Responsibilities and rewards: Reasoning about design patterns

Design patterns provide guidance to system designers on how to structure individual classes or groups of classes, as well as constraints on the interactions between these classes, to enable them to implement flexible and reliable systems. Patterns are usually described informally. While such informal descriptions are useful and even essential, if we want to be sure that designers precisely and unambiguously understand the requirements that must be met when applying a given pattern, and be able to reliably predict the behaviors the resulting system will exhibit, we also need formal characterizations of the patterns. In this paper, we develop an approach to formalizing design patterns. The requirements that a designer must meet with respect to the structures of the classes, as well as with respect to the behaviors exhibited by the relevant methods, are captured in the responsibilities component of the pattern’s specification; the benefits that will result by applying the pattern, in terms of specific behaviors that the resulting system will be guaranteed to exhibit, are captured in the rewards component. One important aspect of many design patterns is their flexibility; our approach is designed to ensure that this flexibility is retained in the formalization of the pattern. We illustrate the approach by applying it to a standard design pattern.