Concept and design of the program animation system JAN are described. JAN visualizes the execution of a Java program by dynamically unfolding an object diagram and an interaction diagram. Several features distinguish JAN from existing program visualization systems and visual debuggers. Annotations in the program code can be used to control the animation by selecting the relevant events and customizing the visual appearance. In addition, the user can interactively steer the animation in various ways. JAN is an integrated visualization system which includes an elaborate graphical user interface, a preprocessor for annotated Java source code and a visualization engine that runs in a separate Java virtual machine. The design of the system is described in detail.
