During maintenance, systems are updated to correct faults, improve functionality, and adapt the software to changes in its execution environment. The typical software-update process consists of stopping the system to be updated, performing the update of the code, and restarting the system. For systems such as banking and telecommunication software, however, the cost of downtime can be prohibitive. The situation is even worse for systems such as air-traffic controllers and life-support software, for which a shut-down is in general not an option. In those cases, the use of some form of on-the-fly program modification is required. In this paper, we present a new technique for dynamic updating of Java software. Our technique is based on the use of proxy classes and requires no support from the runtime system. The technique allows for updating a running Java program by substituting, adding, and deleting classes. We also present DUSC (Dynamic Updating through Swapping of Classes), a tool that we developed and that implements our technique. Finally, we describe an empirical study that we performed to validate the technique on a real Java subject. The results of the study show that our technique can be effectively applied to Java software with only little overhead in both execution time and program size.