Software package metrics

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This article describes various software package metrics. They have been mentioned by Robert Cecil Martin in his Agile Software Development: Principles, Patterns, and Practices book (2002).

The term software package, as it is used here, refers to a group of related classes (in the field of object-oriented programming).

Number of Classes and Interfaces: The number of concrete and abstract classes (and interfaces) in the package is an indicator of the extensibility of the package.
Afferent Couplings (Ca): The number of other packages that depend upon classes within the package is an indicator of the package's responsibility.
Efferent Couplings (Ce): The number of other packages that the classes in the package depend upon is an indicator of the package's independence.
Abstractness (A): The ratio of the number of abstract classes (and interfaces) in the analyzed package to the total number of classes in the analyzed package. The range for this metric is 0 to 1, with A=0 indicating a completely concrete package and A=1 indicating a completely abstract package.
Instability (I): The ratio of efferent coupling (Ce) to total coupling (Ce + Ca) such that I = Ce / (Ce + Ca). This metric is an indicator of the package's resilience to change. The range for this metric is 0 to 1, with I=0 indicating a completely stable package and I=1 indicating a completely instable package.
Distance from the Main Sequence (D): The perpendicular distance of a package from the idealized line A + I = 1. This metric is an indicator of the package's balance between abstractness and stability. A package squarely on the main sequence is optimally balanced with respect to its abstractness and stability. Ideal packages are either completely abstract and stable (x=0, y=1) or completely concrete and instable (x=1, y=0). The range for this metric is 0 to 1, with D=0 indicating a package that is coincident with the main sequence and D=1 indicating a package that is as far from the main sequence as possible.
Package Dependency Cycles: Package dependency cycles are reported along with the hierarchical paths of packages participating in package dependency cycles.