XML Topic Maps

Mapping the XTM Syntax to the XTM Conceptual Model

The diagrams used in this section are 'class diagrams', using the conventions of the Unified Modelling Language (UML). In a class diagram, each rectangle represents a class of objects (a kind of thing that can exist), and the words in the rectangl are the name of that class. The lines and arrows between the rectangles represent relationships that exist or can exist between instances of those classes (individual things of those kinds). In an object diagram, each rectangle represents an individual object, and the words in the recangle are the name of the individual, followed by a colon, followed by the name of the class of which it is an instance. The lines between the rectangles represent relationships that exist between those individual objects.

Topic-identifying elements

There are several element types in the XTM syntax whose purpose is to identify a particular Topic. We shall create an abstract class called "Topic-identifying Element", from which these are all derived.

The XTM syntax provides three mechanisms for identifying a Topic.

• Referencing a <topic> Element that represents the Topic

  Referencing a <topic> element

• Referencing a Resource that is a Subject Indicator for the Topic

  Referencing a Subject Indicator

• Referencing a Resource that is the Subject of the Topic

  Referencing a reified Resource

Resource-identifying elements

There are several element types in the XTM syntax whose purpose is to identify a Resource related to a Topic. We shall create an abstract class called "Resource-identifying Element", from which these are derived.

Resource-identifying elements

The XTM syntax providestwo mechanisms for identifying such a Resource:

• If a Resource-identifying Element has a <resourceData> Element child, that <resourceData> Element is the identified Resource.
• If the Resource-identifying Element has a <resourceRef> Element child, the xlink:href attribute of the <resourceRef> Element references the identified Resource.

Identifying a Resource

Representing an Occurrence of a Topic

Representing the types of Topic Map Components

The XTM syntax allows for the explicit typing of Topics, Occurrences and Associations. The mechanism used is the same for Topics and Associations (which are Topic Map Nodes) and for Occurrences (which are Topic Characteristics). We therefore create an abstract class called "Topic Map Component" from which Topic Map Node and Topic Characteristic are derived, and an abstract class called "Typed Element", from which the <topic> Element, <occurrence> Element and <association> Element are derived.