The Case for Non-Cohesive Packages

Cohesion and coupling principles have been first defined for procedural languages with a black box model of control flow in mind [Stevens et al. 1974; Yourdon and Constantine 1979]. These principles state that a module should have high (internal) cohesion and low (external) coupling so that it implements a well defined functionality and can be easily reused [Briand et al. 1998; Briand et al. 1999]. We argue that when porting the principles to the object-oriented paradigm, some intrinsic properties of the object-oriented paradigm got ignored. We claim that packages with low cohesion are not necessarily packages with low quality. This is due in particular to the presence of inheritance and late-binding, which are the cornerstones of incremental definitions of classes. Classes in one package can extend the behavior of classes defined in other packages and just define a small increment in functionality. Late-binding is also the key mechanism to build white-box frameworks [Pree 1995] using the Hollywood Principle (“don’t call us, we’ll call you”). The application of this principle leads to the situation where an extending package may exhibit low cohesion and still be a well designed package. In this paper, an experiment is performed on various software systems to understand package cohesion and coupling when they are developed as extensions of frameworks. The analysis is performed by studying package dependencies of extender applications with their frameworks. The contributions of the paper are the following: (1) identification of a common cohesion/coupling misunderstanding for object-oriented programming; (2) a validation of the hypothesis on several real cases; and (3) a simple model to represent cohesion/coupling in a context of frameworks where no