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   XML and inheritance and transformation of representations

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  • From: "Tolkin, Steve" <Steve.Tolkin@fmr.com>
  • To: xml-dev@lists.xml.org
  • Date: Wed, 12 Jul 2000 15:34:41 -0400

What kinds of "inheritance" are possible in XML, and in DTD and XML Schema?
Consider this XML, an excerpt from [1] that illustrates a general issue.

<OBJECT ID="JTextField0" CLASS="JTextField"> 
    <OBJECT PROPERTY="nextFocusableComponent" IDREF="JButton0"/> 
    <OBJECT PROPERTY="bounds" CLASS="Rectangle"> 
        <OBJECT CLASS="Integer" VALUE="30"/> 
        <OBJECT CLASS="Integer" VALUE="50"/> 
        <OBJECT CLASS="Integer" VALUE="200"/> 
        <OBJECT CLASS="Integer" VALUE="20"/> 

There is no semantic information about the role of the four integers.
I think that these correspond to two points, 
and are the x and y coordinates, and are in pixels.  
But I cannot know that withotu reading some external documentation.
Perhaps it is the coordinates of one point plus a delta x and delta y.

What techniques are available in XML to increase flexibility and clarity?  
Wearing an object oriented hat we might want to allow CLASS=rectangle 
to be have various possible implementations:
1a. As above
1b. As above, but also labelled as e.g. x1, y1, x2, y2, with either tags or
another attribute of Object e.g. ROLE="x1", or with multiple attributes
2. As one point and two distance deltas
3. As two point objects (with perhaps some constraints e.g. x1 < x2 and y1 <
4. As subtype of a polygon class, constrained to have 4 sides which form a

What can I say in a DTD or in XML Schema to allow this kind of flexibility?

What must/can I do, in e.g. XSLT, to transform one of those other
implementations to
the original one?


[1] The sample XML is a small excerpt from "File Formats" at
which is part of "Long-Term Persistence for JavaBeans" 
by Philip Milne & Kathy Walrath at 
They do say "This is just one possible XML format" but I have chosen a
published example to illustrate my point.
Presumably this format was chosen because these map directly into the
executable call.  For example here is the same information represented in
BeanScript format:
   let JTextField0 = Class.forName("javax.swing.JTextField").new(); 
   JTextField0.nextFocusableComponent := JButton0; 
   JTextField0.bounds := Rectangle.new(30, 50, 200, 20);  

I know why C has unlabelled parameters, but I am hoping that XML can do

Steven Tolkin          steve.tolkin@fmr.com      617-563-0516 
Fidelity Investments   82 Devonshire St. R24D    Boston MA 02109
There is nothing so practical as a good theory.  Comments are by me, 
not Fidelity Investments, its subsidiaries or affiliates.


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