Playing roles in design patterns: An empirical descriptive and analytic study

This work presents a descriptive and analytic study of classes playing zero, one, or two roles in six different design patterns (and combinations thereof). First, we answer three research questions showing that (1) classes playing one or two roles do exist in programs and are not negligible and that there are significant differences among the (2) internal (class metrics) and (3) external (change-proneness) characteristics of classes playing zero, one, or two roles. Second, we revisit a previous work on design patterns and changeability and show that its results were, in a great part, due to classes playing two roles. Third, we exemplify the use of the study results to provide a ranking of the occurrences of the design patterns identified in a program. The ranking allows developers to balance precision and recall.