Since classical OO programming languages do not support assocations as first class citizens, but only classes and reference properties, which represent unidirectional associations (but without any explicit visual rendering), we have to eliminate all explicit associations for obtaining an OO design model.
The starting point of our restricted association elimination procedure is an information design model with various kinds of unidirectional associations, such as the model shown in Figure 11.6 above. If the model still contains any non-directional associations, we first have to turn them into directional ones by making a decision on the ownership of their ends, as discussed in Section 5.
A unidirectional association connecting a source with a target class is replaced with a corresponding reference property in its source class having
the same name as the association end, if there is any, otherwise it is set to the name of the target class (possibly pluralized, if the reference property is multi-valued);
the target class as its range;
the same multiplicity as the target association end,
a uniqueness constraint if the unidirectional association is inverse functional.
This replacement procedure is illustrated for the case of a unidirectional one-to-one association in Figure 11.3 above.
For the case of a unidirectional one-to-many association, Figure 11.8 below provides an illustration
of the association elimination procedure. Here, the non-functional association end at the
target class Point is turned into a corresponding reference property with name
points obtained as the pluralized form of the target class name
In the case of our running example, the Publisher-Book-Author information design model, we
have to replace both unidirectional associations with suitable reference properties. In the
first step, we replace the many-to-one association Book-has-Publisher in the model of Figure 11.6 with a functional reference property
publisher in the class Book, resulting in the following
association-free model:
Notice that since the target association end of the Book-has-Publisher association has the multiplicity 0..1, we have to
declare the new property publisher as optional by appending the multiplicity
0..1 to its name.
In the second step, we replace the many-to-many association Book-has-Author in the model of Figure 11.7 with a multi-valued reference property
authors in the class Book, resulting in the following
association-free model:
After the platform-independent association-free information design model has been completed, one or more platform-specific data models, for a choice of specific implementation platforms, can be derived from it. Such a platform-specific data model can still be expressed in the form of a UML class diagram, but it contains only modeling elements that can be directly encoded in the chosen platform. Thus, for any platform considered, the tutorial contains two sections: 1) how to make the platform-specific data model, and 2) how to encode this model.