TABLE OF CONTENTS This section introduces the first of the 13 UML diagrams: the class diagram. The class diagram was introduced in Chapter 3 in order to illustrate structural modelling, so this section should really only serve as a recap. It is useful to examine class diagrams again, however, as they are the diagrams that are used in order to realise the meta-model.
Class diagrams realise a structural aspect of the model of a system and show what conceptual 'things' exist in a system and what relationships exist between them. The 'things' in a system are represented by classes and their relationships are represented, unsurprisingly, by relationships.
Class diagrams are used to visualise conceptual, rather than actual, aspects of a system. Classes themselves represent abstractions of real-life entities. The actual real-life entities are represented by objects and, therefore, any object must have an associated class that defines what it looks like and what it does.
Class diagrams are made up of two basic elements: classes and relationships. Both classes and relationships may have various types and have more detailed syntax that may be used to add more information about them. However, at the highest level of abstraction, there are just the two very simple elements that must exist in the diagram.
Classes describe the types of 'things' that exist in a system, whereas relationships describe what the relationships are between various classes.
Figure 5.6 shows a high-level meta-model of class diagrams. Ironically, the meta-model...
