Software Design Methodologies: UML in Action

1. Software Design Methodologies: UML in Action Dr. Mohamed Fayad, J.D. Edwards Professor Department of Computer Science & Engineering University of Nebraska, Lincoln Ferguson Hall, P.O. Box 880115 Lincoln, NE 68588-0115 http://www.cse.unl.edu/~fayad ISISTAN Research Institute – Tandil, Argentina

2. Lesson 5: Object Identification - 2 2 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

3. Lesson Objectives + Learn how to identify: Associations and aggregations Attributes Behaviors Inheritance + Understand how to use the following approaches: Use Case CRC Questioning Techniques + Understand how to refine objects and associations + Learn how to define responsibility & collaborations + Learn how to eliminate unnecessary classes, associations, and attributes 3 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

4. Automated Teller Machine (ATM) Stop bothering me! I told you I don’t have any money! 4 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

5. Actors In Use Cases, Everything that interacts with the system will be modeled as an actor, such as persons as well as machines. Customer Automated Teller Machine Bank System ATM Operator 5 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

6. Use Cases in ATM Automated Teller Machine Cash Withdrawal Customer Transfer Funds Bank System Deposit Funds Balance Inquiry 6 System Start ATM Operator ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

7. Use Case: Cash Withdrawal When a customer inserts a card in the ATM, the machine reads the code from the card and checks if it is a valid card. If the card is valid then the machine queries the customer for a PIN number, else the card is ejected. When the machine matches customer coded in the PIN number, the machine checks the validity of the PIN number. If the PIN number is correct and matches the card number then the machine asks for the desired transaction the customer wishes to perform. When the customer selects cash withdrawal the machine asks for the desired amount with a warning indicating only multiple of $10 is allowed. When .... A Use Case Description: Cash Withdrawal 7 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

8. Candidate Objects • Account • ATM • Balance Inquiry • Bank Card Reader • Cancel Key • Cash Dispenser • Deposit Slot • Deposit Funds • Display Screen (Bank System Interface) • Menu (Graphical User Interface) • User Message • Numeric Keypad • Numeric Input Key • PIN • Cash • Receipt Printer • Special Input Key • Transfer Funds • Cash Withdrawal 8 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

9. Define Responsibilities What are the goals of the system What must objects know to meet goals What steps must each object accomplish Determine Collaborations Decompose responsibilities into interactions among objects Define clients and servers Where should knowledge be held System Responsibilities & Collaborations 9 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

10. CRC Cards • General • Each class is described on a separate 3X5 or 4X6 card • The cards are known as CRC card. They have 3 sections: • Class • Responsibilities • Collaborations ATM (role) Collaboration Responsibility Server Clients • Access & modify account balance Account (role) Balance Inquiry • Deposit Transaction • Funds Transfer • Withdrawal Transaction 10 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

11. Generalize and Specialize objects Associate Objects Recognize Accidental Objects Challenging and Testing Objects Ask Questions Other Techniques Help Refine Objects 11 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

12. Generalization exposes commonalities Exercise helps to identify new classes Considerations for generalizations and specializations Is it in the problem domain? Is it within the system’s responsibilities? Will there be inheritance? Explore Generalizations and Specializations 12 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

13. Essential objects represent genuine high-level abstractions Accidental objects represent qualitative judgments Avoid Accidental Objects 13 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

14. Needed Remembrance -- attributes Needed Behavior -- methods Usually Multiple Services per Object Usually More than One Object per Class Challenge Objects 14 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

15. Uniformity Test Each instance must have the same set of characteristics and be subject to the same rules - Car license More than a Name Test Every object has attributes, if not it is probably an attribute of another object -- home address Or Test If inclusion criteria should not use “OR” in any significant way -- driver’s license number or learner’s permit number More Than a List Test If inclusion criteria is only a list of instances -- decadent foods includes croissant , cappuccino, chocolate pie, ice cream. Other Object Tests 15 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

16. Something universal and real for reuse Should encapsulate some reasonably complex behavior to justify existence Methods that don’t make use of its current class’s own attributes is probably encapsulated in the wrong object. Small and simple stable interfaces Self sufficient and complete Final Object Checklist 16 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

17. Play Twenty Questions Is it animal, vegetable, or mineral? Does it have fur or feathers? Can it fly? Define Boundaries What else? What about..? Quantify Qualities as Attributes How fast? How hot? Questioning Techniques Help Elicit Domain Knowledge 17 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

18. Irrelevant Associations outside problem domain Implementation Associations Examples: concurrent process, contains a list Associations Between Eliminated Classes Eliminating Unnecessary Associations 18 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

19. Actions or Transient Events Examples: “Interacts with the Robot”, “ATM accepts cash card.” Ternary Associations Decompose as binary associations or rephrase to one binary association. Derived Associations These are redundant Examples: “Younger than ..” derived from age Eliminating Unnecessary Associations (cont’d) 19 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

