Extending OCL with Null-References

Achim D. Brucker and Matthias P. Krieger and Burkhart Wolff

From its beginnings, OCL is based on a strict semantics for undefinedness, with the exception of the logical connectives of type Boolean that constitute a three-valued propositional logic. Recent versions of the OCL standard added a second exception element, which, similar to the null references in object-oriented programming languages, is given a non-strict semantics. Unfortunately, this extension has been done in an ad hoc manner, which results in several inconsistencies and contradictions.

In this paper, we present a consistent formal semantics (based on our HOL-OCL approach) that includes such a non-strict exception element. We discuss the possible consequences concerning class diagram semantics as well as deduction rules. The benefits of our approach for the specification-pragmatics of design level operation contracts are demonstrated with a small case-study.

Keywords: HOL-OCL, UML, OCL, null reference, formal semantics
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Please cite this article as follows:
Achim D. Brucker and Matthias P. Krieger and Burkhart Wolff. Extending OCL with Null-References. In MoDELS 2009 Workshops. Lecture Notes in Computer Science (6002), pages 251-275, Springer-Verlag, 2010. Selected best papers from all satelite events of the MoDELS 2009 conference.
Keywords: HOL-OCL, UML, OCL, null reference, formal semantics
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