Does an 80:20 rule apply to Java coupling?

Objective. To explore whether an 80:20 rule exists in Java from six coupling metrics over multiple versions of open-source software and, if so, whether that relationship is exacerbated over time. Methods. We used the automated tool JHawk to extract the 6 different coupling metrics from four Open-Source Systems. We then ranked the classes on each of these 6 coupling metrics and then analysed the top 20% of classes to see whether 80% of total coupling was contained therein. Conclusions. Only one metric appeared consistently to have an 80:20 relationship and that was the ‘fan-in’ metric. Evidence suggests that fan-in and fan-out have a complementary relationship. We found many of the other metrics had few, if any such relationships. The RFC was typical in this sense - no 80:20 relationship was found in any of the systems or any version in those systems. We also found no evidence to support the view that over time, the 80:20 is exacerbated.