We illustrate generalization with the following example, which extends the information model of Part 4 by adding the object type Employee and associating employees with publishers.

Figure 17.2. The object types Employee and Author share several attributes

After adding the object type Employee we notice that Employee and Author share a number of attributes due to the fact that both employees and authors are people, and being an employee as well as being an author are roles played by people. So, we may generalize these two object types by adding a joint supertype Person, as shown in the following diagram.

Figure 17.3. The object types Employee and Author have been generalized by adding the common supertype Person

When generalizing two or more object types, we move (and centralize) a set of features shared by them in the newly added supertype. In the case of Employee and Author, this set of shared features consists of the attributes name, dateOfBirth and dateOfDeath. In general, shared features may include attributes, associations and constraints.

Notice that since in an information design model, each top-level class needs to have a standard identifier, in the new class Person we have declared the standard identifier attribute personId, which is inherited by all subclasses. Therefore, we have to reconsider the attributes that had been declared to be standard identifiers in the subclasses before the generalization. In the case of Employee, we had declared the attribute employeeNo as a standard identifier. Since the employee number is an important business information item, we have to keep this attribute, even if it is no longer the standard identifier. Because it is still an alternative identifier (a "key"), we declare it to be unique. In the case of Author, we had declared the attribute authorId as a standard identifier. Assuming that this attribute represents a purely technical, rather than business, information item, we dropped it, since it's no longer needed as an identifier for authors. Consequently, we end up with a model which allows to identify employees either by their employee number or by their personId value, and to identify authors by their personId value.

We consider the following extension of our original example model, shown in Figure 17.4, where we have added two class hierarchies:

Figure 17.4. The complete class model containing two inheritance hierarchies