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Slide 12.1
CHAPTER 12
OBJECT-ORIENTED ANALYSIS PHASE
Slide 12.2
Overview
Object-oriented analysis Use-case modeling Class modeling Dynamic modeling Testing during the object-oriented analysis phase CASE tools for the object-oriented analysis phase Air Gourmet case study: Object-oriented analysis Challenges of the object-oriented analysis phase
Slide 12.3
Object-Oriented Analysis Phase
Object-oriented paradigm– Reaction to perceived shortcomings in structured
paradigm– Problem of larger products– Data and action treated as equal partners
Slide 12.4
Object-Oriented Paradigm
Object consists of– Data – Actions
Objects are independent units– Conceptual independence– Physical independence
Slide 12.5
Object-Oriented Analysis (OOA)
Semi-formal specification technique Many methods exist
– Booch– OMT– Objectory– Shlaer-Mellor– Coad-Yourdon
All essentially equivalent Most of OOA is done these days using UML
(Unified Modeling Language) Just In Case You Wanted to Know on pg. 367
Slide 12.6
The Three Steps of OOA
1. Use-case modeling– Determine how the various results are computed by the product
(without regard to sequencing)– Also referred to as functional modeling– Generate use-case diagram and associated scenarios– action oriented
2. Class modeling (“object modeling”)– Determine the classes and their attributes– Determine the interrelationships and interactions among the
classes– Generate class diagram - similar to ER diagram– data oriented
3. Dynamic modeling – Determine the actions performed by or to each class or subclass– Generate state diagram– action-oriented
Iterative process
Slide 12.7
Elevator Problem: OOA
1. Use-Case Modeling– Use case: Generic description of overall functionality
– Scenario: Instance of a use case
Slide 12.8
Normal Scenario
Slide 12.9
Exception Scenario
Slide 12.10
More on Scenarios
Sufficient scenarios should be generated to give the OOA team a comprehensive insight in the behavior of the system to be modeled
Scenarios are used in the class modeling step to determine the classes (objects)
Scenarios are also used in the OO design
Slide 12.11
2. Class Modeling
Extract classes and their attributes Represent them using an entity-relationship
diagram (ERD) Deduce the classes from use cases and their
scenarios Only the attributes of a class are determined
(the methods are determined in the OO design)
Often there are many scenarios– Possible danger: too many candidate classes
Slide 12.12
Two Approaches to Class Modeling
Noun Extraction– Three-stage process to extract candidate classes– Always works
CRC cards– Class-responsibility-collaboration (CRD) cards– Need to have domain expertise
Slide 12.13
Noun Extraction
Stage 1. Concise Problem Definition– Define the product as briefly and concisely
as possible, preferably in a single sentence– Elevator problem example
» Buttons in elevators and on the floors control the motion of n elevators in a building with m floors.
Slide 12.14
Noun Extraction (contd)
Stage 2. Informal Strategy– Incorporate constraints, express result in a
single paragraph » Buttons in elevators and on the floors control
movement of n elevators in a building with m floors. Buttons illuminate when pressed to request the elevator to stop at a specific floor; illumination is canceled when the request has been satisfied. When an elevator has no requests, it remains at its current floor with its doors closed.
