Chap 05 - object and class...that we discover in Analysis by scanning the users’ business. 11/ 86 5.1. Real-World vs Data-World Objects Jacobson’s Three Types There are three kinds

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Chapter 5Chapter 5

Objects and ClassesObjects and Classes

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Chapter 5: Objects and Classes

�� 5.1. Real-World vs Data-World Objects

�� 5.2. Classes and Classification

�� 5.3. Transient and Persistent Objects

�� 5.4. Objects: Classes or Instances?

�� 5.5. Associations

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Here is what it’s all about:�We Analysts must understandunderstand and

documentdocument the real world real world where our users

live.

�We then create a product in the computer in the computer

world,world,

�To help the users do their job help the users do their job in the real

world.

��Effective analysts Effective analysts must ensure the

computer-world product accurately

mirrors the users’ realreal--world needs.world needs.

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� What An Object Is

� In the Real World

� In the Data World

� Jacobson’s Three Types

� Entity Objects

Concrete objects

Conceptual objects

Event and State objects

� Interface Objects

� Control Objects

� Attributes

� Behavior

� Identity

� Encapsulation

� Object States

Now let’s look at these in detailNow let’s look at these in detail. . .. . .

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The World is full of

THINGSTHINGS

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THINGSTHINGS� An object is some ThingThing in the world of the user

� Car, Train, Elephant, Sale, Invoice, Division, Account, etc.

� Physical or Conceptual

� It has AttributesAttributes that describe it:

� Make, Model, Year,

� Color, Weight,

� Serial Number, License Number

� It has RelationshipsRelationships to other objects:

� e.g., to a Person object, i.e., the Owner.

� It has BehaviorBehavior that it can do:

� Create

� Change Color

� Change Owner

� Destroy Itself (Suicide?)

EntityEntityEntityEntity

ObjectObjectObjectObject

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in the DATA WORLD

� We have some kind of record in the computer for each real-world object

� It carries data for the attributes� Make, Model, Year, Color, Weight, Serial Number, License

Number

� The set of attribute values represents the STATE of the object

� It carries a link of some kind for each relationship� Foreign Key or pointer

� It carries program code for each behavior� Create, Change Owner, Change Color, Delete

� CRUD: Create, Read, Update, Delete.

EntityEntityEntityEntity

ObjectObjectObjectObject

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A Data Object Data Object is an abstractionabstraction of some thingthing in the real world, that

carries both the datadata describing the real-world object , and the

operationsoperations (i.e., program codeprogram code) that have the

only allowable access only allowable access to that data.to that data.


We Define:We Define:

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�Entity Objects


�Control Objects


Jacobson’s Three Types

In the interests of stability and flexibility to

change, Jacobson classifies objects into three

types:

Let’s look at these in detail. . .Let’s look at these in detail. . .

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Definition:Definition:

Entity Objects Entity Objects are the fundamental objects and classes

that we discover in Analysis by

scanning the users’ business.scanning the users’ business.

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There are three kinds of Entity Objects:

��ConcreteConcrete objects

��ConceptualConceptual objects

��EventEvent and StateState objects. . .

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.Concrete Objects:

�Solid, tangible and physical.

�Things you can grab hold of.

�People, animals and other beings

�Land, buildings, equipment, vehicles

�Goods, products, tools, appliances,

materials

These are easily understood by both These are easily understood by both

analysts and users.analysts and users.

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Concrete Entity Objects

Person Apartment Elevator Food

Employee Warehouse ForkliftProduct

Customer House CarClothes

Student College Cell phone Pen

Citizen Street Truck Gravel

Engineer Office LatheSteel

Animal Farm Tractor Beer

Doctor Hospital ScalpelBlood

BeingsBeings Land & BuildingsLand & Buildings EquipmentEquipment GoodsGoods

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Jacobson’s Three Types .

Conceptual Objects:

� Intangible, nonphysical

�More difficult for users to grasp,

�But they are still thingsthings and objectsobjects

�Typically defined in terms of other

object classes . . .

e.g., by breaking out a M:M

relationship

See Chapter 9.

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Corporation Strategy Lease

Church Plan Mortgage

Regiment Map Ceasefire

Sports club Schedule Membership

Government dept Proposal

Approval

Professional assn Qualification

Charter

Union Bylaws

Contract


Conceptual Entity Objects

OrganizationsOrganizations AbstractionsAbstractions AgreementsAgreements

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� These are conceptualconceptual in the sense that a

contract does notnot consist of a piece of paper.

