Given a set of classes as input, a class exerciser, with its easy-to-use graphical user interface, allows users to create objects for the given classes, invoke methods on specified objects and examine the contents of objects interactively. In other words, one can use a class exerciser to exercise and assess the various functions of classes conveniently. The class exerciser has many applications in object-oriented software development. In particular, it can be used as a tool for demonstration testing and maintenance all together. We also describe our design and implementation of a class exerciser, RunClass, for the programming language C++.