How to find objects. 9.1. The Importance of This Step. Jacobson’s Three Types Entity Objects - things in the users ’ real world Entity Objects - things

How to find objectsHow to find objects

9.1. The Importance of This Step. 9.1. The Importance of This Step.

Jacobson’s Three TypesJacobson’s Three Types Entity Objects Entity Objects - things in the users’ real world- things in the users’ real world

ConcreteConcrete objects - Employee, Product, Tool, etc.objects - Employee, Product, Tool, etc.

ConceptualConceptual objects - Corporation, strategy, objects - Corporation, strategy, membership, approval, etc. membership, approval, etc.

Event and State Event and State objects - Purchase, delivery, objects - Purchase, delivery, arrival, ownership, status, etc. arrival, ownership, status, etc.

Interface ObjectsInterface Objects - - To encapsulate the details of To encapsulate the details of GUIs, GUIs, communication protocols, and the like.communication protocols, and the like.

Control ObjectsControl Objects - - To carry complex methods that don’t have a class they To carry complex methods that don’t have a class they obviously belong in.obviously belong in.

The KRB Seven-Step Method The KRB Seven-Step Method

Step Step Candidate ClassesCandidate Classes Several ways to find Several ways to find nounsnouns that might turn out to be names of that might turn out to be names of

classes that matter to your project.classes that matter to your project.

Step Step Define Classes Define Classes Here we check for the meaning the Here we check for the meaning the usersusers attach to each noun, and attach to each noun, and

decide whether it is important decide whether it is important for this project.for this project.

Step Step Establish Associations Establish Associations Checking how these things interact gives us a picture of the Checking how these things interact gives us a picture of the

pathways that exist throughout the datapathways that exist throughout the data

Step Step Expand Many-to-Many Associations Expand Many-to-Many Associations An essential step that generates much discussion and An essential step that generates much discussion and

more classes.more classes.

9.2. The KRB Seven-Step Method 9.2. The KRB Seven-Step Method OverviewOverviewStep Step Attributes Attributes

We list out all the data fields the users need to keep We list out all the data fields the users need to keep aboutabout each each thingthing (i.e., (i.e., aboutabout each type of entity).each type of entity).

Step Step NormalizationNormalization A technical consideration, with a different relevance in the A technical consideration, with a different relevance in the

object paradigm. This step is done behind closed doors object paradigm. This step is done behind closed doors without the users.without the users.

Step Step Operations (i.e., Behavior)Operations (i.e., Behavior) This is the additional step that distinguishes objects from This is the additional step that distinguishes objects from

entities and earlier models. This part is the subject of entities and earlier models. This part is the subject of Chapters 10 and 11.Chapters 10 and 11.

9.2. The KRB Seven-Step Method. 9.2. The KRB Seven-Step Method.

Step Step Candidate ClassesCandidate Classes

Definition:Definition:

Candidate Classes:Candidate Classes:

NounsNouns that are possible class names, that are possible class names, being considered for inclusion in the being considered for inclusion in the

project.project.

Just find all the names!Just find all the names!

The sources of entity objectsThe sources of entity objects

1. Client Interviews1. Client Interviews2. Nouns from the Documentation2. Nouns from the Documentation3. Brainstorming3. Brainstorming

The sources of interface objectsThe sources of interface objects

User Interfaces:User Interfaces: Your classes will be defined by your GUI tool.Your classes will be defined by your GUI tool.

Data Communications Interfaces:Data Communications Interfaces: If you buy a communications class library, your classeIf you buy a communications class library, your classe

s are already defined for you.s are already defined for you. The published comm protocol should be filed with or rThe published comm protocol should be filed with or r

eferred from your project documentation.eferred from your project documentation. Real-world systems to control or monitor:Real-world systems to control or monitor:

Industrial Processes, etc.Industrial Processes, etc. Define a single-instance class to represent the externDefine a single-instance class to represent the extern

al system.al system.

Will mostly be discovered in the Design Phase of the Will mostly be discovered in the Design Phase of the OODLC.OODLC.

As with anything else, if the topic arises earlier during the As with anything else, if the topic arises earlier during the Analysis Phase,Analysis Phase,

Use you judgement;Use you judgement; Either do it now, orEither do it now, or Make serious notes to Make serious notes to ensureensure it gets done later. it gets done later. (Don’t (Don’t

ever miss it!)ever miss it!)

Control classesControl classes


Step Step Define ClassesDefine Classes

A Real-World A Real-World IdentifierIdentifier

(“How do I tell one . . . from another?”)(“How do I tell one . . . from another?”)

A A DefinitionDefinition (“What is a . . . ?”) (“What is a . . . ?”)

Sample Sample AttributesAttributes and and BehaviorsBehaviors

(“What might I need to know (“What might I need to know aboutabout a . . .?” a . . .?”

or “What can happen to a . . .?”)or “What can happen to a . . .?”)

Each Candidate Class must undergo three checks:



Objects have Objects have Identity,Identity, So they are distinct from each other,So they are distinct from each other, And there is usually some way to And there is usually some way to tell them apart.tell them apart. How do we tell one How do we tell one StudentStudent from another? from another?

- Different names - Different names - won’t do, not unique.- won’t do, not unique.

-- Also, names and such are too sensitive to errors Also, names and such are too sensitive to errors Punctuation and casePunctuation and case Spelling and spacing Spelling and spacing

- Different faces - Different faces - scanning technology not yet - scanning technology not yet good enoughgood enough

Question 1: A Real-World IdentifierQuestion 1: A Real-World Identifier



Student No.Student No. Customer No.Customer No. Account No.Account No. Licence No.Licence No. Serial No.Serial No. Product CodeProduct Code Category CodeCategory Code Vendor IDVendor ID Employee No.Employee No.


Some Examples of Identifiers:Some Examples of Identifiers: Building ID Street Name City Name State/Province ID Vehicle ID No. (VIN) Inventory No. Room No. Legal Description (Land) Policy No.



So we try to find a unique identifier for our So we try to find a unique identifier for our Candidate Class.Candidate Class.

If we can’t, then it’s definitely not a class.If we can’t, then it’s definitely not a class. If we eventually do find one, then we move on If we eventually do find one, then we move on

to the next test,to the next test,

Question 2: The Definition . . .Question 2: The Definition . . .


Employee: A person who works at XYZ Widgets.Employee: A person who works at XYZ Widgets. Sounds reasonable, but do we simply delete the records when (s)he Sounds reasonable, but do we simply delete the records when (s)he

quits? Try this:quits? Try this: Employee: A person who works, or has worked, at XYZ Widgets.Employee: A person who works, or has worked, at XYZ Widgets. Better, and this will work for most businesses.Better, and this will work for most businesses. But you may find a company that wants But you may find a company that wants prospectiveprospective employees to be kept employees to be kept

in the database! Try this:in the database! Try this:

Employee: A person who works, has worked, or may in the Employee: A person who works, has worked, or may in the future work at XYZ Widgets.future work at XYZ Widgets.



Question 2: The DefinitionQuestion 2: The Definition


Step Step Establish AssociationsEstablish Associations

Here we ask the questions:Here we ask the questions: ““What does a Student What does a Student DODO to a Course?” to a Course?” Enrols in Enrols in it, of course.it, of course. ““What does a Customer What does a Customer DODO to a Product?”to a Product?” A Customer A Customer buysbuys a Product.a Product.

““What does a Teacher What does a Teacher DODO to a Course?”to a Course?” A Teacher A Teacher teachesteaches a Course.a Course.



Here we ask the questions:Here we ask the questions: ““What does a Student What does a Student DODO to a Course?” to a Course?” Enrols in Enrols in it, of course.it, of course.

““What does a Customer What does a Customer DODO to a Product?”to a Product?” A Customer A Customer buysbuys a Product.a Product.

““What does a Teacher What does a Teacher DODO to a Course?”to a Course?” A Teacher A Teacher teachesteaches a Course.a Course.



An An associationassociation is an is an interinteractionaction between instances of two between instances of two

classes,classes,

Represented by a Represented by a verb.verb. The ones we want are those that describe The ones we want are those that describe

things that happen in the users’ business.things that happen in the users’ business. We need a sentence that goes We need a sentence that goes

““Object Object verbverb Object” Object” And makes senseAnd makes sense in the users’ world. in the users’ world.



There are two important parts to this There are two important parts to this sentence:sentence:

The The VerbVerb, , and and

The The MultiplicityMultiplicity..

The KRB Seven-Step Method. The KRB Seven-Step Method.


Draw a line against the list of classes,Draw a line against the list of classes,

each time you check out a possibility.each time you check out a possibility.

This way you check out This way you check out each classeach class

against against every classevery class below itbelow it in the list. in the list.

