Notes

Outline

Object-Oriented and Classical Software Engineering   Fifth Edition, WCB/McGraw-Hill, 2002 Stephen R. Schach srs@vuse.vanderbilt.edu

CHAPTER 12

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

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

Object-Oriented Paradigm Object consists of Data (attributes, state variables, instance variables, fields, data members), and Actions (methods, member functions) Objects are independent units Conceptual independence Physical independence

Object-Oriented Analysis (contd) Semi-formal specification technique Multiplicity of different methods Booch OMT Objectory Shlaer-Mellor Coad-Yourdon All essentially equivalent Nowadays, we represent OOA using UML (unified modeling language)

The Three Steps of OOA 1. Use-case modeling Determine how the various results are computed by the product (without regard to sequencing) Largely action oriented 2. Class modeling (“object modeling”) Determine the classes and their attributes Purely data-oriented 3. Dynamic modeling Determine the actions performed by or to each class Purely action-oriented Iterative process

Elevator Problem: OOA 1.  Use-Case Modeling Use case: Generic description of overall functionality Scenario: Instance of a use case Get comprehensive insight into behavior of product

Normal Scenario

Exception Scenario

Class Modeling Extract classes and their attributes Represent them using an entity-relationship diagram Deduce the classes from use cases and their scenarios Often there are many scenarios Possible danger: too many candidate classes

Two Approaches to Class Modeling Noun extraction Always works CRC classes Need to have domain expertise

Noun Extraction Stage 1. Concise Problem Definition Define product in single sentence Buttons in elevators and on the floors control the motion of n elevators in a building with m floors.

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.

Noun Extraction (contd) Stage 3.  Formalize the Strategy Identify nouns in informal strategy.  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

First Iteration of Class Diagram Problem Buttons do not communicate directly with elevators We need an additional class: Elevator Controller

Second Iteration of Class Diagram All relationships are now 1-to-n Makes design and implementation easier

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

3.  Dynamic Modeling Produce UML state diagram State, event, predicate distributed over state diagram UML “guards” are in brackets

Testing during the OOA Phase CRC cards are an excellent testing technique

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 be 1. Send message to Elevator Button to turn itself on

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

Second Iteration of CRC Card

Third Iteration of Class Diagram

Second Iteration of Normal Scenario

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

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, and 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

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 Rose

Air Gourmet Case Study: OOA Use-case model for making a reservation

Making a Reservation: Extended Scenario

Air Gourmet Case Study: OOA Use-case for returning and scanning a postcard

Postcards: Extended Scenario

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.

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.

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

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

Second Iteration of Class Diagram (contd) Cause of our problems Flight should have been a candidate class BUT, we all have 20–20 hindsight

Air Gourmet Case Study: Dynamic Model State diagram

Challenges of the OOOA Phase Do not class the boundary into object-oriented design Do not allocate methods to classes yet