Behavioral Subtyping is Equivalent to Modular Reasoning for Object-oriented Programs

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2006-12-22
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Leavens, Gary
Naumann, David
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Computer Science

Computer Science—the theory, representation, processing, communication and use of information—is fundamentally transforming every aspect of human endeavor. The Department of Computer Science at Iowa State University advances computational and information sciences through; 1. educational and research programs within and beyond the university; 2. active engagement to help define national and international research, and 3. educational agendas, and sustained commitment to graduating leaders for academia, industry and government.

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The Computer Science Department was officially established in 1969, with Robert Stewart serving as the founding Department Chair. Faculty were composed of joint appointments with Mathematics, Statistics, and Electrical Engineering. In 1969, the building which now houses the Computer Science department, then simply called the Computer Science building, was completed. Later it was named Atanasoff Hall. Throughout the 1980s to present, the department expanded and developed its teaching and research agendas to cover many areas of computing.

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1969-present

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Behavioral subtyping is an established idea that enables modular reasoning about behavioral properties of object-oriented programs. It requires that syntactic subtypes are behavioral refinements. It validates reasoning about a dynamically-dispatched method call, say E.m(), using the specification associated with the static type of the receiver expression E. For languages with references and mutable objects the idea of behavioral subtyping has not been rigorously formalized as such and the standard informal notion has inadequacies. This paper formalizes behavioral subtyping and introduces a new formalization of modular reasoning, called supertype abstraction. A Java-like sequential language is considered, with classes and interfaces, recursive types, first-class exceptions and handlers, and dynamically allocated mutable heap objects; the semantics is designed to serve as foundation for the Java Modeling Language (JML), a widely used specification language. Behavioral subtyping is characterized as sound and semantically complete for reasoning with supertype abstraction.

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Copyright © by Gary T. Leavens and David A. Naumann.

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