1. IS514 Lecture Week 9
CRC Cards

2. So far Behaviour model Structural Model This week Use Case Diagrams
Use Case Descriptions
Structural Model
Class Diagrams
This week
Continue with Behavioural Model
Class Responsibility Collaboration Cards

3. Background Created by Tektronix, in Portland, Oregon (1989)
Large users of Smalltalk
Attributed with the development of several key ideas in object technology
Ward Cunningham and Kent Beck
Presented as a technique to help ‘think objects’
NOT part of UML

4. Responsibilities and Collaborations
Knowing
Doing
Collaboration
Objects working together to service a request
Object oriented systems are made up of objects working together to provide functionality
CRC cards can help to form a “bridge” from structural model to behavioral model

5. The essentials of CRC cards
Anthropomorphism
Pretending classes have human characteristics
Extremely simple technique
Rather than use diagrams to develop models use 4 x 6 index cards
Rather than indicate attributes and operations on the cards, they identify responsibilities and collaborators
Two uses:
Examining domain
Based upon use cases and use case descriptions from the domain

6. CRC Cards Many templates exist (see Dennis for another)
On the index card, record the following information:-
the name of the class, at the top
the responsibilities of the class, on the left-hand side
the collaborators of the class, which help to carry out each responsibility, on the right-hand side of the card
Many templates exist (see Dennis for another)

7. Example Simple CRC card
Student
Responsibility
Collaborators
Complete Assessment
Assessment
Submit Assessment
School office
Attend Lecture
Lecture
Attend Seminar
Seminar
Quick exercise – What have I missed? What other responsibilities and collaborators are there?

8. Exercise 1
Consider a public house. Consider two classes in the public house. Think about the working of a pub and the interactions between
Bar person
Customer
Draw CRC cards for these two classes
This may involve you identifying other classes
This should take about 10 minutes

9. Exercise 1 – One Possible Answer
Bar Person
Responsibility
Collaborators
Take orders from customers
Order
Customer
Serve orders
Product
Take payments and give change
Till
Check stock
Stock

10. Exercise 1 – One Possible Answer
Customer
Responsibility
Collaborators
Given orders
Bar Person
Order
Receive orders
Product
Make payments

11. What have we seen Identification of “new” classes
Order
Product
Till
Stock
How classes interact to provide functionality
A feel for behavioural aspects of the system
Each of these would also need a CRC card

12. CRC Card Responsibilities
The responsibilities of the class describe at a high-level the purpose of the class’s existence
They are connected to the operations the class provides, but are more general than that might imply :
e.g. in a library system “Make Payment” is acceptable as a description of a responsibility
there may well be many associated operations

13. Coupling & Cohesion A criteria for Good Analysis / Design Coupling
Degree of Interconnectedness
Number of links & degree of interaction
Cohesion
Contribution to single purpose
Not mutually exclusive
You should note that Coupling and Cohesion are among the criteria for defining good analysis / design, in fact it is good practice (during the analysis phase) to prepare models that satisfy these criteria. Coupling describes the degree of interconnectedness between design components and is reflected by the number of links an object has and by the degree of interaction the object has with other objects. Cohesion is a measure of the degree to which an element contributes to a single purpose. Coupling consists of Interaction coupling and Inheritance coupling, but more on this later. While cohesion consists of Operation cohesion, Class cohesion and Specialisation cohesion, again but more on this later. You should be aware that the concepts of coupling and cohesion are not mutually exclusive but actually support each other.

14. CRC Good Practice If a class has lots of responsibilities :-
Can describe them more concisely, and, if not
Would be better to split the responsibilities and have two or more classes
Too many responsibilities:
Bad- low cohesion
Too many collaborators:
Bad -high coupling
Using CRC cards should help to identify and fix both these faults in a model

15. Why use CRC cards
At the start, you may not know which other classes a class has to collaborate with in order to fulfill a responsibility
Using CRC cards facilitate this
If an object of one class collaborates with an object of another class, it does so by using operations of the other
A class has a relationship with each of it’s collaborators
Using CRC cards is helps to establish in which direction associations should be navigable

16. Class Diagrams – navigable relationships
So far all relationships in class diagrams and use cases have been navigable in two ways
This is not always the case
Officer is investigating a crime
Crime doesn’t know which officer is investigating it or indeed if it is being investigated!

17. CRC cards in practice (1 of 2)
One of the main benefits of CRC cards is that they encourage animated discussion among developers
Especially effective when walking through a Use Case to see how the classes are going to implement it (using role play)
We will see an example of this in the seminars
Anthropomorphism
Developers pick cards as each class collaborates in the Use Case
As ideas about responsibilities are formed the cards are annotated

18. CRC cards in practice (2 of 2)
Stops thinking about classes as dumb data holders and moves thinking towards understanding the higher-level behavior of each class
CRC cards work on ‘object’ communication (not classes)
Can provide a draft class model
Apart from the technical benefits, stimulates
Team discussion and hence team spirit
Technique suitable for highlighting opportunities to refactor the class model
Refactoring is the process of altering the class model of an object-oriented design without altering it’s visible behavior

19. Exercise 2 – Have another go
Consider a Library. A library has Borrowers, Books Copies of Books and Reservations. Borrowers can borrow copies of books and make reservations.
Draw CRC cards for the library
Hint classes:
Borrower
Book
Copy
Reservation

20. Exercise 2 – One possible answer
Borrower
Responsibility
Collaborators
Borrows books
Book
Copy
Return books
Make reservations
Reservation

21. Exercise 2 – One possible answer
Book
Responsibility
Collaborators
To have copies
Copy
To be borrowed
Borrower
To be reserved
Reservation

22. Exercise 2 – One possible answer
Copy
Responsibility
Collaborators
Provides book info
Book
Provides borrower info
Borrower
Provides reservation info
Reservation

23. Exercise 2 – One possible answer
Reservation
Responsibility
Collaborators
To create a reservation for a book
Book
Borrower
To hold copy
Copy

24. This weeks reading ESSENTIAL READING
Dennis A, Wixom B, and Tegarden D (2005) System Analysis and Design with UML version 2 second edition, Wiley
Chapter 7 ( especially )
Further reading
Bennett, S., McRobb, S. and Farmer, R. (2002) Object-Oriented Systems Analysis and Design using UML, 2nd Edition, McGraw-Hill
Pages

25. Summary
What CRC cards are
How to use CRC cards