Content roles in XML

[Jim Amsden]

I'm very new to XML, and in developing a DTD for JAR files, JavaBeans, and
Rational Rose petal files, I experienced a recurring problem. The EventSet
element of the JavaBeans DTD is exemplary. Here's a fragment of the JavaBeans
DTD I came up with:

<!ELEMENT EventSet (Annotation*, Method, Method, Method+)>
<!ATTLIST EventSet
  %FeatureDescriptor;

  listenerType CDATA #REQUIRED
  isInDefaultEventSet (true | false) "false"
  isUnicast (true | false) "false"
>

The content of an Event set includes two required methods, and a collection of
other methods. In the DTD, there's no way that I know of to indicate the roles
these methods play in the EventSet. I would like to say something like:

<!ELEMENT EventSet (Annotation*, addListenerMethod, removeListenerMethod,
eventMethod+)>
<!ATTLIST EventSet
  %FeatureDescriptor;

  listenerType CDATA #REQUIRED
  isInDefaultEventSet (true | false) "false"
  isUnicast (true | false) "false"
>

where addListenerMethod, removeListenerMethod, and eventMethod are all Method
elements. This more clearly describes the content of an EventSet and avoids
using positioning only to capture the meaning of element content. I could use
parameter entities to achieve this effect as in:

<!ENTITY % addListenerMethod "Method">
<!ENTITY % removeListenerMethod "Method">
<!ENTITY % eventMethod "Method">

<!ELEMENT EventSet (Annotation*, %addListenerMethod;, %removeListenerMethod;,
%eventMethod;+)>
<!ATTLIST EventSet
  %FeatureDescriptor;

  listenerType CDATA #REQUIRED
  isInDefaultEventSet (true | false) "false"
  isUnicast (true | false) "false"
>

Is this reasonable? Good XML DTD style? Not too much of a runtime overhead? A
common practice? Note that this probably wouldn't help with the parsed XML as
there would be a Method element for each method. You couldn't ask an EntitySet
element for it's addListenerMethod content like you could ask it for it's
isUnicast attribute. You'd have to know to get the first Method in the content.
Of course an extensible parser with factory methods for constructing parse tree
nodes could hide the position dependence and provide more meaningful accessors.

I guess what I'm looking for is a way to capture (using UML terms) the
association roles between the EventSet Class and the Method Class. There are 3
associations between these two classes, and I need a way to distinguish them.

Anyone have any other ideas? Has anyone else experienced this situation?





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