Slide 12.15
Noun Extraction (contd)
Stage 3. Formalize the Strategy– Identify nouns in informal strategy (excluding those that
lie outside of the problem boundary). Use nouns as candidate classes
Nouns– button, elevator, floor, movement, building, illumination,
illumination, door
– floor, building, door are outside problem boundary — exclude
– movement, illumination, illumination are abstract nouns — exclude (may become attributes)
Candidate classes: Elevator and Button Subclasses: Elevator Button and Floor Button
Slide 12.16
First Iteration of Class Diagram
Problem– Buttons do not communicate directly with elevators– We need an additional class: Elevator Controller
Slide 12.17
Second Iteration of Class Diagram
All relationships are now 1-to-many – Makes design and
implementation easier
Slide 12.18
CRC Cards
Used since 1989 for OOA For each class, fill in card showing
– Name of Class– Functionality (Responsibility)– List of classes it invokes (Collaboration)– Now automated (CASE tool component)
Strength– When acted out by team members, powerful tool
for highlighting missing or incorrect items Weakness
– Domain expertise is needed
Slide 12.19
3. Dynamic Modeling
Produce UML state diagram
State, event, predicate distributed over state diagram
UML “guards” are in brackets
Slide 12.20
Testing during the OOA Phase
CRC cards are an excellent testing technique
Slide 12.21
CRC Cards
Consider responsibility– 1. Turn on elevator button
Totally unacceptable for object-oriented paradigm
Responsibility-driven design ignored Information hiding ignored Responsibility
1. Turn on elevator button
should be1. Send message to Elevator Button to turn itself on
Slide 12.22
CRC Cards (contd)
A class has been overlooked– Elevator doors have a state that changes during
execution (class characteristic)– Add class Elevator Doors
– Safety considerations Reconsider class model Then reconsider dynamic model, use-case
model
Slide 12.23
Second Iteration of CRC Card
Slide 12.24
Third Iteration of Class Diagram
Slide 12.25
Second Iteration of Normal Scenario
Slide 12.26
Elevator Problem: OOA (contd)
All three models are now fine We should rather say:
– All three models are fine for now We may need to return to the object-
oriented analysis phase during the object-oriented design phase
Slide 12.27
Why Is All This Iteration Needed?
Perhaps the method is not yet mature?– Waterfall model (explicit feedback loops)– Rapid prototyping model (aim: to reduce iteration)– Incremental model– Spiral model
Latter two explicitly reflect iterative approach Iteration is an intrinsic property of all software
production– Especially for medium- and large-scale products– Expect iteration in the object-oriented paradigm
Slide 12.28
CASE tools for OOA phase
Diagrams play a major role Diagrams often change
– Need a diagramming tool– Many tools go further
All modern tools support UML– Example
» Rational Rose» Smart Draw
Slide 12.29
Air Gourmet Case Study: OOA
Use-case model for making a reservation
Slide 12.30
Making a Reservation: Extended Scenario
Slide 12.31
Air Gourmet Case Study: OOA
Use-case for returning and scanning a postcard
Slide 12.32
Postcards: Extended Scenario
Slide 12.33
Air Gourmet Case Study: Class Modeling
Stage 1. Concise Problem Definition– Define product in single sentence
» A computerized system is needed to provide information regarding the efficacy of a special meals program.
Slide 12.34
Air Gourmet Case Study: Noun Extraction (contd)
Stage 2. Informal Strategy– Incorporate constraints, express result in a single
paragraph » Reports are to be generated to document the
efficacy of the special meals program. The reports concern meals loaded on flights, flights boarded by passengers, names and addresses of passengers, meal quality, and low-sodium meals.
Slide 12.35
Air Gourmet Case Study: Noun Extraction (contd)
Stage 3. Formalize the Strategy– Identify nouns in informal strategy. Use nouns as
candidate classes Nouns
– report, efficacy, program, percentage, meal, flight, boarding, passenger, name, address, quality
– efficacy, program, percentage, boarding, quality are abstract nouns — exclude (may become attributes)
– name, address are attributes of passenger
– Question: Should meal and flight be classes? It is easier to add classes than to remove them Candidate classes: Report and Passenger
Slide 12.36
First Iteration of Class Diagram
Problems with this class diagram– Data for reports are needed on a per-flight basis– Each report has to access multiple flights– Each flight has multiple passengers– Six reports (not four) are needed
Slide 12.37
Second Iteration of Class Diagram
Cause of our problem– Flight should
have been a candidate class
Slide 12.38
Air Gourmet Case Study: Dynamic Model
State diagram
Slide 12.39
Challenges of the OOA Phase
Do not cross the boundary into object-oriented design
Do not allocate methods to classes yet