A Laboratory For Teaching Object-Oriented Thinking

It is difficult to introduce both novice and experienced procedural programmers to the anthropomorphic perspective necessary for object-oriented design. We introduce CRC cards, which characterize objects by class name, responsibilities, and collaborators, as a way of giving learners a direct experience of objects. We have found this approach successful in teaching novice programmers the concepts of objects, and in introducing experienced programmers to complicated existing designs.

The most difficult problem in teaching objectoriented programming is getting the learner to give up the global knowledge of control that is possible with procedural programs, and rely on the local knowledge of objects to accomplish their tasks. Novice designs are littered with regressions to global thinking: gratuitous global variables, unnecessary pointers, and inappropriate reliance on the implementation of other objects.

Because learning about objects requires such a shift in overall approach, teaching objects reduces to teaching the design of objects. We focus on design whether we are teaching basic concepts to novices or the subtleties of a complicated design to experienced object programmers.

Rather than try to make object design as much like procedural design as possible, we have found that the most effective way of teaching the idiomatic way of thinking with objects is to immerse the learner in the “object-ness” of the material. To do this we must remove as much familiar material as possible, expecting that details such as syntax and programming environment operation will be picked up quickly enough once the fundamentals have been thoroughly understood.

It is in this context that we will describe our perspective on object design, its concrete manifestation, CRC (for Class, Responsibility, and Collaboration) cards, and our experience using these cards to teach both the fundamentals and subtleties of thinking with objects.

Procedural designs can be characterized at an abstract level as having processes, data flows, and data stores, regardless of implementation language or operating environment. […]

r.Laboratory.Kent.Ward