A unidirectional association between a source and a target class can be represented as a reference property of the source class. This is illustrated in Figure 1.3 below for two unidirectional associations: a many-to-one and a many-to-many association.
Notice that, in a way, we have eliminated the two explicit associations and replaced them with corresponding reference properties resulting in a class model that can be coded with a classical OOP language in a straightforward way. OOP languages do not support associations as first class citizens. They do not have a language element for defining associations. Consequently, an OOP class design model, which we call OO class model, must not contain any explicit association.