We study a language construct superimpose for methodcall interception (MCI). The construct enables a programmer to attach additional functionality to certain join points along the execution of specified method calls. This is done in a completely type-safe manner where the additional functionality shares its state with the registry location. We present the static and the dynamic semantics of MCI. We use a disciplined style of Natural semantics for an accessible specification of MCI. We organize the constructs in a design space so that we can add expressiveness to a simple objectoriented language core in a stepwise fashion. It is one simple extension to enable interactive MCI, that is, MCI code can interact with the objects involved in a method call. Another simple extension is to allow for collective MCI by using patterns for the calls to be intercepted.

From an object-oriented programming perspective, an important class of aspects (in the sense of aspect-oriented programming) can be modelled via method call interception (MCI). This concept allows us to intercept transitions from a caller to a callee in a way that extra functionality can be superimposed on the actual method call. We describe fundamental language constructs for MCI. The style of Natural semantics is used to specify the meaning of the constructs. The semantical formulation (as opposed to a formulation based on program transformation) shows that static type safety and separate compilation can be preserved when MCI is added to an object-oriented language. 1.