Object-Oriented Paradigm

1. Object-Oriented Paradigm • Object-oriented paradigm • Reaction to perceived shortcomings in structured paradigm • Problem of larger products • Data and action treated as equal partners Lecture 14 & 15

2. 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 Lecture 14 & 15

3. 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) Lecture 14 & 15

4. 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 Lecture 14 & 15

5. Elevator Problem: OOA • 1. Use-Case Modeling • Use case: Generic description of overall functionality • Get comprehensive insight into behavior of product Lecture 14 & 15

6. Normal Scenario Lecture 14 & 15

7. Exception Scenario Lecture 14 & 15

8. Use Case • Use Case Name • Actors • Pre-Conditions • Normal Flow • Post-Conditions • Exceptions or Alternate Flows Lecture 14 & 15

9. 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 Lecture 14 & 15

10. Two Approaches to Class Modeling • Noun extraction • Always works • CRC classes • Need to have domain expertise Lecture 14 & 15

11. 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. Lecture 14 & 15

12. 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. Lecture 14 & 15

13. 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 • movementand illuminationare abstract nouns — exclude (may become attributes) • Candidate classes: Elevator and Button • Subclasses: Elevator Button and Floor Button Lecture 14 & 15

14. First Iteration of Class Diagram • Problem • Buttons do not communicate directly with elevators • We need an additional class:Elevator Controller Lecture 14 & 15

15. Second Iteration of Class Diagram • All relationships are now 1-to-n • Makes design and implementation easier Lecture 14 & 15

16. 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 Lecture 14 & 15

17. 3. Dynamic Modeling • Produce UML state diagram • State, event, predicate distributed over state diagram • UML “guards” are in brackets Lecture 14 & 15

18. Testing during the OOA Phase • CRC cards are an excellent testing technique Lecture 14 & 15

19. 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 Lecture 14 & 15

20. 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 Lecture 14 & 15

21. Second Iteration of CRC Card Lecture 14 & 15

22. Third Iteration of Class Diagram Lecture 14 & 15

23. Second Iteration of Normal Scenario Lecture 14 & 15

24. 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 Lecture 14 & 15

25. 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 Lecture 14 & 15

26. 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 Lecture 14 & 15

27. Air Gourmet Case Study: OOA • Use-case model for making a reservation Lecture 14 & 15

28. Making a Reservation: Extended Scenario Lecture 14 & 15

29. Air Gourmet Case Study: OOA • Use-case for returning and scanning a postcard Lecture 14 & 15

30. Postcards: Extended Scenario Lecture 14 & 15

31. 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. Lecture 14 & 15

32. 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. Lecture 14 & 15

33. 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 Lecture 14 & 15

34. 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 Lecture 14 & 15

35. Second Iteration of Class Diagram (contd) • Cause of our problems • Flight should have been a candidate class • BUT, we all have 20–20 hindsight Lecture 14 & 15

36. Air Gourmet Case Study: Dynamic Model • State diagram Lecture 14 & 15

37. Challenges of the OOA Phase • Do not class the boundary into object-oriented design • Do not allocate methods to classes yet Lecture 14 & 15