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Chapter 4. Measuring Evolving Software
In the context of object-oriented software a range of measures of class
complexity have been defined [117]. In general, there are two perspectives
used when measuring the complexity of a class. In the first perspective,
the complexity of a class can be computed by measuring the
internal structure. A widely used metric to capture this internal structure
is the Weighted Method Count (WMC) metric [46] where cyclomatic
complexity [190] is used as the weight [177]. Th WMC metric reflects
the degree of control flow present within a class and has been shown
to be an indicator of fault-proneness [19].
The second perspective computes complexity of a class by measuring
its coupling with other classes in the software. Two commonly used
metrics to capture the degree of coupling are the In-Degree Count and
Out-Degree Count [21,54,55,77,117,165,209,223,287,294,299]. The
In-Degree Count metric measures the number of classes a particular
class provides services to (that is, a measure of its popularity), while
the Out-Degree Count metric measures how many classes that a particular
class depends upon, respectively. These measures capture the
level of coupling within a software system which serves as an indicator
of the difficulty developers potentially face during maintenance and
evolution [31, 52, 205]. For example, a class X with a high In-Degree
Count (relative to other classes in the system) is considered complex,
since any changes made to X have the potential to significantly impact
other classes that depend on X. Similarly, a class Y that has a very high
Out-Degree Count is also considered complex, since Y makes use of a
large number of different functional aspects of the system in order to
satisfy its responsibilities. As a consequence, developers cannot alter
Y in a meaningful way before they understand classes that Y uses.
In our study, we collect complexity metrics for each class from both perspectives.
Specifically, we measure the internal structural complexity of
a class as well as the coupling for a class (the specific metrics and their
definitions are described in the sections that follow). Furthermore, we
use the term “complexity” to imply structural complexity rather than
volumetric complexity.
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