1. Introduction to Unified Modelling Language (UML) (part 2 Class diagram, Associations)

2. Objects What are Objects Observations
“a discrete entity with a well-defined boundary that encapsulates state and behaviour;
an instance of a class” UML Reference Manual, 2nd Edition-2004
Observations
an instance of a class
state is maintained in fields
behaviour is implemented via methods
every object is uniquely identified

3. UML Object Notation

4. Classes What are Classes
“the descriptor for a set of objects that share the same attributes, operations, methods, relationships, and behaviour” UML Reference Manual, 2nd Edition-2004
template for objects with similar features

5. UML Class Notation

6. UML Class Notation

7. How Much to Show? Scenarios scenario 1: inter-class relationship
scenario 2: behaviour of a class or classes
scenario 3: entity classes representing data entities

8. How Much to Show? Scenarios scenario 1: inter-class relationship
class boxes are probably enough with name compartment only
scenario 2: behaviour of a class or classes
class boxes with name & operations compartment

9. Analysis Classes What is an Analysis Class?
represents an important concept of the problem domain
maps on to real-world concepts
an attempt to capture the big picture

10. Analysis Classes

11. Analysis Classes Observations name reflects the intent of abstraction
focus clearly defined/understood
small & well defined set of responsibilities
high cohesion & low coupling

12. Classes The three class types in the Unified Process are UML notation
Entity classes
Boundary classes
Control classes
UML notation

13. Entity Classes An entity class models information that is long lived
Examples:
Account Class in a banking information system
Mortgage Class and Investment Class in the information system
Instances of all these classes remain in the information system for years

14. Boundary Classes
A boundary class models the interaction between the information system and its actors
Boundary classes are generally associated with input and output
Examples:
Sales Report Class in the E-Commerce information system
Mortgage Listing Class and Investment Listing Class in the information system

15. Control Classes
A control class models complex computations and algorithms
Examples:
Compute GPA Student Class, in the PSU information system

16. Stereotypes UML notation for these three types of classes
These are stereotypes (extensions of UML)
UML allows us to define additional constructs that are not part of UML but which we need in order to model a system accurately

17. The Unified Process and Classes
The Unified Process does not describe how classes are extracted
Users of the Unified Process are expected to have a background in object-oriented analysis and design
We temporarily suspend this discussion of the Unified Process to explain how classes are extracted, and then return to the Unified Process

18. Extracting Entity Classes
Entity class extraction consists of three steps that are carried out iteratively and incrementally:
Functional modeling
Present scenarios of all the use cases (a scenario is an instance of a use case)
Class modeling
Determine the entity classes and their attributes
Determine the interrelationships and interactions between the entity classes
Present this information in the form of a class diagram
Dynamic modeling
Determine the operations performed by or to each entity class
Present this information in the form of a statechart

19. Flowchart for Extracting Entity Classes

20. Initial Class Diagram: Painting System Case Study
The second step in extracting the entity classes is class modeling
The aim of this step is to extract the entity classes, determine their interrelationships, and find their attributes
Usually, the best way to begin this step is to use the two-stage noun extraction method
Stage 1: Describe the information system in a single paragraph
Stage 2: Identify the nouns in this paragraph, then select the entity classes from those nouns

21. Noun Extraction: Painting System Case Study
Stage 1: Describe the information system in one paragraph:
Reports are to be generated in order to improve the effectiveness of the decision-making process for buying works of art. The reports contain buying and selling information about paintings, which are classified as masterpieces, masterworks, and other paintings

22. Noun Extraction: Painting System (contd)
Stage 2: Identify the nouns in this paragraph
Reports are to be generated in order to improve the effectiveness of the decision-making process for buying works of art. The reports contain buying and selling information about paintings, which are classified as masterpieces, masterworks, and other paintings
The nouns are report, effectiveness, process, buying, work of art, selling, information, painting, masterpiece, and masterwork

23. Noun Extraction: Painting System (contd)
effectiveness, process and information are abstract nouns and are therefore unlikely to be entity classes
Abstract nouns identify things that have no physical existence
Nouns buying and selling are derived from the verbs “buy” and “sell”
They will probably be operations of some class

