Object-oriented encapsulation for dynamically typed languages

ABSTRACT

Encapsulation in object-oriented languages has traditionally been based on static type systems. As a consequence, dynamically-typed languages have only limited support for encapsulation. This is surprising, considering that encapsulation is one of the most fundamental and important concepts behind object-oriented programming and that it is essential for writing programs that are maintainable and reliable, and that remain robust as they evolve.

In this paper we describe the problems that are caused by insufficient encapsulation mechanisms and then present object-oriented encapsulation, a simple and uniform approach that solves these problems by bringing state of the art encapsulation features to dynamically typed languages. We provide a detailed discussion of our design rationales and compare them and their consequences to the encapsulation approaches used for statically typed languages. We also describe an implementation of object-oriented encapsulation in Smalltalk. Benchmarks of this implementation show that extensive use of object-oriented encapsulation results in a slowdown of less than 15 percent.

REFERENCES

• 1: O. Agesen, L. Bak, C. Chambers, B.-W. Chan, U. Hölzle, J. Maloney, R. B. Smith, D. Ungar, and M. Wolczko. The SELF 4.0 Programmer's Reference Manual. Sun Microsystems, 1995.
• 2: Jonathan Aldrich , Valentin Kostadinov , Craig Chambers, Alias annotations for program understanding, Proceedings of the 17th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, November 04-08, 2002, Seattle, Washington, USA
• 3: P. S. Almeida. Balloon types: Controlling sharing of state in data types. In Proceedings ECOOP '97, pages 32--59. Springer Verlag, June 1997.
• 4: Ken Arnold , James Gosling , David Holmes, The Java Programming Language, Addison-Wesley Longman Publishing Co., Inc., Boston, MA, 2000
• 5: M. Barnett and D. A. Naumann. Friends need a bit more: Maintaining invariants over shared state. In Proceedings MPC 2004, July 2004. To appear.
• 6: Kent Beck, Extreme programming explained: embrace change, Addison-Wesley Longman Publishing Co., Inc., Boston, MA, 1999
• 7: Edwin Blake , Steve Cook, On including part hierarchies in object-oriented languages, with an implementation in Smalltalk, European conference on object-oriented programming on ECOOP '87, p.41-50, October 1987, Paris, France
• 8: Chandrasekhar Boyapati , Barbara Liskov , Liuba Shrira, Ownership types for object encapsulation, Proceedings of the 30th ACM SIGPLAN-SIGACT symposium on Principles of programming languages, p.213-223, January 15-17, 2003, New Orleans, Louisiana, USA
• 9: John Boyland , James Noble , William Retert, Capabilities for Sharing: A Generalisation of Uniqueness and Read-Only, Proceedings of the 15th European Conference on Object-Oriented Programming, p.2-27, June 18-22, 2001
• 10: Gilad Bracha, The programming language jigsaw: mixins, modularity and multiple inheritance, University of Utah, Salt Lake City, UT, 1992
• 11: Kim Bruce , Luca Cardelli , Giuseppe Castagna , Gary T. Leavens , Benjamin Pierce, On binary methods, Theory and Practice of Object Systems, v.1 n.3, p.221-242, Fall 1995
• 12: Craig Chambers , David Ungar , Bay-Wei Chang , Urs Hölzle, Parents are shared parts of objects: inheritance and encapsulation in SELF, Lisp and Symbolic Computation, v.4 n.3, p.207-222, July 1991 [doi>10.1007/BF01806106]
• 13: David G. Clarke , John M. Potter , James Noble, Ownership types for flexible alias protection, Proceedings of the 13th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, p.48-64, October 18-22, 1998, Vancouver, British Columbia, Canada
• 14: The E Language. http://www.erights.org/.
• 15: Erich Gamma , Richard Helm , Ralph Johnson , John Vlissides, Design patterns: elements of reusable object-oriented software, Addison-Wesley Longman Publishing Co., Inc., Boston, MA, 1995
• 16: Adele Goldberg , David Robson, Smalltalk-80: the language and its implementation, Addison-Wesley Longman Publishing Co., Inc., Boston, MA, 1983
• 17: John Hogg, Islands: aliasing protection in object-oriented languages, Conference proceedings on Object-oriented programming systems, languages, and applications, p.271-285, October 06-11, 1991, Phoenix, Arizona, United States
• 18: John Hogg , Doug Lea , Alan Wills , Dennis deChampeaux , Richard Holt, The Geneva convention on the treatment of object aliasing, ACM SIGPLAN OOPS Messenger, v.3 n.2, p.11-16, April 1992 [doi>10.1145/130943.130947]
• 19: Dan Ingalls , Ted Kaehler , John Maloney , Scott Wallace , Alan Kay, Back to the future: the story of Squeak, a practical Smalltalk written in itself, Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, p.318-326, October 05-09, 1997, Atlanta, Georgia, United States
• 20: Günter Kniesel , Dirk Theisen, JAC—access right based encapsulation for Java, Software—Practice & Experience, v.31 n.6, p.555-576, May 2001 [doi>10.1002/spe.372]
• 21: P. Müller and A. Poetzsch-Heffter. Universes: A type system for alias and dependency control. Technical Report 279, Fernuniversität Hagen, 2001.
• 22: Oscar Nierstrasz, A survey of object-oriented concepts, Object-oriented concepts, databases, and applications, ACM Press, New York, NY, 1989 [doi>10.1145/63320.66468]
• 23: James Noble, Iterators and Encapsulation, Proceedings of the Technology of Object-Oriented Languages and Systems (TOOLS 33), p.431, June 05-08, 2000
• 24: James Noble , David Clarke , John Potter, Object Ownership for Dynamic Alias Protection, Proceedings of the 32nd International Conference on Technology of Object-Oriented Languages, p.176, November 22-25, 1999
• 25: James Noble , Jan Vitek , John Potter, Flexible Alias Protection, Proceedings of the 12th European Conference on Object-Oriented Programming, p.158-185, July 20-24, 1998
• 26: N. Schärli, S. Ducasse, O. Nierstrasz, and A. Black. Traits: Composable units of behavior. In Proceedings ECOOP 2003, pages 248--274. Springer Verlag, July 2003.
• 27: N. Schärli, S. Ducasse, O. Nierstrasz, and R. Wuyts. Composable encapsulation policies. In Proceedings ECOOP 2004. Springer Verlag, June 2004. To appear.
• 28: Alan Snyder, Encapsulation and inheritance in object-oriented programming languages, Conference proceedings on Object-oriented programming systems, languages and applications, p.38-45, September 29-October 02, 1986, Portland, Oregon, United States
• 29: Patrick Steyaert , Carine Lucas , Kim Mens , Theo D'Hondt, Reuse contracts: managing the evolution of reusable assets, Proceedings of the 11th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, p.268-285, October 06-10, 1996, San Jose, California, United States
• 30: Mario Wolczko, Encapsulation, delegation and inheritance in object-oriented languages, Software Engineering Journal, v.7 n.2, p.95-101, March 1992

INDEX TERMS

Primary Classification:
  D. Software
  D.3 PROGRAMMING LANGUAGES
      D.3.3 Language Constructs and Features
          Subjects: Classes and objects

Additional Classification:
  D. Software
  D.3 PROGRAMMING LANGUAGES
      D.3.3 Language Constructs and Features
          Subjects: Inheritance

General Terms:
Languages

Keywords:
dynamic typing, encapsulation, encapsulation policies, information hiding, smalltalk