In this paper we address the problem of performing a class static analysis in a modular fashion, i.e. by just analyzing the class code and not the full program. In particular we show two things: the first one is how starting from a class C we can derive an approximation C^a to be used either as a class documentation or as a tester for a client using C; the second one is how to discover, in a fully automatic way, a class invariant. Two methods for class invariant computation are presented, proved correct and their usage is discussed.