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Abstract: An important use of the Unified Modelling Language (UML) is modelling synchronous object-oriented software systems. State diagrams are used to model interesting object behaviour, including method invocation. However, almost all previous work formalising state diagrams has assumed asynchronous communication. We show that UML’s “run to completion” semantics leads to anomalous behaviour in the synchronous case, and in particular that it is not possible to model recursive calls, in which an object receives a second synchronous message whilst still in the process of reacting to the first. We propose a solution using state diagrams in two complementary ways.

Binary associations between classifiers are among the most fundamental of UML concepts. However, there is considerable room for disagreement concerning what an association is, semantically; it turns out that at least two different notions are called Associations. This confusion of concepts gives rise to unnecessary complication and ambiguity in the language, which have implications for the modeller because they can result in serious misunderstandings of static structure diagrams; similarly, they have implications for tool developers. In this paper we explore these issues, suggest improvements and clarifications, and demonstrate side-benefits that would accrue.

The following is a light-hearted attempt to expose some difficulties in the use of patterns, including some places where naive ways of thinking about how they are used may be insufficient. We'd very much welcome comments, to feed into a more "academic" paper on the same subject.

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