6.2.2 Coupling

Coupling is the degree to which the elements in a design are connected.

Impact on quality

Coupling connections cause dependencies between design elements, which, in turn, have an impact on system qualities such as maintainability (a modification of a design element may require modifications to its connected elements) or testability (a fault in one design element may cause a failure in a completely different, connected element). Thus, a common design principle is to minimize coupling.

Most coupling dependencies are directed - the coupling usually defines a client-supplier relationship between the design elements. Therefore, it is useful to distinguish import coupling ("using", "fan-out") and export coupling ("used", "fan-in"), which we discuss in the following.

Import coupling

Import coupling measures the degree to which an element has knowledge of, uses, or depends on other design elements. High import coupling can have the following effects:

• Decreased maintainability: changes to the supplier may necessitate follow-up changes (ripple effects) to the client.
  The stability of the supplier is a factor to consider here. High coupling to elements that are not likely to change is less harmful than coupling to "hot spots".
• Decreased understandability, increased fault-proneness: elements with high import coupling operate in large context, developers need to know all the services the element relies on, and how to use them.
• Decreased reusability: To reuse a class or package with high import coupling in a new context, all the required services must also be made available in the new context.

Export coupling measures the degree to which an element is used by, depended upon, by other design elements. High export coupling is often observed for general utility classes (e.g., for string handling or logging services) that are used pervasively across all layers of the system. Thus, high export coupling is not necessarily indicative of bad design.

Again, an important issue to consider here is stability. High export coupling elements that are likely to change in the future can have a large impact on the system if the change affects the interface. Therefore, high export classes should be reviewed for anticipated changes, to ensure that these changes can implemented with minimal impact.

Empirical results

Coupling metrics have consistently been found to be good indicators of fault-proneness. It seems worthwhile to investigate different dimensions of coupling: import and export coupling, different coupling mechanisms, distinguishing coupling to COTS libraries and application-specific classes/packages. Coupling metrics are suitable to identify design elements with high fault density. Therefore, coupling metrics greatly help to identify small parts of a design that contain a large number of faults.