Object diagrams model instances of classes and are used to describe the system at a par- ticular point in time. Just as an object is an instance of a class we can view an object dia- gram 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.