Abstractness measures the proportion of abstract types in a package relative to the total number of types in that package.

A highly abstract package (where the metric value is close 1) that is furthermore unstable is likely to be useless: the class hierarchy has been over-engineered, and all those abstract types are not heavily used.

One should try to design packages that are abstract in proportion to their incoming dependencies, and concrete in proportion to their outgoing dependencies. That way, making changes is likely to be easy. See this excellent summary of Robert C. Martin's work for further discussion of these metrics and their use. Distance of a Package from Main Line is a combined metric that captures the balance between abstractness (this page) and instability.

Pre-packaged Queries

Query name = "Semmle/Metrics/Packages/Packages that have abstractness > 0.2"

Reports packages in source where the abstractness value is greater than 0.2, in decreasing order of abstractness as a bar chart.

from MetricPackage p, float c
where p.fromSource() and c = p.abstractness() and c > 0.20
select p, c order by c desc

Query name = "Semmle/Metrics/Packages/Concrete packages"

Reports packages in source that are completely concrete, with abstractness=0.

from MetricPackage p, float c
where p.fromSource() and c = p.abstractness() and c = 0
select p, c order by c desc

.QL Source of Metric

This metric is defined in MetricPackage. Its definition is a direct translation of the formula:

    float abstractness() {
          exists ( int i, int j | i = count(RefType t | t.getPackage() = this) and
                                  j = count(RefType t | t.getPackage() = this and
                                                  t.hasModifier("abstract")) and
                                  result = j / i and i > 0 )
    }

References

Robert C. Martin. Object-Oriented Design Metrics Ensure Robust Software. January 6, 2003.

Robert C. Martin. OO Design Quality Metrics. October 24, 1994.

Robert C. Martin. Agile Software Development, Principles, Patterns and Practices. Addison Wesley, 2002.

Bill Venners. How Useful are Code Metrics? Artima Developer, April 26, 2006.

Wikipedia. Software Package Metrics . 2007.