Re: MML Questions



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Tony Simons (a.simons@dcs.shef.ac.uk)
Wed, 20 Dec 2000 10:24:03 +0000 (GMT)


Kerry, I think your concerns are valid and there is lack of clarity here. I'm confused about the type/kind problem too. I'm also confused about the new senses in which "containment" is being used. Originally, UML distinguished "aggregation" (unmarked whole-part relationship, with no restrictions on sharing) from "composition" (marked whole-part relationship, with exclusive ownership and mutual lifetime dependencies implied) and from "containment" (unmarked homeomeric repository-element relationship, such as sets or lists of things). The documents you cite appear to use "containment" in senses that intuitively come close to "composition" (attributes are necessarily co-existent with the structures they define). Originally, containment had no marking for value/reference semantics of its contained elements, so it would be possible eg for a list to contain references to lists which reference the original list. But I doubt if it's correct to imagine the circular object/type relationships, nor that this is what we should call "containment". Brian Henderson-Sellers wrote a good paper distinguishing all the different kinds of aggregation, containment and composition. (Java uses "Container" in a different sense, as the superior part of a Composite design pattern (eg in the AWT), which should strictly have a different name like Aggregate or Composite.) Ho-hum! --Tony ========================================================================== Dr Anthony J H Simons a.simons@dcs.shef.ac.uk Lecturer in Computer Science http://www.dcs.shef.ac.uk/~ajhs Director of Teaching Department of Computer Science tel: (+44) 114 22 21838 University of Sheffield dept: (+44) 114 22 21800 Regent Court, 211 Portobello Street fax: (+44) 114 22 21810 SHEFFIELD, S1 4DP univ: (+44) 114 22 22000 United Kingdom ==========================================================================


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