Metamodelling for Software Engineering

4.1 TERMINOLOGY AND CULTURE CLASHES

Taking UML as an exemplar use of the four-layer, strict metamodelling architecture, we first note the ambiguity engendered when one also applies the standard Type–Object model of object technology. Figure 4.1 shows that the object book1 is of type Book, i.e. there is an instance-of relationship from book1:Book to Book. In contrast, the analyst is not concerned with individual book objects but wishes to model, for a library information system, the requirement that a library contains many books. The simple model shown in Figure 4.2 illustrates this by the use of two classes, Library and Book, related with a one-to-many association relationship. Since both Library and Book are classes, they can be shown as instances of the M1 class, Class (Figure 4.3). This is standard object-oriented modelling (with UML or any equivalent modelling language).

Figure 4.1: In traditional object-oriented modelling, an object (book1) is an instance of a class (Book)¹

Figure 4.2: Simple object-oriented model in which a library houses many books

Although this is widely accepted as the standard approach to creating an (M1-level) object-oriented class model, further consideration of Figure 4.3 leads to the realization that since (M2) Class is in fact a class, its instances ...