6.2.4 Complexity

Complexity measures the degree of connectivity between elements of a design unit. Whereas size counts the elements in a design unit, and coupling the relationships/dependencies leaving the design unit boundary, complexity is concerned with the relationships/dependencies between the elements in the design unit. For instance, counting the number method invocations among the methods within one class can be considered a measure of class complexity, or the number of transitions between the states in a state diagram.

Impact on quality

High complexity of interactions between the elements of a design unit can lead to decreased understandability and therefore increased fault-proneness. Also, testing such design units is more difficult.

Empirical results

In practice, complexity metrics are often strongly correlated with size measures. Large design units that contain many design elements within are also more likely to have a large number of connections between the design elements.

Thus, while complexity metrics are good indicators of qualities such as fault-proneness, they provide no new insights in addition to size metrics.