Measuring Cohesion and Coupling of Object-Oriented Systems

Cohesion and coupling are considered amongst the most important properties to evaluate the quality of a design. In the context of OO software development, cohesion means relatedness of the public functionality of a class whereas coupling stands for the degree of dependence of a class on other classes in OO system. Given two lines of code, A and B, they are coupled when B must change behavior only because A changed. They are cohesive when a change to A allows B to change so that both add new value. In this paper, a new metric has been proposed that measures the class cohesion on the basis of relative relatedness of the public methods to the overall public functionality of a class. The proposed metric for class cohesion uses a new concept of subset tree to determine relative relatedness of the public methods to the overall public functionality of a class. A set of metrics has been proposed for measuring class coupling based on three types of UML relationships, namely association, inheritance and dependency. The reasonable metrics to measure cohesion and coupling are supposed to share the same set of input data. Sharing of input data by the metrics encourages the idea for the existence of mutual relationships between them.