CSC/ECE 517 Fall 2011/ch4 4i sd

"CRC Card Tools"

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Introduction

CRC Cards are the Class Responsibility Collaboration Cards which is a useful way to document collaborative design decisions. CRC Cards were invented by Kent Back and Ward Cunningham in 1989.<ref name="paper"> Beck, Kent; Cunningham, Ward (October 1989), "A laboratory for teaching object oriented thinking", ACM SIGPLAN Notices (New York, NY, USA: ACM) 24 (10): 1–6, doi:10.1145/74878.74879, ISBN 0-89791-333-7</ref> This wiki article will discuss about CRC cards, how they are made and a few software tools that are used to implement CRC cards.

CRC Cards

CRC Cards are the Class Responsibility Collaboration Cards which are used to interactively brainstorm an initial design of a program or a program segment. These are used as a tool used in the design on object oriented software.<ref name="crc">CRC Cards - Wikipedia</ref> It supports a rapid and thorough exploration of design alternatives. It is used during initial model construction as a brainstorming technique where it is used to determine which classes are needed and how they will interact among themselves, and again later to evaluate the design. Normally, it could be done by one person or may require up to five people.<ref name="alistair">CRC Cards</ref>

CRC cards explicitly represent multiple objects simultaneously. However, rather than simply tracing the details of a collaboration in the form of message sending, CRC cards place the designer’s focus on the motivation for collaboration by representing (potentially) many messages as a phrase of English text.<ref name="paper"></ref>

Description of CRC

The CRC cards are commonly created from 4 by 6 inch index cards and they contain:

• Class - the name of the object-oriented class
• Responsibility - the responsibilities of the class
• Collaborator - the relationship the class has with other classes in order to collaborate and fulfil its responsibilities

To identify the name of the classes, we look for the nouns in the requirements document. The class should be a singular noun, does not really have the same functionality as some other class, and is not a primitive type.

Responsibility-driven modelling can be done using CRC cards. These could also be used to form object interaction diagram. The advantage of using a small index card is that they are cheap, portable, readily available, and familiar.<ref name="paper" /> Since these cards are small in size, they help reduce the complexity of the design to minimum. It focuses the designer on the essentials of the class and prevents her/him from getting into its details and inner workings at a time when such detail is probably counter-productive. It also forces the designer to refrain from giving the class too many responsibilities. Because the cards are portable, they can easily be laid out on a table and re-arranged while discussing a design with other people.

Steps in making CRC Cards

1. Identify and assign candidate classes
2. Determine a set of specific scenarios
3. Walk through the scenario, naming cards and responsibilities
4. Write the main responsibility for each class
5. Identify the responsibilities for each class
6. Identify one or more collaborators for each class

Then, essentially, these index cards become your class diagram. In your class diagram, you create a class for each card that was used. The responsibilities on the card become the method of the class. All collaborators indicate that an association between the two classes must be drawn (and the class will need to have an instance of the collaborating classes). Finally, the data members on the back of the card become the attributes of the class. You’ll probably need to make some adjustments to this initial design – for example recognizing and handling inheritance – but its usually a great start!

Example of a CRC Card

Here is an example of how a CRC Card looks.<ref name="dennis">Dennis, Wixom, Roth, "Systems Analysis and Design", 3rd Edition</ref> The CRC card here is for a Patient class. The Description of the class indicated the main responsibility associated with the class. Along with this information, the other responsibilities performed by this class is indicated in a separate Responsibilities section. If a collaborator is required to perform the responsibilities of the class, the name of the collaborator class is indicated in the Collaborators section associated with a particular responsibility. The back side of the card essentially displays the Attributes of the class and the Relationships of the class with other classes.

• Front Side of a CRC Card

• Back Side of a CRC Card

CRC Card Tools

There are a couple of CRC card tools available which are implemented in software packages to help users manage the different stories.

QuickCRC

QuickCRC is a tool developed by Excel Software for responsibility driven design of object-oriented software using CRC cards. CRC cards are used to discover and document classes, responsibilities, attributes and collaborations between classes. Design scenarios can be identified and simulated. Complex designs can be partitioned into multiple diagrams with easy navigation through a contents view. The inheritance graph automatically shows the class structure of the evolving design. The user can assign the attribute access of each card responsibility and the tool presents an attribute access graph.<ref name="quickcrc1">QuickCRC Tool</ref>

Conclusion

CRC cards are used to give a useful and convincing experience with objects and the mechanisms of objects but do not yet see their value. Using CRC cards, one can speculate the designs, ensure that they are concrete and establish an explicit relationship between objects. This makes it easier to understand, evaluate, and modify a design. One of the major problem for using this is the integration of the cards with larger design methodologies and with particular language environments. The need to retain the value of physical interaction points to the need for a new kind of user interface and programming environment as far beyond what we have today as our current systems are beyond the tool-oriented environments of the past.<ref name="paper" />

See Also

• Unified Modeling Language
• Responsibility-driven Design
• Object-Oriented Design

References

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External Links

• Introduction to CRC Cards
• CRC Models
• CRC: Class Responsibility Collaboration cards
• CRC Cards for ATM example