Object Diagram

An Object Diagram can be referred to as a screenshot of the instances in a system and the relationship that exists between them. Since object diagrams depict behavior when objects have been instantiated, we are able to study the behavior of the system at a particular instant. Object diagrams are vital to portray and understand functional requirements of a system. In other words, “An object diagram in the Unified Modeling Language (UML), is a diagram that shows a complete or partial view of the structure of a modeled system at a specific time.”

Overview of Object Diagrams
In the Unified Modeling Language (UML), an object diagram focuses on some particular set of objects and attributes, and the links between these instances. A correlated set of object diagrams provides insight into how an arbitrary view of a system is expected to evolve over time. In early UML specifications the object diagram is described as:

"An object diagram is a graph of instances, including objects and data values. A static object diagram is an instance of a class diagram; it shows a snapshot of the detailed state of a system at a point in time. The use of object diagrams is fairly limited, namely to show examples of data structure."

The latest UML 2.5 specification does not explicitly define object diagrams, but provides a notation for instances of classifiers.

Object diagrams and class diagrams are closely related and use almost identical notation. Both diagrams are meant to visualize static structure of a system. While class diagrams show classes, object diagrams display instances of classes (objects). Object diagrams are more concrete than class diagrams. They are often used to provide examples or act as test cases for class diagrams. Only aspects of current interest in a model are typically shown on an object diagram.

Purpose of Object Diagrams
The purpose of a diagram should be understood clearly to implement it practically. The purposes of object diagrams are similar to class diagrams. The difference is that a class diagram represents an abstract model consisting of classes and their relationships. However, an object diagram represents an instance at a particular moment, which is concrete in nature. It means the object diagram is closer to the actual system behavior. The purpose is to capture the static view of a system at a particular moment. The purpose of the object diagram can be summarized as −

• Forward and reverse engineering.
• Object relationships of a system
• Static view of an interaction.
• Understand object behavior and their relationship from practical perspective

Examples of Object Diagrams
The following are the application areas where the object diagrams can be used.

• To build a prototype of a system.
• To model complex data structures.
• To perceive the system from a practical perspective.
• Reverse engineering.

References