An object diagram is a UML structural diagram that shows the instances of the classifiers in models. Object diagrams use notation that is similar to that used in class diagrams. However, while class diagrams show the actual classifiers and their relationships in a system, object diagrams show specific instances of those classifiers and the links between those instances at a point in time. You can create object diagrams by instantiating the classifiers in class, deployment, component, and use-case diagrams.
The following figure shows a simple class diagram that consists of two classes connected by an association.
The following figure shows the corresponding object diagram that consists of two instance specifications connected by a link relationship.
Object diagrams are helpful in the following situations:
- During the analysis phase of a project, you might create a class diagram to describe the structure of a system and then create a set of object diagrams as test cases to verify the accuracy and completeness of the class diagram.
- Before you create a class diagram, you might create an object diagram to discover facts about specific model elements and their links, or to illustrate specific examples of the classifiers that are required.
The following topics describe the elements in object diagrams: