Domain Models

About Domain Models1. Domain model is the most important—and classic—

model in OO analysis.2. Illustrates noteworthy concepts in a problem

domain. 3. Acts as a source of inspiration for software objects.

Value of OOA/D knowledge over UML notation; – Basic notation is trivial, but there are subtle modeling

guidelines for a useful model.– Will explore basic skills in creating domain models.

Domain Model• Key idea: Domain Model – A visual dictionary of

abstractions about the real world.• Class diagrams with no operations specified. • Attributes and relationships YES.• Conceptual classes: idea, thing, or object. • Classes don’t necessarily correspond to software

classes.

Store Register Sale

How to identify classes?1. Use a conceptual class category list.

2. Linguistic Analysis: identify noun phrases in use cases.

Conceptual Class Category List• Physical or tangible objects

– Register, Airplane• Specifications, or descriptions of things

– ProductSpecification, FlightDescription• Places

– Store, Airport• Transactions

– Sale, Payment, Reservation• Transaction items

– SalesLineItem• Roles of people

– Cashier, Pilot• Containers of other things

– Store, Hangar, Airplane• Things in a container

– Item, Passenger• Computer or electro-mechanical systems

– CreditPaymentAuthorizationSystem, AirTrafficControl• Catalogs

– ProductCatalog, PartsCatalog• Organizations

– SalesDepartment, Airline

Same category list applies to

many problems.

Finding noun phrases in use casesMain Success Scenario (or Basic Flow):1. Customer arrives at POS checkout with goods and/or services to

purchase.2. Cashier starts a new sale.3. Cashier enters item identifier.4. System records sale line item and presents item description, price,

and running total. Price calculated from a set of price rules.5. System presents total with taxes calculated.

6. Cashier tells Customer the total, and asks for payment.7. Customer pays and System handles payment.8. System logs completed sale and sends sale and payment

information to the external Accounting system (for accounting and commissions) and Inventory system (to update inventory).

…

Candidate Conceptual Classes

• Register (cat list)• Item (noun)• Store (cat list)• Sale (noun)• Payment (noun)• ProductCatalog (cat list)

• ProdSpecification (cat list)

• SaleLineItem (noun)• Cashier (noun)• Customer (noun)

Attribute or class?If we don’t think of some conceptual class X as a number or string, then X is probably a class, not an attribute.

Need for specification classes• E.g.

– An Item instance represents a physical item in a store.– An Item has a description, price, and itemID, which aren’t

recorded anywhere else.– Everyone in store has amnesia.– Every time a real physical item is sold, an Item is deleted.

• What can go wrong?1. When some product is all sold out there is no info

anymore about it.2. The data about price and description is replicated at each

item. • This brings update anomalies. Why?

Solution: add specification class

Even if all inventoried items are sold and their corresponding Item software

instances are deleted, theProductDescription still remains.

Another Specification Example (Flights)

Associations• Associations record relationships for which there

is a need to preserve some memory. Usually when: – A is subpart/member of B. (SaleLineItem - Sale)– A uses or manages B. (Cashier – Register, Pilot-

airplane)– A communicates with B. (Student - Teacher)– A is transaction related to B. (Payment - Sale)– A is next to B. (SaleLineItem - SaleLineItem)– A is owned by B. (Plane-Airline)– A is an event related to B. (Sale-Store)

Roles and Multiplicity• Each end of an association is called a role.

• Multiplicity defines how many instances of a class A can be associated with one instance of class B.

• E.g., a single instance of a Store can be associated with “many” (zero or more) Item instances.

ItemStore Stocks

*

multiplicity of the role

1

Some examples of multiplicityzero or more;"many"T*

one or moreT1..*

one to 40T1..40

exactly 5T5

T3, 5, 8

exactly 3, 5, or 8

POS Domain Model

Register

Item

Store

addressname

Sale

datetime

Payment

amount

LineItem

quantity

Stocked-in

*

Houses

1..*

Contained-in

1..*

Records-sale-of

0..1

Paid-by

1

1

1

1

1

1

1

1

Captured-on 4

Multiple associations

Flight Airport

Flies-to

Flies-from

*

* 1

1

• It’s non uncommon to have more than one associations between two classes.

Attributes vs. Associations

Cashier

namecurrentRegister

Cashier

name

Register

numberUses

Worse

Better

not a "simple" attribute

1 1

• Relate conceptual classes with an association, not with an attribute.

Conceptual vs. Implementation• How the associations are implemented in Java or

C++?• Very often by references or pointers, which are

attributes of the software classes. • This is a source of confusion…but they are

conceptually different. • So, we keep the associations distinct from

attributes in the Domain Model.

In practice…keep the implementation in mind

• Suppose you have two Personobjects, o1 and o2 whose names are “Jill Smith.”

• These people are different, and so are the memory addresses (refs) of o1 and o2.

• Thus, when comparing we do so with respect to memory addresses and we are Ok.

• Now, suppose you want to compare the phone numbers of two people.

• You care about the values, not about their memory address!

• The same for name, address, etc.

Bottom lineAttributes are data values.Associations are refs.

• What about values that can get too complex, like address?

• The choice is yours, with a slight preference for creating separate classes for them.

• Also, no attributes as foreign keys.

Register

ItemStore

addressname

Sale

date

time

Payment

amount

SalesLineItem

quantity

CashierCustomer

Manager

ProductCatalog

ProductSpecification

descriptionpriceitemID

Stocks

*

Houses

1..*

Used-by

*

Contains1..*

Describes

*

Captured-on

Contained-in

1..*

Described-by

*

Records-sale-of

0..1

Started-by

Paid-by Initiated-by

Logs-completed6

*

3 Records-sales-on

1

1

1

1

1

1..*

11

1

1

1

1

1

1

1

1 1

1

What about some DM for the airline?