Class-Responsibility-Collaboration (CRC) card

A Class-Responsibility-Collaboration (CRC) card is a [modeling] tool used in [object-oriented software design].

CRC cards are simple index cards that capture essential information about a class:

  • Class: The class’s name.

  • Responsibilities: The main tasks or roles that the class performs.

  • Collaborations: Other classes that the class interacts with to fulfill its responsibilities.

At the top of the index card is the name of the class. On the left side are listed the class’s responsibilities, and on the right the names of other classes it collaborates with.

Example
┌──────────────────────────────┬─────────────┐
│ Order                        │ Customer    │
| -----                        │ Payment     │
│ Check if items in stock      │ Inventory   │
│ Determine price              │             │
│ Check for valid payment      │             │
│ Dispatch to delivery address │             │
│                              │             │
└──────────────────────────────┴─────────────┘

CRC cards were originally proposed as a teaching aid by [Ward Cunningham] and [Kent Beck] in a 1989 paper. They subsequently became popular as a system design tool. [Martin Fowler] referenced them in his book "UML Distilled", and [Fred Brooks] mentioned them as a design tool in the 20th anniversary edition of "The Myth Man-Month".

CRC cards are particularly handy for early brainstorming of the structure of an object-oriented system, between translating use-case descriptions into class diagrams.

CRC cards are noted for their simplicity and flexibility. Using paper index cards makes it easy to create, update, and discard cards, and rearrange the index, as the design evolves. The cards can be laid out, or pinned to a board, to visualize the hierarchy of classes and their relationships.

Using small cards helps to place useful constraints on the design. It minimizes the complexity of each class, by putting a physical limit on the number of responsibilities that can be listed, and on the number of collaborators that can be referenced.

To create a CRC card:

  1. Write the name of the class at the top of a new index card.

  2. List the main responsibilities of the class on the left, without going into implementation details.

  3. List the names of other classes that the class collaborates with on the right.

  4. Pull out the index cards for the collaborating classes, and reference the new class as a collaborator on each of them (so creating bi-directional references between collaborators).