Understanding the interplay between the logical and structural coupling of software classes

- Structurally coupled classes, will likely be co-evolving in future.
- No linear correlation between the degree of structural and change coupling.
- Over 60% of the structural coupling links remain stable over time.

During the lifetime of object-Oriented (OO) software systems, new classes are added to increase functionality, also increasing the inter-dependencies between classes. Logical coupling depicts the change dependencies between classes, while structural coupling measures source code dependencies induced via the system architecture. The relationship or dependency between logical and structural coupling have been debated in the past, but no large study has confirmed yet their interplay.In this study, we have analysed 79 open-source software projects of different sizes to investigate the interplay between the two types of coupling. First, we quantified the overlapping or intersection of structural and logical class dependencies. Second, we statistically computed the correlation between the strengths of logical and structural dependencies. Third, we propose a simple technique to determine the stability of OO software systems, by clustering the pairs of classes as “stable” or “unstable”, based on their co-change pattern.The results from our statistical analysis show that although there is no strong evidence of a linear correlation between the strengths of the coupling types, there is substantial evidence to conclude that structurally coupled class pairs usually include logical dependencies. However, not all co-changed class pairs are also linked by structural dependencies. Finally, we identified that only a low proportion of structural coupling shows excessive instability in the studied OSS projects.