In object-oriented testing literature, a class is considered to be a basic unit of testing. A major characteristic of classes is the interaction between data members and member functions. This interaction is represented as definitions and uses of data members in member functions and can be properly modeled with finite state machines (FSM). In this paper, we discuss how FSMs can be effectively used for class testing. We demonstrate how to specify the behavior of classes using FSMs and present a test case generation technique based on FSMs. In our technique, FSMs are transformed into a flow graph from which we can explicitly identify data flows of the FSM. Then we generate test cases using conventional data flow testing techniques upon the flow graph.