class STACK ( T ) is { { Implementation of STACK

Embedded speciications in object-oriented (OO) languages such as Eiiel and Sather are based on a rigorous approach towards validation, compatibility and reusability of sequential programs. The underlying method of \design-by-contract" is based on Hoare logic for which concurrency extensions exist. However concurrent OO languages are still in their infancy. They have inherently imperative facets, such as object identity , sharing, and synchronisation, which cannot be ignored in the semantics. Any marriage of objects and concurrency requires a trade-oo in a space of intertwined qualities. This paper summarises our work on a type system, calculus and an operational model for concurrent objects in a minimal extension of the Eiiel and Sather languages (cSather). We omit concurrency control constructs and instead use assertions as synchronisa-tion constraints for asynchronous functions. We show that this provides a framework in which subtyping and concurrency can coexist.