� Rather, it is an agreement that comes into

existence,

� The moment the two parties agreeagree, verbally or

otherwise.

� Signing the piece of paper documentsdocuments the

existence of the contract, to make it easier to

enforce (e.g. in court).


Jacobson’s Three Types Conceptual Objects:

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�Also very abstract in nature.

�When an EventEvent occurs, some object

will change its StateState.

�When a business event business event occurs,

it is documented, and that is a fact

we must record; i.e., it is it is DataData..


Jacobson’s Three Types Event and State Objects:

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�A statestate is a condition or situation.

�At any time an object exists in a certain

state.

�Sometimes the state is more important

to our project than the object, in which

case, for us,

the state itself is a thingthing, and an

object.object.


Jacobson’s Three Types State Objects:

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Purchase Deposit Ownership Enrolment

Delivery Loan Suspense Assignment

Negotiation Return Birth

Termination

Arrival Hire Status

Immigrant

Departure Rental Licence

Registration

Transaction Sale In Transit

Employment


Event and State Entity Objects

EventsEvents StatesStates

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Most software systems need

to exchange data with

other systems.



Interface Objects:

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These might be:�Other information systems, such as

Human Resources, Accounting, etc., etc.

�A real-world system controlled by our

software

Industrial processes, refineries, etc.

Machinery, robots, assembly lines, etc.

�Monitoring systems

Security systems, Intensive Care Units, etc.


Jacobson’s Three Types Interface Objects:

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�These days, they might also be:

�Communications Interfaces Point-to-Point

Modem and phone line

Fax and Fax-back

LAN, WAN, etc.

Internet and/or Intranet

Peripheral devices e.g. printers, scanners, sound and video Input/Output.

�Graphical User Interfaces (GUIs)



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�Suppose you developed a system that needed to communicate over modem lines.

�For simplicity, during testing you might just plug two serial ports together from one machine to another.

�When the software is deployed, we find things have changed - it must communicate over a LAN.

�And then what if the needs grew and the users wanted it to work across the Internet?



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�To handle this, we would hide all the

processing to do with the communication

link within an object.

�When the changes happen, we rewrite

some of the methods within this class,

�And add an attribute or two.

Nothing outside this class Nothing outside this class

needs to be touchedneeds to be touched!!!!



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�The complexity of the interface is

hiddenhidden within the interface object.

�All the interface processing is

hidden,

�And can be changed can be changed to handle

changes such as the communications

medium . . . . Without upsetting the rest Without upsetting the rest

of the systemof the system


Jacobson’s Three Types Interface Objects, Advantages:

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�This is an example of

EncapsulationEncapsulation, , or Information Hiding, Information Hiding,

which we explore later in this chapter.


Jacobson’s Three Types Interface Objects, Advantages:

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�� 5.1. Real5.1. Real--World vs DataWorld vs Data--World ObjectsWorld Objects

Jacobson’s Three Types Jacobson’s Three Types Interface Objects, Advantages:

OtherOther

InterfacesInterfaces

OutputsOutputs

User InterfaceUser Interface

Object BehaviorObject Behavior

Object AttributesObject Attributes

Object ClassesObject ClassesObject ClassesObject Classes

� One reason this

form of

encapsulation is

so important is

that

The interfaces to other The interfaces to other

systems are the systems are the

least stable aspect least stable aspect

of any information of any information

system. . .system. . .

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�Sometimes we find a method

(subroutine, function) that uses data,

and calls other methods, from many many

different different objects and classes.

�These methods don’t easily fit don’t easily fit or don’t don’t

logically belong logically belong

in any of our entity classes entity classes or interface interface

classes.classes.


Jacobson’s Three Types Control Objects:

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�So we create a class just to have

somewhere to put this method;

�This we call a

Control Class.Control Class.



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�Sometimes this control class will have

just this one method.one method.

�Nothing else, no attributesno attributes, no nothing!no nothing!

�Other times it will need a few attributes

for intermediate results, etc.

�And perhaps a few other methods.



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Definition:Definition:

Control Objects Control Objects are created during Analysis or Design to give us

somewhere to put behavior that

doesn’t easily fit doesn’t easily fit into existing

interface or entity objects.



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We’ve looked at these, Now let’s look at these in detail. . .





� Entity Objects

Concrete objects

Conceptual objects



� Control Objects

� Summary

�Attributes

�Behavior

�Identity

�Encapsulation�Violating Encapsulation

�Object States

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Attributes

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�A child concludes there are objects

out there

�And notices some properties they

have� e.g., Color, softness, taste, warmth, etc.

