1.6.3 Shared References in UML Object Diagrams

Overview

An object diagram shows particular objects and the references that connect them. Two fields can point to the same object. In an airline example, a Flight and a Gate may both refer to the same assigned Aircraft; drawing one Aircraft object with two incoming arrows communicates shared identity.

flight.AssignedAircraft = aircraft;
gate.CurrentAircraft = aircraft;

Both assignments store the same object reference. They do not create a second Aircraft. A later change to a field on that Aircraft is visible through either access path because both reach the same instance.

Read Two Paths to One Object

Trace flight.AssignedAircraft and gate.CurrentAircraft in the diagram. Both arrows end at one aircraft object. If the diagram showed two separate Aircraft boxes, matching field values alone would not prove that they were the same instance.

Trace the Outcome

In the Zoo assignment, ComoZoo.FeaturedAnimal and ComoZoo.BirthArea.Mother similarly reach Dolly after the shared-reference assignment. Draw Dolly only once and add both labeled arrows.

An object diagram is a snapshot of references at one moment. Before the second assignment, Gate.CurrentAircraft may be null; after it, both fields point to one Aircraft. The diagram shows the later state. If an Aircraft field changes through Gate.CurrentAircraft, reading the same field through Flight.AssignedAircraft sees that change because there is only one Aircraft instance.