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

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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

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Object-Oriented Paradigm

Object consists of– Data – Actions

Objects are independent units– Conceptual independence– Physical independence

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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

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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

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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.

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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

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First Iteration of Class Diagram

Problem– Buttons do not communicate directly with elevators– We need an additional class: Elevator Controller

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Second Iteration of Class Diagram

All relationships are now 1-to-many – Makes design and

implementation easier

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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

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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

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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

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Second Iteration of CRC Card

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Third Iteration of Class Diagram

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Second Iteration of Normal Scenario

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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

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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

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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

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Air Gourmet Case Study: OOA

Use-case model for making a reservation

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Making a Reservation: Extended Scenario

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Air Gourmet Case Study: OOA

Use-case for returning and scanning a postcard

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Postcards: Extended Scenario

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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.

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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.

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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

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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

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Second Iteration of Class Diagram

Cause of our problem– Flight should

have been a candidate class

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Air Gourmet Case Study: Dynamic Model

State diagram

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Challenges of the OOA Phase

Do not cross the boundary into object-oriented design

Do not allocate methods to classes yet