�The child is gathering data about

these objects

�We will call each such data item an

Attribute

�These Attributes fit our earlier


Attributes

In the real world:

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� Each object instance carries a set of attribute

values

� These represent the data items that a user

might need to know

about the object at any time

� Data types and sizes are defined for the class

� And apply to every instance.

� The domaindomain of an attribute is its set of all all permissible values.permissible values.


Attributes

In the data world:

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Behavior

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�A child makes two observations about

the objects surrounding her:

�They have attributesattributes, and

�They DODO things.�And she learns to influence that

behavior by sending them messagesmessages


Behavior

In the real world:

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�An object (a parent) responds to the

child’s message

by performing the behavior that goes

with that message

�And in the process performs some

serviceservice that the child needed.� This view of the behavior as a serviceservice is

important in ‘Responsibility‘Responsibility--Driven Design’Driven Design’

which we will examine in later chapters.


Behavior

In the real world:

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� Throughout life we use this method to

manipulate objects around us

� We issue commandscommands to people, computers,

cars, graphic objects, and all kinds of things,

� Sometimes we get the resultsresults we desire.

� Everyone, including your users, manipulates

their world with this “send“send--aa--messagemessage--getget--aa--

service” service” approach to life.


Behavior

In the real world:

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� OvertOvert behavior

� PassivePassive behavior

� ResponsiveResponsive behavior

Now let’s look at these in detail. . .Now let’s look at these in detail. . .


Behavior

In the data world, there are three types of

behavior that may concern us:

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�Where we actually instruct the object

to do something:do something:

�“Graphic object, display thyself”

�“Heating valve, open yourself”

�“Window, minimize yourself”

etc. . .


Behavior In the real worldBehavior In the real world

Overt Behavior:

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�This is really just recordrecord--keeping:keeping:

��The recording of the data generated by The recording of the data generated by

some action or event in the real world.some action or event in the real world.

�Here we do not make something happen

but merely record what happened:

�“Purchase Order, change your status.”

�“Customer, change your address”

etc. . .



Passive Behavior:

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�The providing of information

(i.e., attribute values)

in response to a message requesting

them.

�“Here is a customer number, give me

the name and address”

�“How many sales exist for this

customer?”

etc. . .



Responsive Behavior:

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We’ve looked at these, and these,





� Entity Objects

Concrete objects

Conceptual objects



� Control Objects

� Summary

�Attributes

�Behavior

Now let’s look at these in detail. .

�Identity

�Encapsulation�Violating Encapsulation

�Object States

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Identity

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� Identity is inherentinherent in an object.

�Oxford: inherentinherent - “Involved in the

essential character of something.”

�� InherentInherent means that an object has

identity just because it is that object.


Identity

In the real world:

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� A child has identity from the moment of birth,

or perhaps before.

� It doesn’t get a name until after its birth.

� So an identifier does not conferconfer identity

� But simply documentsdocuments or recognizes recognizes identity

that already existed.

� So do identifiers such as customer numbers,

social security numbers, etc.


Identity

In the real world:

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�We must be cautious about using real-

world identifiers in our software and

ensure they are truly unique

�Many O-O systems generate globally

unique OObject IDIDentifiers or “OIDs”“OIDs”

�Remember, only objects have identity,

attribute values don’t.


Identity

In the data world:

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Encapsulation

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� What does matter is

��Consistency in the published interface.Consistency in the published interface.� The Model T Ford had the clutch between the brake

and gas pedals! But. . .

� Every car made these days has the standard pedal

layout.

� Now we have standard user interfaces (or close) for

all kinds of products:

� Touch phones, keyboards, cars, motorcycles,

microwave ovens, thermostats, TV remote controls,

etc., etc., etc.


Encapsulation

In the real world:

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� What all of these do, is

Hide the details.Hide the details.� The complexity is enclosed within the product

� Leaving a small, easily-understood set of

commands to operate it.


Encapsulation

In the real world:

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� We hide both data and program code within

the object

� Giving a level of encapsulation better than

ever before,

Leading to

StabilityStabilityand

Portability.Portability.


Encapsulation

In the datadata world:

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�� StabilityStability is improved since once a class is built, it will survive the years with very few changes.

� It can be reused in new projects with few changes

� But all this only works if

We do our analysis accurately We do our analysis accurately

in the first place. in the first place.


Encapsulation

In the data world:

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�� PortabilityPortability is enhanced because:

Only the narrow published interfacenarrow published interface

needs to be learned

by the new developers,

i.e., the reuse.


