Company

Employee Company

Manager

BoardOfDirectors

BoardOfDirectors


Analysing and Validating Associations…

• One-to-one—For each company, there is exactly one board

of directors—A board is the board of only one company—A company must always have a board

Company BoardOfDirectors



Avoid unnecessary one-to-one associations

Avoid this Do this

Person

nameaddressemailbirthdate

Person

name

PersonInfo

addressemailbirthdate



•Many-to-many—A secretary can work for many managers—A manager can have many secretaries—Secretaries can work in pools—Managers can have a group of secretaries—Some managers might have zero secretaries.

*supervisor

*****1..*Secretary Manager


Generalisation

Specialising a superclass into two or more subclasses

• The discriminator is a label that describes the criteria used in the specialization

Animal Animal

habitat typeOfFood

HerbivoreCarnivoreLandAnimalAquaticAnimal


Handling Multiple Discriminators

Animal

habitat

LandAnimalAquaticAnimal

AquaticCarnivore AquaticHerbivore LandCarnivore LandHerbivore

typeOfFood typeOfFood

Creating higher-level generalisation


More Advanced Features: Aggregation

• Aggregations are special associations that represent ‘part-whole’ relationships.

—The ‘whole’ side is often called the assembly or the aggregate

—This symbol is a shorthand notation association named isPartOf

****

****** Region

VehiclePart

Country

Vehicle


When to Use an Aggregation

As a general rule, you can mark an association as an aggregation if the following are true:

• You can state that—the parts ‘are part of’ the aggregate—or the aggregate ‘is composed of’ the parts

• When something owns or controls the aggregate, then they also own or control the parts


• A composition is a strong kind of aggregation —if the aggregate is destroyed, then the parts are

destroyed as well

Composition

***** RoomBuilding


Aggregation Hierarchy

* *

*WheelTransmissionEngineFrame

DoorBodyPanelChassis

Vehicle


Summary

At the completion of this chapter, you have learnt:

• What UML is• Features and functionality of OOD• Application in software design


Review Questions…

Draw diagrams to link the following classes using aggregation, inheritance and multiplicity where appropriate

(a) Village, Street, House, Shop, Road(b) University Staff, Academic, Administrator,

Technician (c) Subscriber, Paying Subscriber, Complimentary

Subscriber, Individual Paying Subscriber, Corporate Paying Subscriber

(d) Zoon, Animal, Bird, Mammal, Reptile, Cage, Keeper

(e) Estate, Building, House, Shop Unit, Room, Window, Door, Wall, Floor, Ceiling


Review Questions

Draw a class diagram, including class attributes, to represent the information given in the paragraph below:

A dental surgery keeps information about its patient, who may be either private or health service. For each patient the surgery records the name, address, phone number, date of birth, and either the health service number or the payment method.

object orientation in design

Documents

object layer

algorithmic design

design details

object management group

class hierarchies

classesa class

software engineers

software developmentchapter