1. CRC: Classes, Responsibilities, and Collaborations
CS 4311
12/09

2. OO Design
Identify or define collections of data and operations on the data
Define classes
Determine how classes interact

3. Objects Should

4. Characteristics of a Good Class

5. Responsibilities
responsibilities

6. Aside: Objects are not just “data and methods”
At this point in design, you must consider objects at the conceptual level.
An object is something that has responsibilities.

7. CRC
CRC is a design method.
Why CRC?
Intuitive.
Easy to teach.

8. CRC Process Identify classes. Identify responsibilities.
Assign responsibilities to classes.
Identify collaborations.
Collect responsibilities into contracts.
Define protocols for contracts.

9. CRC Card One class per card Write subclasses on lines below the name
List attributes and purpose on back of card
Assign responsibilities
Concrete Class: Numeric Keypad
Superclass: Keypad
Subclasses: None
Responsibilities Collaborations

10. Collaboration
A collaboration occurs when one class requires the services of another class in order to fulfill its responsibilities.
Indicates classes are coupled.

11. Contracts A contract is a set of related responsibilities.
A contract specifies a service provided by one class to other classes

12. Protocols A protocol contains
method name and type
method parameters and types
preconditions
postconditions
A protocol is a formalization of a contract

13. Finding Classes A class is a collection of objects. Objects = “things”
Look for nouns in documents such as the requirements (SRS)

14. Finding Classes: Advice
Be careful of nouns outside the system.
Does an object need to act on behalf of the user?
Model known interfaces to world (user interfaces, interfaces to programs).
Categories may become abstract classes, but just model them as classes for now.
The interfaces will evolve to more complex things as time goes by. 14

15. Identifying Responsibilities
Responsibilities are doing and knowing
This suggests verbs in English
Look for verbs in documents
Actions that must be performed
Modes of maintaining information

16. Assigning Responsibilities
Assign every responsibility to some class
Evenly distribute the intelligence and work-load
State responsibilities as generally as possible
Keep behavior with related information
Collect information about 1 thing in 1 place
Pp Wirfs-Brock 16

17. Example
There is a voting system with several types of voting machines. These include touch screen machines, voice activated machines (for the sight impaired), and a mouse/keyboard machine (prototyping an interface for web-based voting). There are several different ballots for the machines, one for the city, one for the county, and one for the state. A voter approaches the machine, enters the keycode given to her or him by the election clerk, and the system presents the ballots, one at a time. The voter makes selections, and then casts each of the ballots. Once cast, a vote cannot be modified.
Purpose is to give a feel for the process: the details come a little later. 17

18. Example: Nouns system Voting machine Machine Touch Screen Machine
Voice Activated Machine
Mouse/Keyboard Machine
Interface
Ballot
One
Keycode
City
County
State
Her/Him
Election Clerk
Time
Voter
Selection
Each
Vote

19. Example: Classes Likely Questionable Machine keycode selection Ballot
Touch Screen
Voice Activated
Mouse Keyboard
Ballot
State
County
City
Voter
Vote
Questionable
keycode
selection

20. Example: Responsibilities
Machine
Knows which ballots are in the election
Can present ballots to a voter
Can validate a keycode
Knows what selections a voter has made
Can cast a ballot
Vote
Knows what ballot is being voted on
Knows a voters selection for items on the ballot
Ballot
Knows the type of item (issue or candidate)
Knows the seats and candidates
Knows the issues and the choices
Printer, Print Server, and Queue are a subsystem 20

21. Example: Collaborations
Machine and Vote are clients of Ballot
Machine is a client of Vote

22. Problem #1 Missing Classes
If you can’t easily assign a responsibility, you may be missing a class.
Try to reduce complexity
Example: grouping elements in a drawing.
The element need not know it’s part of a group.
The drawing need not know about groups.
But a group class can handle the responsibilities. The group class knows about its members and propagates actions to them.

23. Problem #2 Arbitrary Assignment
If you don’t know what class to assign a responsibility to, consider
What are the possibilities (what classes can you assign to)?
What are the implications?
Which is most obvious?
Which is most efficient?

24. Two Approaches on Distributing Intelligence

25. Assigning Responsibilities-1
Heuristic: Evenly distribute intelligence
Intelligence is what the system knows, actions that it can perform, and impact it has on other systems and users with which it interacts
A class gets intelligence based on how much it knows or can do, and how many other objects it can affect.

26. Assigning Responsibilities-2
Heuristic: State responsibilities as generally as possible.
Can find common responsibilities that can be shared
Heuristic: Keep behavior with related information.
If information changes, no update messages must be sent between objects
The object that needs to know that something has changed will know it.

27. Assigning Responsibilities-3
Heuristic: Keep information about one thing in one place
Do not share responsibility for maintaining specific information!!
Minimize duplication

28. Advice
Try to express responsibilities in general terms. You are still exploring.
Try to keep all of a class’s responsibilities at the same conceptual level (same level of abstraction).

29. Walk-through What is happening when the machine is idle?
What happens when a card is inserted?
What happens when the user chooses “withdrawal” from main menu?
What happens when the user presses the cancel key?

30. Finding abstract classes
Re-examine list of candidate classes and group related classes
Identify those that share attributes
Identify those that share behavior
Goal is to identify as many abstract classes as possible