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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Inheritance in XML [^*]
 At 1:46 PM -0700 4/23/98, Lisa Rein wrote: >> >> What are two properties? Type and class? Did you shift from discussing >> properties "concerning" type and class and type and class *as* properties. >> >> Can you also please defend the distinction between type and class? It >> makes sense in object oriented programming languages (mostly for >> performance reasons), but I don't know that there is any such distinction >> in common usage or in most ontologies. I am prepared to be convinced >> otherwise, but I think that the class/type distinction is specific to OOP >> and is not useful except as an arbitrary distinction, to avoid confusion, >> as it is used in the DSSSL spec. (node class, flow object class vs. >> element type ... you could as easily reverse them and talk of element >> classes and node types...) > >It was my understanding that they are all just different names for the >same "things". At least they are for RDF, right? In fact, in the RDF >Schema group, as far as the typing (classing) models were concerned, >types and classes were exactly the same (so much so, in fact, that we >went back and forth on which to call them and, conceptually anyway, the >words became so interchangible -- Resulting in a "type system" of >classes :-). Also, the types/classes and their respective resources (of >which RDF's mission is to describe) WERE classes and node types (that >was my understanding anyway...). > >Just when I thought I finally had a clear understanding of the above, >suddenly I'm not so sure. Someone reassure me....please. > Whether there's a distinction between "type" and "class" depends on the context. Loosely speaking, as well as in many contexts, they're generally considered synonymous. RDF uses them pretty much interchangeably. As Paul suggests, the distinction is often made in the context of OOPLs. In this context, roughly speaking, the term "type" is often used to mean a protocol shared by a group of objects, while the term "class" is often used to mean an implementation shared by a group of objects. The same distinction can be important in other contexts (e.g., when talking about implementation aspects of distributed objects) when you need to distinguish between an interface that may supported by a group of objects, as opposed to an implementation that may be shared by a group of objects. Some object analysis and design methodologies make this distinction as well. However, given the various usages, it's not a good idea to count on this distinction unless you know that everyone accepts it. It's not really set in concrete. You might want to look at the descriptions of "type and class" in various object models in the X3H7 Object Model Features Matrix (it's Section 7) at http://www.objs.com/x3h7/fmindex.htm --Frank ----------------------------------------------------------------------- Frank Manola www: http://www.objs.com Object Services and Consulting, Inc. email: fmanola@o... 151 Tremont Street #22R voice: 617 426 9287 Boston, MA 02111 xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ To (un)subscribe, mailto:majordomo@i... the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@i... the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@i...) 
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