Object Diagrams

In the Unified Modeling Language (UML), an object diagram is a diagram that shows a complete or partial view of the structure of a modeled system at a specific time. This snapshot focuses on some particular set of object instances and attributes, and the links between the instances. A correlated set of object diagrams provides insight into how an arbitrary view of a system is expected to evolve over time. Object diagrams are more concrete than class diagrams, and are often used to provide examples, or act as test cases for the class diagrams. Only those aspects of a model that are of current interest need be shown on an object diagram.

An object diagram may be considered a special case of a class diagram. Object diagrams use a subset of the elements of a class diagram in order to emphasize the relationship between instances of classes at some point in time. They are useful in understanding class diagrams. They don’t show anything architecturally different to class diagrams, but reflect multiplicity and roles.

Although we design and define classes, in a live application classes are not directly used, but instances or objects of these classes are used for executing the business logic. A pictorial representation of the relationships between these instantiated classes at any point of time (called objects) is called an "Object diagram". It looks very similar to a class diagram, and uses the similar notations to denote relationships.

If an object diagram and a class diagram look so similar, what is an object diagram actually used for? Well, if you looked at a class diagram, you would not get the picture of how these classes interact with each other at runtime, and in the actual system, how the objects created at runtime are related to the classes. An object diagram shows this relation between the instantiated classes and the defined class, and the relation between these objects, in the logical view of the system. These are very useful to explain smaller portions of your system, when your system class diagram is very complex, and also sometimes recursive.

Class and Object Elements

The following diagram shows the differences in appearance between a class element and an object element. Note that the class element consists of three parts, being divided into name, attribute and operation compartments; by default, object elements don’t have compartments. The display of names is also different: object names are underlined and may show the name of the classifier from which the object is instantiated.

Run Time State

A classifier element can have any number of attributes and operations. These aren’t shown in an object instance. It is possible, however, to define an object’s run time state, showing the set values of attributes in the particular instance.

Example Class and Object Diagrams

The following diagram shows an object diagram with its defining class diagram inset, and it illustrates the way in which an object diagram may be used to test the multiplicities of assignments in class diagrams. The car class has a 1-to-many multiplicity to the wheel class, but if a 1-to-4 multiplicity had been chosen instead, that wouldn’t have allwed for the three-wheeled car shown in the object diagram.