Object Diagrams provide a snapshot of system execution at a point in time using objects and links.

An object diagram is most useful in the early phases of a development effort to model examples from the problem domain which supports the investigation of requirements. Later in the development effort, to validate class diagrams, the object diagram can be used to model test cases for correctness and completeness.

An object diagram is a variant of a class diagram and basically uses the same notation with the difference being that the object diagram shows a set of instances and not actual classes.

Tools need not support a separate format for object diagrams. The class diagram canvas can contain object specifications, so a class diagram with objects and no classes is an “object diagram.” The phrase is useful, however, to characterize a particular usage achievable in various ways.