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Safe and Flexible Objects with Subtyping

Lorenzo Bettini, Dipartimento di Sistemi ed Informatica, Università di Firenze,
Viviana Bono and Silvia Likavec, Dipartimento di Informatica, Università di Torino
This work has been partially supported by MIUR project EOS.


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We design a calculus where objects are created by instantiating classes, as well as mixins. Mixin-instantiated objects are “incomplete objects”, that can be completed in an object-based fashion. The combination of class-based features with object-based ones offers some flexible programming solutions. The fact that all objects are created from fully-typed constructs is a guarantee of controlled (therefore reasonably safe) behavior. Furthermore, the calculus is endowed with width subtyping on complete objects, which provides enhanced flexibility while avoiding possible conflicts between method names.

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About the authors


Lorenzo Bettini holds a PostDoc position at the Dipartimento di Sistemi e Informatica, Università di Firenze, Italy. His research focuses on theory, extension and implementation of mobile code and object-oriented languages, and on distributed applications. He can be reached at or


  Viviana Bono is an Associate Professor in Computer Science at the University of Torino, Italy. Her main research interests are theoretical foundations, semantics, and design of object-oriented and functional languages. She can be reached at or

    Silvia Likavec got her PhD from University of Torino, Italy and Ecole Normale Supérieure de Lyon, France in February 2005. Her research interests are: lambda calculus, theory of objects, type theory, semantics of programming languages, application of logic in computer science. She can be reached at or

Cite this article as follows: Lorenzo Bettini, Viviana Bono and Silvia Likavec,“Safe and Flexible Objects with Subtyping”, in Journal of Object Technology, vol. 4, no. 10, Special Issue: OOPS Track at SAC 2005, Santa Fe USA, December 2005, pp. 5-29

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