Lecture 14

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Lecture 14 15 Object-Oriented Analysis

• Anita S. Malik
• anitamalik_at_umt.edu.pk
• Adapted from Schach (2004) Chapter 12

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Object-Oriented Paradigm

• 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 (contd)

• 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

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Object-Oriented Analysis (OOA)

• 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)

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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

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Elevator Problem OOA

• 1. Use-Case Modeling
• Use case Generic description of overall
  functionality
• Get comprehensive insight into behavior of product

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Normal Scenario
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Exception Scenario
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Use Case

• Use Case Name
• Actors
• Pre-Conditions
• Normal Flow
• Post-Conditions
• Exceptions or Alternate Flows

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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

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Two Approaches to Class Modeling

• Noun extraction
• Always works
• CRC classes
• Need to have domain expertise

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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.

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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. Use nouns
  as candidate classes
• Nouns
• button, elevator, floor, movement, building,
  illumination, door
• floor, building, door are outside problem
  boundary exclude
• movement and 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-n
• Makes design and implementation easier

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CRC Cards

• Class Responsibility Collaboration (CRC)
• 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

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Testing during the OOA Phase

• CRC cards are an excellent testing technique

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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

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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, 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

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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
• Rose

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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 (contd)

• Cause of our problems
• Flight should have been a candidate class
• BUT, we all have 2020 hindsight

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Air Gourmet Case Study Dynamic Model

• State diagram

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Challenges of the OOA Phase

• Do not class the boundary into object-oriented
  design
• Do not allocate methods to classes yet