We propose an object-oriented calculus with internal concurrency and
class-based inheritance that is built upon the join calculus. Method calls,
locks, and states are handled in a uniform manner, using labeled
messages. Classes are partial message definitions that can be combined and
transformed. We design operators for behavioral and synchronization
inheritance. We also give a type system that statically enforces basic
safety properties. Our model is compatible with the JoCaml implementation
of the join calculus.