Towards a Fully Abstract Semantics for Object-Oriented Program Components

Behavioral semantics for components abstract from implementation details and describe the components’ behavior independent of the components’ implementations. It provides an important foundation for behavioral substitutability and interface specifications. In this paper, we develop and investigate a behavioral semantics for a sequential class-based object-oriented language with aliasing, subclassing, and dynamic dispatch. The implementation of a component includes an owner class C and the classes and interfaces used by C. A component instance consists of a dynamically evolving set of objects with a clear boundary to the environment. Behavioral semantics is expressed in terms of the messages crossing the boundary. The types of objects that can cross a boundary are described by the component signature. We develop a light-weight object-oriented component model and discuss substitutability. We present an object-oriented core language supporting this model and define its operational semantics in a standard way. As a central step towards a behavioral semantics, we develop a message semantics that assigns to each component its part of the heap and stack, localizes object creation to components, and treats inter-component communication by messages. We show that the message semantics is equivalent to the standard semantics. By abstracting from the component states, we obtain a behavioral semantics. Component behaviors are defined as partial functions from incoming messages to outgoing messages. The semantic domain of these behaviors is independent of possible component environments and implementations. We proof that the behavioral semantics is fully abstract with respect to the operational semantics.