One of the subjects that undergraduate students learning Object Oriented (OO) design find hard to apply is the construction of class hierarchies in general, and the use of interface classes in particular. The design process requires decomposition and reconstruction of problems in order to model software classes. The common attributes and behaviors are usually modeled using class inheritance except when the common behaviors do not belong to the same class hierarchy; in such case, an interface class is preferred. In order to be able to properly design, the designer has to demonstrate the abstraction abilities on various levels. In this study, we aimed at examining the students' demonstration of abstraction levels during design of class hierarchy in general, and whether or not they use interface classes in particular. The results of the study reveal that a majority of students were able to build a reasonable class hierarchy; however, many of them did not use interface classes as a tool for expressing common behaviors. Nevertheless, all of them could identify and choose the best solution for a particular problem among four different solutions, which included the use of interface class. (Contains 1 table and 13 figures.)