CIE 203 Software Engineering Lecture 9: Class Modeling II

1. CIE 203 Software Engineering Lecture 9: Class Modeling II Mohammad El-Ramly, PhD 2016 Read Chapter 5 from OO SE 2/e, Slides by: Timothy Lethbridge

2. Last Lectures • What is UML? • UML Class Diagrams • Associations • Generalization • Aggregations Chapter 5: Modelling with classes

3. Outline • Modelling with Classes Chapter 5: Modelling with classes

4. Chapter 5: Modelling with classes

5. 5.9 The Process of Developing Class Diagrams • Create UML models at different stages for different purposes and levels of details • Exploratory domain model: Developed in domain analysis to learn the domain • System domain model: Models aspects of the domain represented by the system • System model: Includes also classes used to build the UI and system architecture Chapter 5: Modelling with classes

6. System domain model vs. System model • The system domain modelomits many classes that are needed to build a complete system. It contains domain classes. • Can contain less than half the classes of the system. • Should be developed to be used independently of particular sets of • userinterface classes • architectural classes Chapter 5: Modelling with classes

7. System domain model vs. System Model • The complete system modelincludes • The system domain model • Userinterface classes • Architectural classes • clients, servers, files and databases. • Utility classes Chapter 5: Modelling with classes

8. There are different types of Objects • Entity Objects • Represent the persistent information tracked by the system (Application domain objects, also called “Business objects”) • Boundary Objects • Represent the interaction between the user and the system • Control Objects • Represent the control tasks performed by the system.

9. Suggested sequence of activities • Identify a first set of candidate classes • Add associations and attributes • Find generalizations • List main responsibilities of each class • Decide on specific operations • Iterate over the model till it is satisfactory • Add or delete classes, associations, attributes, generalizations, responsibilities or operations • Identify interfaces • Apply design patterns (Chapter 6) Chapter 5: Modelling with classes

10. 1. Identifying classes • When developing a domainmodel you tend to discover classes • When you work on the user interface or the systemarchitecture, you tend to invent classes • Reuse should always be a concern • Frameworks • System extensions • Similar systems Chapter 5: Modelling with classes

11. A simple technique for discovering domain classes • Look at a source materials like SRS, etc. • Extract the nouns and noun phrases • Eliminate nouns that: • are redundant • represent instances • are vague or highly general • not needed in the application • Pay attention to classes in a domain model that represent types of usersor other actors Chapter 5: Modelling with classes

12. Example – Identify Classes • Ootumlia Airlines runs sightseeing flights from Java Valley, the capital of Ootumlia. The reservation system keeps track of passengers who will be flying in specific seats on various flights, as well as people who will form the crew. • For the crew, the system needs to track what everyone does, and who supervises whom. The airlines runs several daily numbered flights on a regular schedule. • Ootumlia Airlines expects to expand in the future, therefore the system needs to be flexible; in particular, it will be adding a frequent-flier plan. Chapter 5: Modelling with classes

13. 2. Identifying associations and attributes • Start with most central / important classes • Decide the clear and obvious data it must contain and its relations with other classes. • Work outwards towards the classes that are less important. • Avoid adding many associations and attributes to a class • A system is simpler if it manipulates less information Chapter 5: Modelling with classes

14. Tips about identifying and specifying valid associations • An association should exist if a class possesses, controls , is connected to , is related to , is a part of , has as parts , is a member of, or , has as members some other class in your model • Specify the multiplicity at both ends • Label it clearly. Chapter 5: Modelling with classes

15. Identifying attributes • Look for information that must be maintained about each class • Several nouns rejected as classes, may now become attributes • An attribute should generally contain a simple value • E.g. string, number Chapter 5: Modelling with classes

16. Tips about identifying and specifying valid attributes • It is not good to have many duplicate attributes • If a subset of a class’s attributes form a coherent group, then create a distinct class containing these attributes

17. Airline example (attributes and associations) Chapter 5: Modelling with classes

18. 3. Identifying generalizations and interfaces • There are two ways to identify generalizations: • bottom-up • Group together similar classes. Create a new superclass • top-down • Look for general classes first, specialize them if needed • Create an interface, instead of a superclass if • The classes are very dissimilar except for having a few operations in common • One or more of the classes already have their own superclasses Chapter 5: Modelling with classes

19. An example (generalization) Chapter 5: Modelling with classes

20. 4. Allocating responsibilities to classes • A responsibility is something that the system is required to do. • Each functional requirement must be attributed to one of the classes • All the responsibilities of a given class should be clearly related. • If a class has too many responsibilities, consider splitting it into distinct classes • If a class has no responsibilities attached to it, then it is probably useless Chapter 5: Modelling with classes

21. Allocating responsibilities to classes • A responsibility is something that the system is required to do. • When a responsibility cannot be attributed to any of the existing classes, then a new class should be created To determine responsibilities • Perform use case analysis • Look for verbs and nouns describing actions in the system description Chapter 5: Modelling with classes

22. Categories of responsibilities • Setting and getting the values of attributes • Creating and initializing new instances • Loading to and saving from persistent storage • Destroying instances • Adding and deleting links of associations • Copying, converting, transforming, transmitting or outputting • Computing numerical results • Navigating and searching • Other specialized work Chapter 5: Modelling with classes

23. An example (responsibilities) • Creating a new regular flight • Searching for a flight • Modifying attributes of a flight • Creating a specific flight • Booking a passenger • Canceling a booking Chapter 5: Modelling with classes

24. Prototyping a class diagram on paper • As you identify classes, you write their names on small cards • As you identify attributes and responsibilities, you list them on the cards • If you cannot fit all the responsibilities on one card: this suggests you should split the class into two related classes. • Move the cards around on a whiteboard to arrange them into a class diagram. • Draw lines among the cards to represent associations and generalizations. Chapter 5: Modelling with classes

25. 5. Identifying operations • Operations are needed to realize the responsibilities of each class • There may be several operations per responsibility • The main operations that implement a responsibility are normally declared public • Other methods that collaborate to perform the responsibility must be as private as possible Chapter 5: Modelling with classes

26. Chapter 5: Modelling with classes

27. 5.11 Difficulties and Risks when creating class diagrams • Modeling is particularly difficult skill • Even excellent programmers have difficulty thinking at the appropriate level of abstraction • Education traditionally focus more on programming than modelling • Resolution: • Ensure that the team have adequate training • Have experienced modeller in the team • Review all models thoroughly Chapter 5: Modelling with classes

28. Chapter 5: Modelling with classes

29. So far in this and last lectures • What is UML? • UML Class Diagrams • Associations • Generalization • Aggregations • OCL • Modelling with Classes Chapter 5: Modelling with classes

30. Readings and Videos • Videos of lectures 8 and 9 • Very Important • R6: Chapter 5 (Modelling with Classes) of OOSE, 2nded by Timothy Lethbridge Chapter 5: Modelling with classes