Re: modeling link creation



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Andy Evans (andye@cs.york.ac.uk)
Fri, 14 Jan 2000 09:54:48 -0000


Gonzalo, Object diagrams are the way in which links between objects are described in UML. However, they are not particularly well described by the current documentation. As you say, they are particularly useful for understanding legal states of a system. They are also essential to understanding the semantics of many aspects of UML, as there is a notion of conformance between valid object diagrams and models. Catalysis (D'souza and Wills) also calls these "snapshots" - they represent a snapshot of the objects and links that conform to a particular model at a point in time. They use them exstensively throughout their book. Andy -----Original Message----- From: Gonzalo Genova <ggenova@ie.inf.uc3m.es> To: puml-list@cs.york.ac.uk <puml-list@cs.york.ac.uk> Date: 13 January 2000 17:33 Subject: modeling link creation >Hello, > >My name is Gonzalo Genova from Carlos III University of Madrid, Spain, and I >have recently registered to pUML list. > >My question may seem very simple, but I have seen it neither clearly >explained nor well resolved in the official UML documentation. It may also >happen that I have not understood some basic principle, and the question >turns to be irrelevant, as someone has already told me. I will try to be >very brief, although if someone is interested, I can delve into the >argument. > >Here comes the question: How do you model the creation of a link between two >object instances? (understanding "link" as "instance of an association", not >as the association itself; once the system is running, the associations do >not change, but the links do, they may appear and disappear). Using the >well-known example, "Acme hires John Doe", where "Acme" is an object of >class Company, "John Doe" is an object of class Person, and "hires" means >the establishing of a link between "Acme" and "John Doe". > >The question seems important to me because the existence or absence of links >between object instances is part of the state of the system; if you want to >model the evolution of the system, which objects exist at every time and >which values have their attributes, and how they change along time, you >should model also the changes in the links between objects. > >In UML we have standard ways to represent the creation and destruction of >object instances within interaction diagrams, via stereotyped messages (see >for example UML User Guide, p. 211). But it seems there is not a standard >way to represent the linking of two objects (at least, UML User Guide shows >none). > >There may be other conceptual problems concerning the UML metamodel >regarding link creation and destruction, but I leave them for those really >interested. > >Thanks, >Gonzalo > > > >To remove yourself from this list please mail puml-list-request@cs.york.ac.uk >with a message containing the word "unsubscribe". > > >


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