Encapsulation

In the data world:

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� All the attributes and their values

should be hidden within the object

and accessible only via the methods.

�There should be a “crisp” boundary “crisp” boundary between what is insideinside the object and

what is outsideoutside it,

�Crossed only by the

narrow published interface.narrow published interface.


Encapsulation

In the data world:

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� Since we have encapsulated all the data and

behavior of our system within the classes,

we reduce the “ripple effect“ripple effect” of change.

� Even though the reduction may be small for

each class, remember. . .

�The number of possible interactions goes up

exponentially with the number of objects.

� In a large system this makes a huge

difference in maintenance.


Encapsulation

In the data world:

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Object States

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A Purchase Order Purchase Order

begins life as a request document, request document,

gets authorizedauthorized at various levels,

goes out to a vendor, out to a vendor,

is fulfilled by receivingreceiving the goods,

and finally is archivedarchived for future

reference.


Object States In the real world:

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Object States

States in the real world:

� Many objects in the real world go through a

predictable cycle predictable cycle

� From their creation,creation,

� Through their useful life,useful life,

� From stage to stage

� Until they die or are somehow disposed of disposed of

(Archived or deleted or whatever.)

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Object States

and something changes about it,

we say that it now exists in a different

state.state.� We can also say that each time an attribute

changes its value,

the object now exists in a new

state.state.

� Each time something happens to the object,

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�� 5.1. Real-World vs Data-World Objects: Object States

As the act or experience

of changing from one state to another.

� We define an object lifecycle object lifecycle asas

the various statesstates that an object may

transition through,

the set of permissible transitionstransitions,

and the sequencingsequencing of those states and

transitions,

as it progresses from its initial creation to its as it progresses from its initial creation to its

eventual disposal.eventual disposal.

�We define a State TransitionState Transition

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�� 5.2. Classes and Classification5.2. Classes and Classification




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Children and adults alike group objects by

some or all of the following criteria:

� Similar values for one or more attributes.

� An attribute value within a specified range.

� All that havehave a certain attribute (regardless

of value.)

� Those that display similar behavior.

� Those that have similar relationships.

�� 5.2. Classes and ClassificationIn the real world

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� We group objects together on the basis

of:

� Shared Attributes

� Shared behavior

� Class of:

� All Employees

� All Products

� All Transactions

� An individual object is then an

INSTANCE INSTANCE

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For our purposes, then,

A classclass is a group of objects with: similar similar propertiesproperties (attributes),

common common behaviorbehavior (operations),

common common associationsassociations to other

objects, and common common semanticssemantics(i.e., meaning.)

�� 5.2. Classes and ClassificationIn the real world

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The class of Fire TrucksThe class of Fire TrucksThe class of Fire TrucksThe class of Fire Trucks

The class of AnimalsThe class of AnimalsThe class of AnimalsThe class of Animals

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�Features not shared by the rest

of the class

�Attributes

�Behavior

�Associations

�Key verb is:

Some instances of a class may be Some instances of a class may be Some instances of a class may be Some instances of a class may be

grouped together on the basis ofgrouped together on the basis ofgrouped together on the basis ofgrouped together on the basis of

“ISAKINDOF”“ISAKINDOF”“ISAKINDOF”“ISAKINDOF”“ISAKINDOF”“ISAKINDOF”“ISAKINDOF”“ISAKINDOF”

�� 5.2. Classes and ClassificationSubclasses In the real world

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The class of Fire TrucksThe class of Fire TrucksThe class of Fire TrucksThe class of Fire Trucks

The class of AnimalsThe class of AnimalsThe class of AnimalsThe class of Animals

Subclass of ElephantsSubclass of ElephantsSubclass of ElephantsSubclass of Elephants

Subclass ofSubclass ofSubclass ofSubclass of

TurtlesTurtlesTurtlesTurtles

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� Yes, of course!

� But Why?

� Because a Customer is a personperson, , and

peoplepeople have names.

� Does a Customer have a Wage Rate?Wage Rate?

� No, only EmployeeEmployee-type people have one of those!

�� 5.2. Classes and ClassificationSubclasses In the real world

In our company, does a Customer have a name?In our company, does a Customer have a name?

