Object diagrams model instances of classes and are used to describe the system at a particular
point in time. Just as an object is an instance of a class we can view an object diagram
as an instance of a class diagram. We referred to this type of diagram as a semantic
net diagram in Chapter 11. Using this technique, we can validate the class diagram (ER
diagram in our case) with ‘real world’ data and record test cases. Many object diagrams
are depicted using only entities and relationships (objects and associations in the UML
terminology). Figure 25.15 shows an example of an object diagram for the Staff Manages
PropertyForRent relationship.