Customer

Product

Vendor

Invoice

Bin

Warehouse


Step Step Expand Many-to-Many Expand Many-to-Many AssociationsAssociations

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Take a look at this card-and-string database:



This will allow us to do sales reports by either:This will allow us to do sales reports by either: Pulling a Customer card, and the strings will Pulling a Customer card, and the strings will

bring all the relevant Product cards,bring all the relevant Product cards, Or vice-versa.Or vice-versa. But there are problems . . .But there are problems . . .

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More realistically, we could perhaps write on the cards More realistically, we could perhaps write on the cards instead of using strings:instead of using strings:

As you can easily see As you can easily see on Jo’s card, we are on Jo’s card, we are

likely to run out of likely to run out of

room!room!



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And,And,The data is recorded in The data is recorded in twotwo places, places,

which is not good design,which is not good design,

since it since it willwill get getout of stepout of step

(Murphy’s Law)(Murphy’s Law)



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Not only that, Not only that, but where would we write the date, but where would we write the date, quantity and selling price for each sale?quantity and selling price for each sale?

On the strings, On the strings,

perhaps?perhaps?



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We replace each string with a card and two strings:We replace each string with a card and two strings:



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This allows us This allows us to write the details to write the details on the card in the on the card in the

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This allows us This allows us to write the details to write the details on the card in the on the card in the

middlemiddle



Now let’s do Now let’s do

the rest:the rest:

Now let’s do Now let’s do

the rest:the rest:

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Customer

*NameAddressPhone No.Balance

Product

*DescriptionUnit PriceQty in Stock

Sale

*Name*DescriptionQty SoldActual Price

Check the strings - the M:M is now 2 x 1:M !!Check the strings - the M:M is now 2 x 1:M !!Check the strings - the M:M is now 2 x 1:M !!Check the strings - the M:M is now 2 x 1:M !!

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isinvolved

in has

** **11 11


Step Step

AttributesAttributes


Step Step

NormalizationNormalization


Step Step Normalization Normalization

Let’s start with an example.Let’s start with an example.

Say you need a sales report something like this:Say you need a sales report something like this:

Customer Catalog Unit Qty Actual ExtendedNo. Name Address No. Description Price Date Sold Price Price

131 Jo Blo 13 May St 3A21 T-Shirt 12.49 03/01/98 45 10.00 450.00179 Yo Yo 271 OK Ave 1B77 Sweats 15.00 01/03/98 12 15.00 180.00212 Mu Mu 32 Saddle Rd 4X21 Pants 23.47 12/11/98 5 21.00 105.00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



is to build a is to build a relational table relational table

that that mimics this report.mimics this report. That is, it has the That is, it has the same columns same columns as this report.as this report. In the object paradigm, the In the object paradigm, the equivalent error equivalent error

would be to create a class with would be to create a class with attributesattributes

that that match the columns match the columns on the report.on the report. But what would we call this class?But what would we call this class? The best name would probably be something like The best name would probably be something like “Sales” “Sales” or or “Sales “Sales

Analysis.”Analysis.”

But . . .But . . .

What the uninitiateduninitiated (read “amateuramateur”) database designer tends to do



We have:We have:

Data that describes a Data that describes a CustomerCustomer (Cust No./Name/Address)(Cust No./Name/Address)

Data that describes a Data that describes a ProductProduct (Cat No/Description/Unit Price)(Cat No/Description/Unit Price)

And data that describes a And data that describes a SaleSale (Date/Quantity/Actual (Date/Quantity/Actual and Extended Prices)and Extended Prices)

Compare this situation with all the earlier models we have looked at,Compare this situation with all the earlier models we have looked at,

You’ll see that You’ll see that CustomerCustomer, , ProductProduct and and SaleSale should each be a should each be a sseparate class . . .eparate class . . .

The problem is that we have three kinds three kinds of data in this report.



Customer

*NameAddressPhone No.Balance

Product

*DescriptionUnit PriceQty in Stock

Sale

*Name*Description*DateQty SoldActual PriceExt. Price

isinvolved

in has

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** **11 11

Like this!Like this!


Step Step

Operations Operations (i.e., Behavior) (i.e., Behavior)


Step Step Operations (i.e., Behavior) Operations (i.e., Behavior)

1. By Inspection1. By Inspection2. Basic 2. Basic CRUDCRUD3. Use Cases3. Use Cases4. Statechart Diagram4. Statechart Diagram5. CRC Cards5. CRC Cards6. 6. CRUDCRUD Revisited Revisited

Six techniques for Six techniques for Finding OperationsFinding Operations

Documents

How to find objects. 9.1. The Importance of This Step. Jacobson’s Three Types Entity Objects - things in the users ’ real world Entity Objects - things