Associations

Abstract

In this chapter, we focus on the fact that classes do not exist in isolation, but are semantically connected. We explain that class properties corresponding to complex and relational characteristics can be developed in a model in the form of associations. An association is composed of two semi-associations, which are inverse of each other. Every semi-association belongs to a class, called the participant class, and connects it to the opposite class. We also explain that every semi-association has a name that distinguishes it from others of the same class, such as LivesIn or BelongsTo, as well as a cardinality, which describes how many instances of the opposite class may exist for the semi-association for each instance of the participant class. Finally, we explain that a semi-association may have a role, which works as an alternative name for the opposite class in the context of the association, such as Residence or Owner. Then, we move on to two particular association cases. The first is that of whole/part semantics and explain that some associations represent the fact that some things are composed of others. This is the case, for example, of a forest and its trees, or a town and its houses. The second case is that of self-associations, that is, associations that connect a class with itself, representing situations where things of the same type are connected to each other. This is the case, for example, of a person and his/her children (who are also persons), or a land division and its subdivisions (which are also land divisions). We close the chapter by linking back to the concept of object and explaining that the links that an object may have, how many, and to which other objects, are determined by the associations of the corresponding class.