These links must be
represented differently: different line thicknesses and colors, arrowheads, and various
decorations.
The different kinds of links also provide an important source of the semantics and
rules of the modeling language: the differences between them are more than skin deep.
As the semantics often depend on the direction of a connection, there must be a
distinction according to the role an object plays in the connection: is it a superclass or
subclass, a container or contained element. If we restrict ourselves to binary
relationships, it is possible to consider this information as part of the relationship.
Each relationship will have its own information, and a set of information for each end:
what object it connects to, and any properties describing that connection. If we look a
little further and allow relationships connecting more than two objects, that set of
information is seen more clearly as a concept it its own right: a role.
N-Ary Relationships In a binary relationship, it appears that the arrowheads are
part of the de?¬?nition of the role the object plays in the relationship, but the line itself is
part of the relationship. Looking at a ternary relationship, a connection between three
objects, we see that the line too is actually a feature of the role. The meeting point of
the three lines in a ternary relationship is the relationship itself, which may have a
graphical symbol.
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