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Using CRC cards

From: Foundations of Programming: Object-Oriented Design

Video: Using CRC cards

Here is an alternate technique commonly found at this stage of an Object-Oriented Design: CRC Cards. CRC stands for Class, Responsibility, Collaboration. Now we're looking for exactly the same information as in the conceptual object diagram, we're just using a different format, and these are another use of index cards, they're simple, they're easy to create, easy to discuss, hand around, spread across the conference table, and they're easy to dispose of if you make a mistake or change your mind. Each CRC card represents one class.

Using CRC cards

Here is an alternate technique commonly found at this stage of an Object-Oriented Design: CRC Cards. CRC stands for Class, Responsibility, Collaboration. Now we're looking for exactly the same information as in the conceptual object diagram, we're just using a different format, and these are another use of index cards, they're simple, they're easy to create, easy to discuss, hand around, spread across the conference table, and they're easy to dispose of if you make a mistake or change your mind. Each CRC card represents one class.

It has three sections, the first C is the name of the class at the top, usually underlined, the R is the responsibilities of the class, the things that needs to take care of, and C is the Collaborators, the other classes it interacts with. Typically, CRC cards use this format with the responsibilities in the left-hand side two-thirds of the card, and the collaborators on what's remaining on the right. And you can start creating these again from using the nouns in your descriptions to help you identify classes, and the verbs and verb phrases to help you identify say the first responsibilities.

Now, with these responsibilities, you're not worried about official method names, you're just using whatever phrases make sense to say what this class needs to take care of. You can refine these later, some will be combined, some will be split apart. If it's obvious what other classes you're writing are collaborators, meaning what other classes we interact with, you can write those too. But here is the great thing, when you physically start to do this and start creating a pile of CRC cards, most people naturally find themselves starting to move related CRC cards together, and that helps a lot in figuring out the natural collaborators and the way these objects interact.

And that's one reason you shouldn't look for an electronic tool for doing this, it's a place where there is a lot of value in the physicality of moving these around, and as an exercise for helping you think in objects, CRC cards can be very useful. When working through CRC exercises, I've seen people rearrange these cards on a desk and then point to the gap where they're going to put this new class they haven't written yet. The other benefit of using the cards is an enforced constraint. As with using index cards for user stories, you can just keep adding more and more to these.

If you need more than one CRC card for a class, particularly if you're using 4 inch by 6 inch index cards, it's a clue that you may need to redesign that class. But whether you use CRC cards, conceptual diagrams, or a method of your own choosing, you should be able to leave this phase of the design process with at least the names and the core responsibilities of the first set of classes you intend to build in code. But before we start writing, we do need to flesh those ideas out, and that's next.

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Foundations of Programming: Object-Oriented Design

47 video lessons · 47723 viewers

Simon Allardice
Author

 
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  1. 11m 35s
    1. Welcome
      1m 25s
    2. Who this course is for
      1m 15s
    3. What to expect from this course
      3m 6s
    4. Exploring object-oriented analysis, design, and development
      1m 41s
    5. Reviewing software development methodologies
      4m 8s
  2. 26m 14s
    1. Why we use object-orientation
      2m 42s
    2. What is an object?
      5m 22s
    3. What is a class?
      4m 43s
    4. What is abstraction?
      2m 45s
    5. What is encapsulation?
      3m 45s
    6. What is inheritance?
      3m 35s
    7. What is polymorphism?
      3m 22s
  3. 12m 16s
    1. Understanding the object-oriented analysis and design processes
      4m 13s
    2. Defining requirements
      6m 9s
    3. Introduction to the Unified Modeling Language (UML)
      1m 54s
  4. 23m 35s
    1. Understanding use cases
      6m 11s
    2. Identifying the actors
      4m 16s
    3. Identifying the scenarios
      5m 7s
    4. Diagramming use cases
      4m 18s
    5. Employing user stories
      3m 43s
  5. 16m 36s
    1. Creating a conceptual model
      1m 59s
    2. Identifying the classes
      2m 27s
    3. Identifying class relationships
      2m 38s
    4. Identifying class responsibilities
      6m 43s
    5. Using CRC cards
      2m 49s
  6. 22m 25s
    1. Creating class diagrams
      6m 11s
    2. Converting class diagrams to code
      4m 57s
    3. Exploring object lifetime
      5m 55s
    4. Using static or shared members
      5m 22s
  7. 19m 49s
    1. Identifying inheritance situations
      6m 49s
    2. Using inheritance
      2m 43s
    3. Using abstract classes
      2m 2s
    4. Using interfaces
      4m 20s
    5. Using aggregation and composition
      3m 55s
  8. 9m 23s
    1. Creating sequence diagrams
      5m 18s
    2. Working with advanced UML diagrams
      2m 3s
    3. Using UML tools
      2m 2s
  9. 10m 39s
    1. Introduction to design patterns
      2m 40s
    2. Example: the singleton pattern
      4m 53s
    3. Example: the memento pattern
      3m 6s
  10. 21m 47s
    1. Introduction to object-oriented design principles
      2m 50s
    2. Exploring general development principles
      3m 55s
    3. Introduction to SOLID principles
      6m 43s
    4. Introduction to GRASP principles
      8m 19s
  11. 7m 1s
    1. Reviewing feature support across different object-oriented languages
      3m 50s
    2. Additional resources
      2m 27s
    3. Goodbye
      44s

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