Chapter 9

1. Chapter 9 Domain Models CS6359 Fall 2012 John Cole

2. Why do this? • If you don’t understand the domain, you can’t program for it effectively • Lower the representational gap between mental model and software model CS6359 Fall 2012 John Cole

3. Objectives • Identify conceptual classes related to the current iteration • Create an initial domain model • Model appropriate attributes and associations CS6359 Fall 2012 John Cole

4. What is a Domain Model? • Illustrates noteworthy concepts in a domain. That is, defines what the system is about • Models the things in your system and the way they relate to each other • A domain model is conceptual, not a software artifact • A visual dictionary CS6359 Fall 2012 John Cole

5. What’s the Difference? Software Artifacts: Conceptual Class: CS6359 Fall 2012 John Cole

6. Business Object Model • Class diagrams with no methods, just fields • May show domain objects • Associations between classes • Attributes of conceptual classes CS6359 Fall 2012 John Cole

7. Domain Model, visually CS6359 Fall 2012 John Cole

8. Conceptual Classes • An idea, thing, or object • Symbol—Words or images representing a conceptual class • Intension—Definition of a conceptual class • Extension—The set of examples to which the class applies • It is not a data model CS6359 Fall 2012 John Cole

9. How to Create a Domain Model • Find the conceptual classes • Draw them as classes in a UML class diagram • Add associations necessary to record relationships • Add attributes necessary for information to be preserved • Use existing names, the vocabulary of the domain CS6359 Fall 2012 John Cole

10. How do I find Conceptual Classes? • Reuse or modify existing models. If there are published models, prior art, or books, use them • Use a category list (Table 9.1) • Identify noun phrases (linguistic analysis) CS6359 Fall 2012 John Cole

11. Noun Phrase Identification • Consider the following problem description, analyzed for Subjects, Verbs, Objects: The ATM verifies whether the customer's card number and PIN are correct. S V O O OIf it is, then the customer can check the account balance, deposit cash, and withdraw cash. S V O V O V O Checking the balance simply displays the account balance. S O V O Depositing asks the customer to enter the amount, then updates the account balance. S V O V O V O Withdraw cash asks the customer for the amount to withdraw; if the account has enough cash, S O V O O V S V O the account balance is updated. The ATM prints the customer’s account balance on a receipt. O V S V O O CS6359 Fall 2012 John Cole

12. Noun Phrases Analyze each subject and object as follows: • Does it represent a person performing an action? Then it’s an actor, ‘R’. • Is it also a verb (such as ‘deposit’)? Then it may be a method, ‘M’. • Is it a simple value, such as ‘color’ (string) or ‘money’ (number)? Then it is probably an attribute, ‘A’. • Which NPs are unmarked? Make it ‘C’ for class. Verbs can also be classes, for example: • Deposit is a class if it retains state information CS6359 Fall 2012 John Cole

13. Where are the Terms? • Some are in the use case • Some come from domain experts • Natural language is imprecise and ambiguous, so you need to use judgment CS6359 Fall 2012 John Cole

14. POS example You can create a list, or you can use a set of class diagrams, per the table below CS6359 Fall 2012 John Cole

15. Monopoly Game Domain Model CS6359 Fall 2012 John Cole

16. Maintaining the Model • Usually the model is a guideline to thinking, not an end in itself. Thus it will change as you learn more • Who will use the updated model? CS6359 Fall 2012 John Cole

17. Report Objects • Including reports in a domain model usually isn’t useful because the information is derived from other objects. • There are special cases, such as receipts, that should be in the model. CS6359 Fall 2012 John Cole

18. Mapmaker (domain terms) Strategy • Use existing names so far as possible. Learn the terms your users use. • Exclude out-of-scope features. If a feature is not in the current iteration, don’t use it. • Model the “unreal” world (i. e. Telecom) by listening carefully to the vocabulary of experts. “Port” means two different things in telecom and shipping. CS6359 Fall 2012 John Cole

19. Attributes vs. Classes • Common mistake: representing something as an attribute when it should have been a class. • “If we do not think of some conceptual class X as a number or text in the real world, it should probably be a class, not an attribute.” • Should “store” be an attribute of Sale or should it be a class? CS6359 Fall 2012 John Cole

20. Description Classes • A description class contains information that describes something else • E. g. Product Description records the price, picture, text description (and what else?) of an item CS6359 Fall 2012 John Cole

21. Why Use Them? • If we keep all the information in, say, a sales line item, once all of that item are sold, there is no record of what the item was • How does this relate to database design? CS6359 Fall 2012 John Cole

22. Associations • Relationship between (instances of) classes that indicates some interesting and meaningful connection • What objects need memory of a relationship? • In Monopoly, where each piece is and which player owns which token • No need to remember numbers on the dice CS6359 Fall 2012 John Cole

23. Associations (cont’d) • Avoid having too many; this adds communication paths and complexity • Will they be implemented in software? • UML: Line between classes with the name of the association • Name with VerbPhrase-ClassName format. E. g. Sale Paid By CashPayment • Each end of an association is called a Role CS6359 Fall 2012 John Cole

24. Associations (cont’d) • Multiplicity: how many instances of class A can be associated with an instance of class B. • E. g. an instance of Store can be associated with many (zero or more) Items • This is at a particular moment, not over time • Discussion of 0..1 or 1 • Multiple associations: Flight FliesTo and FliesFrom an airport • Common Associations List on p. 155 CS6359 Fall 2012 John Cole

25. Attributes • An Attribute is a logical data value of an object • You can also have derived attributes denoted by / before the name • Full syntax: visibility Name : type multiplicity = default {property string} • +pi : Real=3.14159 {read-only} CS6359 Fall 2012 John Cole

26. Attribute Types • Usually primitive data types, as well as things like Color, DateTime, Zip code, etc. The latter are specializations of primitive types • It should not normally be a class, such as Sale or Airport • Relate conceptual classes with an association, not an attribute. No foreign keys • Attributes in code are okay CS6359 Fall 2012 John Cole

27. New Data Type Classes • Things like ItemID or ClaimNumber are not always simple data types, even though they look like them • If the attribute contains separable pieces, it can be its own class. For example, a Claim Number has the state, year, and day encoded in it • If it has operations associated with it CS6359 Fall 2012 John Cole

28. New Data Type Classes (cont’d) • If it has other attributes, such as a sale price • If it has units, such as currency • If it is an abstraction of one or more types with these properties CS6359 Fall 2012 John Cole

29. Quantities with Units • Represent them with distinct classes with an associated unit CS6359 Fall 2012 John Cole

30. Is the Domain Model Correct? • No, but it’s a good approximation • It gets better with each iteration; don’t try to get it all at once • Domain model usually done in the elaboration phase CS6359 Fall 2012 John Cole