6.2.5 Cohesion

Cohesion is the degree to which the elements in a design unit (package, class etc.) are logically related, or "belong together". As such, cohesion is a semantic concept.

Cohesion metrics have been proposed which attempt to approximate this semantic concept using syntactical criteria. Such metrics quantify the connectivity (coupling) between elements of the design unit: the higher the connectivity between elements, the higher the cohesion.

Cohesion metrics often are normalized to have a notion of minimum and maximum cohesion, usually expressed on a scale from 0 to 1. Minimum cohesion (0) is assumed when the elements are entirely unconnected, maximum cohesion (1) is assumed when each element is connected to every other element.

Not normalized metrics are based on counts of connections between design elements in a unit (e.g., method calls within a class). As such, not normalized metrics are conceptually similar to complexity metrics.

Impact on quality

A low cohesive design element has been assigned many unrelated responsibilities. Consequently, the design element is more difficult to understand and therefore also harder to maintain and reuse. Design elements with low cohesion should be considered for refactoring, for instance, by extracting parts of the functionality to separate classes with clearly defined responsibilities.

Empirical results

In practice, cohesion metrics are only of limited usefulness:

• Not normalized cohesion metrics often are strongly related to size metrics. This makes sense since, as discussed in the section on size metrics, large classes or packages may in fact suffer from low cohesion. Such cohesion metrics then are, of course, good quality indicators, but they are redundant with size metrics - they provide no new information about the design element.
• Normalized cohesion metrics do not consistently have a bearing on system qualities. I.e., we cannot conclude from a high or low cohesion value that a class is, e.g., more or less fault-prone. Either, the theoretical negative impact of low cohesion on system quality is not always that critical in practice, or, the cohesion metrics simply fail to identify design elements with unrelated responsibilities.