PRIVATE STATE IN OBJECT-ORIENTED PROGRAMMING LANGUAGES

We develop a core object-oriented programming language named SimpleJ that implements private state for all private elds in objects. We lay out a type system and operational semantics for the data typing and accessibility (assurance of private state) information. The type system statically analyzes the code to produce type and accessibility information, which describes the behavior of the runtime system as laid out in the operational semantics. We provide arguments for type correctness in SimpleJ, and that the static information produced by the type system does indeed accurately describes runtime qualities of the runtime system. The primary purpose of the accessibility requirements are to ensure that references to private elds in objects cannot escape to outside the object.