20. Choose meaningful association names Add role names where appropriate Add attributes or associations which qualify existing associations Example: “Standard Oil of Ohio” uses state attribute to qualify company name. Specify one-to-many and many-to-many associations in the class diagram Add missing associations Not in problem statement from knowledge of application domain Refine Association List 20 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

21. Attributes can be thought as a simple association with a value which is not an object Examples: name, age, weight Usually corresponding to nouns followed by possessive phrases Examples: “color of the car”, “age of the donor” Less likely to be fully described in the problem statement Included in the class box diagram Not as relevant to the problem structure as associations Identifying Attributes 21 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

22. Descriptive Attributes Naming Attributes Referential Attributes Attribute Types 22 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

23. Provide facts intrinsic to each instance of the object. Examples Account.balance, Cat.weight If the value of a descriptive attribute changes, it means only that some aspect of an instance has changed, but the instance is still the same instance. Descriptive Attributes 23 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

24. Are used to name or label instances. Examples:Account.number, Flight.number Names are typically somewhat arbitrary Naming attributes are frequently used as an identifier or part of an identifier. If the vale of a naming attribute changes, it means only that a new name has been given to exactly the same instance Naming Attributes 24 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

25. Are used to tie an instance of one object to an instance of another. Examples: Cat.owner name indicates which person owns this cat. If the vale of a referential attribute changes, it means that different instances are now being associated. Referential Attributes 25 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

26. First Rule: One instance of an object or a class has exactly one value for each attribute at any given time. Employee M/S Phone M. Fayad 171 4356 G. Smee 23 3456 7890 L. Harris 1234 Rules of Attributes OK Not OK Not OK 26 [Shlaer-Mellor 90] ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

27. Second Rule: An attribute must contain no internal structure Examples: Age, balance, size are all OK. A name consists of first name, middle initial, and last name (Not OK) An address contains house number, street name, city, state, zip code, and country name (Not OK) Rules of Attributes (cont’d) 27 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

28. Third Rule: When an object has a compound identifier -- that is, one made up of two or more attributes -- every attribute that is not part of the identifier represents a characteristic of the entire object. Rules of Attributes (cont’d) Juice Transfer storage Tank ID cooking Tank ID gallons plannedTime The juice Transfer.gallons attribute means that the number of gallons transferred from the storage tank to the cooking tank and not the number of gallons in either the storage tank or the cooking tank. 28 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

29. Fourth Rule: Each attribute is not part of an identifier that represents a characteristic of the instance named by the identifier and a characteristic of some other non-identifier attribute Rules of Attributes (cont’d) Batch batch ID recipe ID gallons cookingTime The Batch.cookingTime attribute must represent the actual time the batch was cooked, and not the cooking time specified by the recipe 29 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

30. Do not keep attributes that have an object as a value, they are associations Do not keep attributes that depend on a context, these are qualifiers for associations If an object can have more that one name, then the name qualifies an association of that object with another Eliminating Unnecessary Attributes 30 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

31. Do not put attributes of the association in one or the other of the objects involved in the association, put the attributes in the association itself Eliminate attributes which are only used internally by the object Keep initial analysis of attributes at a high level Eliminate attributes which are too low level Attributes which are in some instances of a class, but not in others, indicate that the class should be split into two or more classes Eliminating Unnecessary Attributes (cont’d) 31 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

32. Identify classes which share common information Three basic approaches: Bottom Up Look for classes with repeated associations, attributes or behaviors, and group together into higher level classes This approach is easier for inexperienced modelers Top Down Look for Noun phrases describing different kinds of things in the problem statement. Examples: Family cars, Sports cars, Luxury cars Combination of the two approaches works the best. Do Top Down when doing initial analysis Identify repeated information in the late passes. Identifying Inheritance 32 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

33. Always use the “AKO” test All inheritance specifications should identify one or more classes which are “A Kind Of” a higher level class. NEVER use inheritance for “Part / Part-of” relationships Use multiple inheritance only when necessary Some object-oriented programming languages do not even have this feature. Identifying Inheritance (cont’d) 33 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

34. Done in latest stages List of behaviors can become large and get detailed quickly. May correspond to queries about attributes and associations Operations to read or write attribute or association value Examples: user name, property value, etc. May correspond to events or activities Examples: begin simulation, alert, calculate balance, computer distance Identifying Behaviors 34 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

35. Experience and Domain Knowledge • Good objects come from language of domain • If you are not an expert -- consult users • Experience will tune decisions • Slowly at first • Much faster later Just do it! 35 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

36. Define with examples: CRC cards, associate objects, referential attributes. Describe the third norm for testing objects What are the differences between essential objects and accidental objects What are questioning techniques and their purposes? Describe how do you identify: associations , aggregations, inheritance, attributes, and behaviors Describe how to refine objects and associations Explain how to define responsibilities and collaborations Discussion Questions 36 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad

37. Define: Guidelines Heuristics What do you think of these heuristics A designer should distribute system intelligence uniformly among the top level classes in the system. A designer should have 4.6 top level classes per 1,000 lines of code. Eliminate classes that are outside the system Questions for the Next Lecture 37 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad