This paper presents experiments on formal validation of Java applets. It describes a tool that has been developed at the Gemplus Research Labs. This tool allows to formally prove Java classes annotated with JML, an annotation language for Java that provides a framework for specifying class invariants and methods behaviours. The foundations and the main features of the tool are presented. The most innovative part of the tool is that it is tailored to be used by Java programmers, without any particular background in formal methods. To reduce the difficulty of using formal techniques, it aims to provide a user-friendly interface which hides to developers most of the formal features and provides a “Java style view” of lemmas.