Aggregation is a form of association in which one element consists of other, smaller con-
stituents. This relationship is depicted by a diamond-shaped white arrowhead. In this
case, if the parent object is deleted, the child object may still continue to exist. Figure 1-12
shows an aggregation relation between an insurance agent and the local insurance office
in which he works. The local insurance office is where insurance agents carry out tasks
such as policy underwriting, depositing premiums for their customers, and various other
functions. So even if the local office is closed down, the agent can report to another
office. Similarly, the agent can de-register from a local office and move to a different
office of the same insurer.
Figure 1-12. Aggregation
Composition is a stronger form of aggregation; as in this case, if the parent is deleted, the
children will also no longer exist. This relationship is depicted by a diamond-shaped solid
arrowhead. Figure 1-13 shows the composition relationship between a party involved in
some policy or claim and their address. If the party is deleted from the system, its address
will also be deleted.
Figure 1-13. Composition