Adding Roles to Relationship Patterns

In this paper we study how roles can be added to
patterns modelling relationships in Object Oriented programming,
and which new relationship patterns can be introduced
using roles. Relationships can be introduced in programming
languages either by reducing them to attributes of the objects
which participate in the relationship, or by modelling the relationship
itself as a class whose instances have the participants of
the relationships among their attributes. However, even if roles
have been recognized as an essential component of relationships,
also in modelling languages like UML, they have not been
introduced in Object Oriented programming when it is necessary
to model relationships. Introducing roles allows to add attributes
and behaviors to the participants in the relationship, rather
than to the relationship itself, and to distinguish natural types
as classes participating in the relationships from the roles the
participants acquire in the relationships. In this paper we show
how the role model proposed in powerJava can be used to endow
relationships with roles, both in the relationship as attribute and
in the relationship object pattern. Finally, since these patterns
have different advantages and limitations, we propose a third
pattern based on roles which benefits from the advantages of the
two previous patterns when modelling relationships.