Interfaces are widely used as central design elements of Java applications. Although interfaces are abstract
types similar to abstract classes, the usage of interfaces in Java applications may considerably differ from
the usage of abstract classes. Unlike abstract classes, interfaces are meant to enable multiple inheritance
in Java programs. Hence, interfaces are meant to encode shared similarities between classes belonging to
different class-type hierarchies. Therefore, it is frequent to use interfaces as partial types, where an
interface specifies one specific aspect or usage of its implementing classes. In this paper, we investigate
interfaces' usage in Java applications from two perspectives. First, we investigate the usage of interfaces as
types of classes belonging to different class-type hierarchies (i.e., interface's type variability). Second, we
investigate the usage of interfaces as partial types of implementing classes (i.e., interface's type
completeness).