Re: modeling link creation



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Stuart Kent (stuart@mclellankent.com)
Fri, 21 Jan 2000 22:24:26 +0000


I think this ongoing discussion reveals the weakness in expressivity of collaboration diagrams. I find them not very useful because it is very hard to see how the state changes as messages are passed around. The problem is that a visual depiction of time is lost. Collaboration diagrams seem to work best when the configuration of objects remains near-enough static throughout the series of interactions. On the other hand, sequence diagrams don't show the connections between objects, which can lead to confusion when trying to relate parameters of actions to the objects whose lifelines are shown on the diagram. Rather than try and squeeze everything into one diagram, which seems to be the case again and again in UML, I prefer to keep concepts simple in one diagram and the use different diagrams to visualise what they're good at visualising. In this spirit, a technique which I use (good on paper, though painful to do in any CASE tool) is to draw a sequence diagram then run a filmstrip (a sequence of object diagrams, as described in Catalysis) down the side. The filmstrip shows the changes in state and connections between objects at any particular time; the sequence diagram shows which messages are being passed to whom. Of course, constructing a case tool that could do this would not be that difficult... We have also been working on 3D visual notations which combines the best of sequence and collaboration diagrams. We also use constraint diagrams as a generalisation of object diagrams. Constraint diagrams allow you to express general conditions on the state (as one might express in a pre or post condition) rather than protypical examples of state, which is the best that can be achieved with object diagrams. Take a look at the paper at http://www.cs.ukc.ac.uk/pubs/1998/790/index.html I'd be interested in any comments. Stuart Kent


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