24. Noun Extraction: Painting System (contd)
Noun report is unlikely to be an entity class, because a report is not long lived
A report is much more likely to be a boundary class
Noun work of art is just a synonym for painting

25. First Iteration of the Initial Class Diagram
This leaves four candidate entity classes:
Painting Class, Masterpiece Class, Masterwork Class, and Other Painting Class

26. Second Iteration of the Initial Class Diagram
Consider the interrelationships between the entity classes
A masterpiece is a specific type of painting, and so is a masterwork and an “other painting”
Painting Class is therefore the base class
Masterpiece Class, Masterwork Class, and Other Painting Class are subclasses of that base class

27. Second Iteration of Initial Class Diagram (contd)

28. Third Iteration of the Initial Class Diagram
The class diagram does not reflect aspects of the pricing algorithm
When dealing with a masterwork
“The information system first computes the maximum purchase price as if it were a masterpiece by the same artist”

29. Third Iteration of the Initial Class Diagram (contd)
That is, a masterwork has to have all the attributes of a masterpiece (so that its maximum purchase price can be computed as if it were a masterpiece) and, in addition, it may have attributes of its own
This is modeled in the next slide

30. Third Iteration of the Initial Class Diagram (contd)

31. Fourth Iteration of the Initial Class Diagram
Another aspect of the pricing algorithm that is not reflected in the current class diagram is
“The information system computes the coefficient of similarity between each painting for which there is an auction record and the painting under consideration for purchase”

32. Fourth Iteration of the Initial Class Diagram (contd)
Auctioned Painting Class is needed to make these comparisons
An auctioned painting must be a subclass of Painting Class
But a painting previously been sold at an auction somewhere in the world has nothing to do with paintings currently on display for sale in Osbert’s gallery

33. Fourth Iteration of the Initial Class Diagram (contd)

34. Fourth Iteration of the Initial Class Diagram (contd)
An instance of Painting Class is either
A painting that Osbert has bought (an instance of Gallery Painting Class), or
A painting sold at some auction (an instance of Auctioned Painting Class)

35. Initial Class Diagram: Painting System Case Study
Why was the first iteration of the class diagram so inadequate?
The Painting System case study appears to be a straightforward data-processing application
The one-paragraph description correctly did not incorporate the pricing algorithm
Unfortunately, the algorithmic details turned out to be critical to the class diagram

36. Initial Class Diagram: Painting System (contd)
The first iteration of the class diagram was no good
However, repeated iteration and incrementation led to a reasonable class diagram
This demonstrates the power of the iterative and incremental approach

37. Initial Class Diagram (contd)
Finally, we add the attributes of each class to the class diagram
the result is shown on the next slide
The empty rectangle at the bottom of each box will later be filled with the operations of that class

38. Fifth Iteration of the Initial Class Diagram (contd)

39. Fifth Iteration of the Initial Class Diagram (contd)

40. Done with the Painting System case study
Still the dynamic modeling – later

41. Design Classes What is a Design Class?
Integrates the problem domain & solution domain
problem domain → refined analysis classes
solution domain → implementation specific classes
are complete, ready to be implemented

42. Design Classes Concerns responsibility inheritance
the public methods imply a contract
code evolution should be considered
inheritance
major concern of design classes (why?)

43. Inheritance Concerns the strongest form of coupling between classes
“fragile base class” problem
changes in the base class impact subclasses
inflexible to change
fixed at runtime

44. Example: Ambitious John

45. Inheritance or Interface
interface & implementation
concerns with reuse
Interface
interface only
concerns with functionality contract
“design by contract”
concerns with reuse (what?)

46. Further Considerations
Student Self-Study
Templates
Nested classes
Multiple Inheritance

47. connection between two things (usually classes)
What is an Association?
connection between two things (usually classes)
“for a link between two object, there must be an association between the class of those objects”
link is an instance of an association
link can be represented by dependency relationship

48. Example

49. Reflexive Association

50. Reflexive Association

51. Aggregation

52. Composition

53. Association Classes

54. Association Classes Observations
association classes are purely analysis classes
convert association class into normal design class
use the various relationships to clarify
association (aggregation, composition, dependency)
additional constraints to refine the model
navigation, multiplicity