CSC/ECE 517 Summer 2008/wiki3 4 bk

Cohesion Explained

Cohesion is an element of object-oriented programming related to how well or how poorly a component of a program is compartmentalized. High cohesion is the preferable state in which a component's implementation and "awareness" is contained within the respective component. Low cohesion is the undesirable state in which the implementation crosses over the boundaries of the component into the rest of the program. In short, the component encapsulates its implementation. By adhering to the principle of encapsulation and forcing a program's components to remain compartmentalized, the process of implementing, testing, and maintaining the application become much more straightforward [Pressman]. Cohesion should not be confused with the closely related object-oriented design concept of coupling. Coupling is the degree to which modules interact with one another. While a high level of cohesion is desirable, a minimal level of coupling in which modules intercommunicate as little as possible is the goal of a well thought out design.

Levels of Cohesion

Cohesion can be broken down into seven main levels [Pressman]:

Most Desirable

• Functional cohesion is the highest level in which a component performs one operation and then returns its result. An example would be a simple method which takes two values, sums them, and then returns the result.
• Layer cohesion occurs when components are functionally layered and the higher layers can access lower layers in a downward fashion, but lower layers cannot in turn access higher layers. This could be illustrated by levels of security clearance. Top secret can view all levels of secrecy which exist below. But lower security levels can only look at their level and those possessing lower security clearance.
• Communicational cohesion exists where operations accessing a particular type of data are contained within the same class. This is apparent in any properly planned and implemented class in which all mutator and accessor methods are available only to objects of the given class.