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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.
In practice, cohesion metrics are only of limited usefulness:
|Section 6.2.4 "Complexity"||Contents||Section 6.3 "Data Analysis Techniques"|