Towards Design – Entity / Relationship Models

1. Towards Design – Entity / Relationship Models Richard Hopkins rph@nesc.ac.uk UML for Use Cases NeSC, Edinburgh, Jan 5/6 2006

2. Contents Goal • An appreciation of how UML class diagrams can be used to elucidate technical detail Contents • Internal vs External class diagrams • Details of class diagrams • Practical

3. Activity Diag. Class Diag. Sequence Diag. Sequence Diag. Activity Diag. Development Process • Requirements level Class Diagram – externally visible entities • Design level Class Diagram – model of external entities + internal entities => OO design Partially articulated requirements to elucidate requirements – what entities exist and how each behaves Capture Requirements Requirements Use Case Diag. Construct Model of Overall system Class Diag. Structure … Behaviour

4. Internal vs External • Similarities – used external classes as starting point; differences, e.g. • A “lendable” entity is may be a more useful abstraction than “Item” – greater commonality but need additional constraints for User-to-Loan User Item Is part of contains has out Is copy of Student Staff Member Journal Article Copy Book has out Ordered print-off to of User Loan Lendable for Student Staff Journal Copy print-off order in of Article Book of

5. Internal vs External • Similarities – used external classes as starting point; differences, e.g. • Relationship entities – need information about the relationship – • when the loan is to be returned • In this case, 1:N, could put that info with the Copy User Item Is part of contains has out Is copy of Student Staff Member Journal Article Copy Book has out Ordered print-off to of User Loan Lendable for Student Staff Journal Copy print-off order in of Article Book of

6. Payment Fine Internal Entities • Introduce Modelling of required internal state need to keep track of fines – what for how much paid Overdue Damage Loss to of User Loan Lendable for Student Staff Journal Copy print-off order in of Article Book of

7. Contents Goal • An appreciation of how UML class diagrams can be used to elucidate techniocal detail Contents • Internal vs External class diagrams • Details of class diagrams • Practical

8. Aggregation and Composition • Special Associations • Aggregation – To model a collection of things • Journal has a collection of Articles • Article has a collection of Authors • Composition • A special case of aggregation – “ownership” / “component” • An article is a component of a journal • Part can only belong to one whole • The part cannot exist without the whole • Role name - Not given explicitly – implicit in the end-point – “is part of” • Multiplicity – by default (1) --- (0..*) Journal Article Author composition general aggregation

9. Payment Fine Multiplicity Compactly communicates a lot of properties/assumptions – one picture of whole design • A payment can relate to several fines • An article can’t be in more than one Journal (issue) …. • Can be very useful as reality check on requirements with user • Can daunting notation for user – may be better to extract all assumptions made! 1..* 0..* Overdue Damage Loss 1 1 1 1 1 1 User Loan Lendable 1 1 Student Staff Journal Copy print-off order 0..* Article Book 1..*

10. Attributes • Attribute – Information associated with each instance of class • Each attribute will have a type (could be omitted) • Attributes are inherited – Title would be attribute of book, not Copy • Show What’s important for understanding • Maybe not all that exists in implementation • Don’t show IsCopyof attribute because duplicates Of association • Could put in virtual information • Title for copy even though only held for Book - ? Copy Book ShelfMark : Int Title : String of IsCopyOf : Book Title : String

11. Specialised Class vs Attribute Vs. Book Book hardBack: bool HardBackBook SoftBackBook • Use specialised class if a significant difference to user, • e.g. hardBack has a reBind operation, but softBack doesn’t

12. Navigability • Arrow-head means the association can be followed in the direction shown • In implementation view, navigability requires attributes to hold the link(s) to associated object(s) • In VP-UML – default is no navigability signs – • either both un-specified or both navigable Is copy of of Copy Copy Book Book One-way from copy to the Book it is a copy of Two-way Also - From a Book to any of its copies

13. Complex Class Structures • The intended structure would be elucidated by giving some examples ! • Particularly if audience is not familiar with this particular view on the possible structure of book • So illustrate the class diagram by one or more instance diagrams Class Diagram 0..* Book 1 by 0..* 1..* 0..* contains Author Division 1 by 1..* 0..* Part Chapter

14. B1: Book Title: UML & WSDL A2: Author Name: Guy A1: Author Name: Richard A2: Author Name: David C111: Chapter Title: Classes C112: Chapter Title: Use Cases Object Diagrams • An Instance diagram • Does not show full range of possibilities • The lines are “links” – instances of associations • Can annotate links with navigability and role if useful Class Diagram 0..* Book 1 by 0..* 1..* 0..* contains Author Division 1 by 1..* 0..* Part Chapter Object Diagram P11: Part Title: UML P12: Part Title: WSDL

15. Questions ? • Can we express/derive full OO design from UML? • Yes • Additional features • operations • using UML-defined classes as attribute types • visibility • interfaces • …. • ?????

16. Contents Goal • An appreciation of how UML class diagrams can be used to elucidate techniocal detail Contents • Internal vs External class diagrams • Details of class diagrams • Practical

17. Practical –Entity/ Relationship Model • Clarifying technical detail – 1 hour • One group member plays role of software developer • draft a class diagram for the design the system you have been working on • quickly! • could be two developers for 4-person group • Others play role of clients • with the developer – discuss and evolve the detail • The point is to explore the potential of these diagrams as communication tool – • Not to come up with the ideal design