From Problem Statement to Design

From Problem Statement to Design

The Object Modeling Process

MIS 83 Notes

MIS 83 - Object Oriented Analysis - 20020801-2

References

Sources for some of the material in this section:

Beginning Java Objects, Jacquie Barker, Wrox Press, Ltd. 2000 ISBN 1861004176

Object-Oriented Systems Analysis, Modeling the World in Data, Sally Shlaer, Stephen Mellor, Yourdon Press, 1988 ISBN 013629023-X

An Example of Object Oriented Design, An ATM Simulation, Russell Bjork, (Website)

http://www.math-cs.gordon.edu/local/courses/cs320/ATM_Example/default.html


The Process

Narrative Problem Statement Class Identification Class Relationships Class behavior (e.g. Public methods required)


Narrative Problem Statement

Potential Sources Forms, reports Engineering drawings Real world object specifications Interviews


Bjork's ATM Example


ATM Narrative (Partial) from Bjork's website

Services one customer at a time Must be able to handle 10 customers an hour Customer inserts ATM card and enters PIN to begin Transactions

Cash withdrawals --- in $20 increments Deposits Transfers Balance inquiries

Immediate transaction validation with the bank Standard error procedures

Pin error, rejection due to inadequate balance Reports


Narrative Problem Statement - Use Cases

A use case is a logical sequence of events from the users first interaction, to the desired outcome.

For each use scenario, and each user (actor) . . . a use case describes The services the system provides to the user The sequence of events The desired outcome

Users of systems can be people organizations other systems


Some Use Cases for the ATM

From the Bjork website:System startup . . .

User: System operatorServices: Load cash dispenser, start ATM

Cash withdrawal . . . User: CustomerServices: Choose type of account, choose dollar

amt, Validate withdrawal with Bank

"The system is started when the operatorturns the switch on the panel to 'on'. The operator will be asked to enter the amountof money in the cash dispenser . . . "


Object Classes Are . . .

Tangible Things Airplane, ATM, person

Roles Professor, student

Incidents and interactions Purchase, withdrawal, flight

Specifications Aircraft characteristics: Weight, wingspan, max speed

Notice that many objects are not physical. (e.g. transactions, like a purchase, are not physical)


Identifying Potential Object Classes -- Exercise -- (Steps from the Barker text)

Identify noun-phrases (nouns and related adjectives) in the narrative descriptions

Eliminate duplicates (including plurals of items in the list) Eliminate state information (e.g. terms like "completion") Eliminate synonyms Assure that the classes are abstractions that a user would

recognize (Not implementation tools) Determine whether actors not in the class list should be.

Check to see if other actors need their services.

Use the requirements statement and use cases to develop a list of objects.


Naming Objects

Avoid roles or states in the names (e.g. ATM dispensing cash)


Testing Potential Classes

Objects instantiated from the classes must have the same set of attributes . . . Dog licenses and vehicle licenses cannot be in the

same class However, they can both be derived from the same class

Objects must have attributes Social security number is not a class, it is only a name Social security account is a class

"Or" must be avoided in defining the class The objects formed from a class must be more

than a list . . . The class must be definable


Exercise Continued

Test the classes you created

Are any actors mentioned in the use cases, not included as classes?

Review: Bjork's class list. Do you agreewith the list? (Do the classes pass the tests?)


Static Class Relationships

Static relationships Inheritance: Class a is derived from class b Association, e.g.

President runs an airlineThere are many students in one UniversityThere may be many Universities associated with

one student. Aggregation (Special case of association): class a "is

comprised of" Class b, e.g.A building has roomsA course has 5 sections

Associations and aggregations are coded the same way.


Static Class Relationships in the Unified Modeling Language (UML)

Animal

Dog Cat

Inheritance

School

University

Aggregation


Static Class Relationships in the Unified Modeling Language (UML) - Continued

Section Professor

Course *

*

1

*

1*

Association relationships possible: 1 * --- One to many* * --- Many to many1 1 --- One to one

What are the specific associations in this example?

Associations


Static Class Relationships in the Unified Modeling Language (UML) - Continued

Section Professor

Course *

*

1

*

1*

PersonInheritance andAssociations in thesame diagram


Exercise --- continued

Consider the following list of ATM classes

Are there any inheritance relationships? Do any classes contain other classes? Are there any other associations --- what are they?

ATM Bank

Card Reader Session

Keyboard Transaction

Display Withdrawal

Cash Dispenser Envelope Acceptor

Operator Panel Deposit

Transfer Inquiry

Draw a UML representation of the relationships.Review: Bjork's UML definition.


Completing the Static Class Definitions

Determine and define attributes of each class

Give an example of a field belonging to thecash dispenser.Review: Bjork's field definitions


Dynamic class behavior (public methods)

State of an object --- Values of all attributes at a specific time

Methods --- Respond to events Handle requests from other objects Send requests to other objects by calling their methods May return values --- Accessor methods (i.e. have a

non-void return value) May modify state of an object --- Mutator methods (i.e.

may have a void return value)


A first step in establishing public methods

Start with a use case or state diagram Construct a sequence of steps required of

objects to complete the sequence of events associated with each use case.

For each step Service description Object requesting the service (client) Object fulfilling the service Next object (or objects) involved in completing the

sequence Identify methods for each object to perform the

services.


Sequence of Steps for a Withdrawal

Withdrawal transaction ATM --> Card Reader checkForCardInserted() ATM --> Session startSession() Session --> Keyboard readPin() Session --> Keyboard readMenuChoice() Session --> WithDrawalTransaction

getTransactionSpecifics() WithDrawalTransaction --> Bank initiateWithdrawal (cardNumber,PIN,ATM number, serial, from,

amount)Some of these steps require instantiation of newobjects by adding them to vectors . . . others do not. What is an example of this?Following the state diagram for a session, what's next?


Next step: From model to program . . .

Inheritance relationships coded as extends Relationships

1:1 becomes instantiation of the related class 1:* becomes instantiation of a Vector or array

containing the related class Public Methods are determined from each

sequence of steps for major uses. The methods are associated with specific

classes. Additional implementation classes

GUI design and classes Data base and other technology-related classes

From Problem Statement to Design

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