1. An informal, team oriented, OO design system
CRC Cards
An informal, team oriented, OO design system
Copyright © 2016 by Curt Hill

2. Copyright © 2016 by Curt Hill
Briefly
Stands for Class, Responsibilities, and Collaboration
Originally a teaching tool for objects
Kent Beck and Ward Cunningham (1989)
Now also an informal OO design technique
Format is name at top
Responsibilities on left
Collaborating classes on right
Super and sub classes are sometimes present
Copyright © 2016 by Curt Hill

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Example Format
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Creating and Using
Consider the scenario or user story
Find classes within this scenario
Find responsibilities and properties
Define collaborators
Move the cards around
Simulate scenarios using the cards
Copyright © 2016 by Curt Hill

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Find Classes
An analysis task
Identifies the building blocks of application
Look for verbs and nouns
A good rule of thumb is to look for the three to five main classes
Later add classes concerning
User interface
Storage
Or other helpers for the main classes
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Responsibilities
What does the class do?
These will become methods
What does the class need to do this?
These may become properties
A responsibility for a class may fulfill the need of another class
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Find Collaborators
A class often does not have sufficient information to fulfill its responsibilities
It must then collaborate with other classes
Collaboration will be in one of two forms:
A request for information
A request to perform a task
Copyright © 2016 by Curt Hill

8. Identifying Collaborators
Ask some questions
Does the class have the ability to fulfill this responsibility?
If not, then look for a class that either has the ability or a class which should have the ability
This will show the need for new responsibilities in other classes or the need for new classes
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Collaborations
Typically one way relationship
A client server relationship
Client class asks server class to:
Perform a service
Return some information
Very seldom does the server turn around and ask the client something
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Move the Cards Around
Place cards on a table
Two cards that collaborate should be close together
Is a and has a relationship should be close as well
Two cards that don't collaborate should be placed far apart
The more two cards collaborate, the closer they should be
This makes it easier to understand the relationships between classes
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The Session
Start with a user story or scenario
Use the cards to simulate how such a system would work
This is a group activity
There should be at least:
A domain expert
An experienced OO designer
Developers
A group leader
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Example
Suppose we want to simulate a library
We will have several scenarios:
Checking out a book
Returning a book
Search for a book
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Finding classes
A brainstorming exercise produced the following candidates:
Library, Librarian, User, Borrower, Article, Material, Item, Due Date, Fine, Lendable, Book, Video and Journal
These were refined into a smaller set:
Librarian, User, Borrower, Lendable, Book, Video and Journal
Next a card for each
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Scenario Questions
Who should have the overall responsibility of check-out?
Librarian
What sub-tasks are part of this task?
Checking the borrower
Assigning a due date
Moving the book from in to out status
Connecting book with borrower
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Simulate
The check out scenario is now simulated
In the simulation responsibilities and collaborators are identified
Assign attributes to fulfill the responsibilities
In our example the following CRC cards were produced
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Results of CRC Cards
Librarian
Borrower
Check out book for borrower
Book
Borrower
Can borrow Know set of books
Book
Book
Date
Know if overdue
Check out
Calculate due date
In or out
Know due date
Know borrower
Date
Compare dates
Date
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Continuing
After this scenario is done work through the next scenarios
This may cause the cards (classes) to change or others to be added
For example a search would require a set of books
None of these classes could as yet handle this
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Good Practices
Start with simplest scenarios
Use meaningful class names
You may be stuck with them
Write a description of the class
Usually after end of scenario
Before that it may not last
If in doubt, then act it out
Be flexible
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Aftermath
When we are done the information needs to be saved
Complete set of cards
The cards may be incomplete during simulations
They should be fully completed now
Optionally: Collaboration diagram
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Actual Classes
Once the design is complete we can get to implementation
New helper classes may be created at this point
Design classes may be split into multiple classes
Neither of these suggests that there were flaws in the design
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Finally
CRC cards are frequently used in the design phase
This is a team activity
Once the basic design is done implementation is comparatively easy
Are we ready for an exercise?
Finally for now
Copyright © 2016 by Curt Hill