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Balance Owing

Amount Overdue 30 days



Credit Rating

Date of Last Payment

Name Name

Address Address

Phone Phone

Date of Birth Date of Birth

Sex Sex

SIN

Marital Status

Number of Dependants

Date HiredWage Rate

Overlapping Sets of AttributesCustomer

Attributes

Customer

Attributes

Person

Attributes

Person

Attributes

Employee

Attributes

Employee

Attributes

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Venn diagram of

ATTRIBUTES

CUSTOMER CUSTOMER CUSTOMER CUSTOMER

ATTRIBUTESATTRIBUTESATTRIBUTESATTRIBUTES EMPLOYEEEMPLOYEEEMPLOYEEEMPLOYEE

ATTRIBUTESATTRIBUTESATTRIBUTESATTRIBUTESPEOPLEPEOPLEPEOPLEPEOPLE

ATTRIBUTESATTRIBUTESATTRIBUTESATTRIBUTES

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venn diagram of

INSTANCES

PEOPLEPEOPLEPEOPLEPEOPLE

CUSTOMERSCUSTOMERSCUSTOMERSCUSTOMERS

EMPLOYEESEMPLOYEESEMPLOYEESEMPLOYEES

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Hierarchy Diagram

(UMLUML notation)PERSONPERSONPERSONPERSON

CUSTOMERCUSTOMERCUSTOMERCUSTOMER EMPLOYEEEMPLOYEEEMPLOYEEEMPLOYEE

Balance

O/Due 30, 60, 90

Credit Rating

Date Paid

CheckCrRating

AgeBalances

Name, Address

Phone, Sex

Date of Birth

ChangeAddress

EnquireDOB&Sex

SIN

Marital Status

No. of Dependants

Date Hired

Wage Rate

GiveRaise

CalcMonthPay

CANBEACANBEACANBEACANBEA

ISAKINDOFISAKINDOFISAKINDOFISAKINDOF

This kind of arrowhead to

indicate that this relationship

is one of subclassing

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�Creating Instances

�Holding the program code

�Holding the attribute definitions

�Data type and size, etc.

�Point of contact for associations.

�� 5.2. Classes and ClassificationClasses In the DATA world

Classes perform such things as:Classes perform such things as:

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�Finding the classes is the core

activity of OOA.

�Chapter 9 has specific methods and

how-tos.

�So we develop a Class Diagram, and

add the attributes, to it

and then add the behaviors.

�� 5.2. Classes and ClassificationMapping Classes from the Real World to the Data World

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�You will add some extra classes as

you go,

�Some at Analysis, and some at

Design.

�Mostly they will be these 3 kinds:

� Interface Classes

�Control Classes

�Abstract classes

�� 5.2. Classes and ClassificationAdditional Data-world Classes

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��Interface ClassesInterface Classes� To encapsulate communication details

�With user (GUI), other systems, etc.

�Reuse from earlier projects.

��Control ClassesControl Classes�A place to put large method(s)

� That may draw data, and call methods, from many classes

� Typically these classes have few attributes, and perhaps only this one method.


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Abstract ClassesAbstract Classes� Often added to take advantage of

inheritance and polymorphism

� Sometimes we invent an “artificial”

superclass

� To make use of some common attributes or

behavior


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Abstract ClassesAbstract Classes� Such classes have no instances other than

the ones provided for them by their

subclasses

� That is, they have no Direct InstancesDirect Instances� For this reason we call them

Abstract ClassesAbstract Classes More in Chapter 8.


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�� 5.3. Transient and Persistent Objects5.3. Transient and Persistent Objects


�� 5.5. Relationships

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�For each class we discover or create,

we must decide whether these

objects need to be:

��Transient, Transient, destroyed at or before the end of the session, or

��Persistent, Persistent, kept in storage for a considerably longer time.

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�� 5.4. Objects: Classes or Instances?5.4. Objects: Classes or Instances?


83 / 86

Many authors use “object” to refer to a

classclass..

Strictly, as used in UMLas used in UML, an “object” “object” is

an InstanceInstance. .


BEWAREBEWAREBEWAREBEWAREBEWAREBEWAREBEWAREBEWARE::::::::

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�� 5.4. Objects: Classes or Instances?5.4. Objects: Classes or Instances?

�� 5.5. Associations5.5. Associations

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� Associations represent (i.e., they abstractabstract)

actions that involve two or more objects;

� That is, they abstract the interinteractionsactionsamong the objects.

� They are modeled by verbsverbs

� That abstractabstract what one object does to does to another.

� More in Chapters 8 and 9.

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End of Chapter 5.

Documents

Chap 05 - object and class...that we discover in Analysis by scanning the users’ business. 11/ 86 5.1. Real-World vs Data-World Objects Jacobson’s Three Types There are three kinds