From Problem Statement to Design The Object Modeling Process MIS 83 Notes.
-
Upload
hilary-ward -
Category
Documents
-
view
216 -
download
0
Transcript of From Problem Statement to Design The Object Modeling Process MIS 83 Notes.
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
MIS 83 - Object Oriented Analysis - 20020801-3
The Process
Narrative Problem Statement Class Identification Class Relationships Class behavior (e.g. Public methods required)
MIS 83 - Object Oriented Analysis - 20020801-4
Narrative Problem Statement
Potential Sources Forms, reports Engineering drawings Real world object specifications Interviews
MIS 83 - Object Oriented Analysis - 20020801-5
Bjork's ATM Example
MIS 83 - Object Oriented Analysis - 20020801-6
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
MIS 83 - Object Oriented Analysis - 20020801-7
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
MIS 83 - Object Oriented Analysis - 20020801-8
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 . . . "
MIS 83 - Object Oriented Analysis - 20020801-9
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)
MIS 83 - Object Oriented Analysis - 20020801-10
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.
MIS 83 - Object Oriented Analysis - 20020801-11
Naming Objects
Avoid roles or states in the names (e.g. ATM dispensing cash)
MIS 83 - Object Oriented Analysis - 20020801-12
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
MIS 83 - Object Oriented Analysis - 20020801-13
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?)
MIS 83 - Object Oriented Analysis - 20020801-14
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.
MIS 83 - Object Oriented Analysis - 20020801-15
Static Class Relationships in the Unified Modeling Language (UML)
Animal
Dog Cat
Inheritance
School
University
Aggregation
MIS 83 - Object Oriented Analysis - 20020801-16
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
MIS 83 - Object Oriented Analysis - 20020801-17
Static Class Relationships in the Unified Modeling Language (UML) - Continued
Section Professor
Course *
*
1
*
1*
PersonInheritance andAssociations in thesame diagram
MIS 83 - Object Oriented Analysis - 20020801-18
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.
MIS 83 - Object Oriented Analysis - 20020801-19
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
MIS 83 - Object Oriented Analysis - 20020801-20
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)
MIS 83 - Object Oriented Analysis - 20020801-21
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.
MIS 83 - Object Oriented Analysis - 20020801-22
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?
MIS 83 - Object Oriented Analysis - 20020801-23
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