The object diagram is not typically included in modeling tools. Unfortunately, its value as a testing tool is often overlooked, especially by those who are new to object modeling. We will see a very simple example in this lesson that illustrates how easy and how valuable the object diagram is for testing your class diagram.

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. 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 obect diagrams is fairly limited, namely to show examples of data structure. An object diagram is a snapshot of the objects in a system at a point in time, because it shows instances rather than classes, an object diagram is often called an instance diagram.

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 was described as:

Use cases show what your system should do. Activity diagrams allow you to specify how your system will accomplish its goals. Activity diagrams show high-level actions chained together to represent a process occurring in your system. For example, you can use an activity diagram to model the steps involved with creating a blog account. Activity diagrams are particularly good at modeling business processes . A business process is a set of coordinated tasks that achieve a business goal, such as shipping customers' orders.
Some business process management (BPM) tools allow you to define business processes using activitydiagrams, or a similar graphical notation, and then execute them. This allows you to define and execute, for example, a payment approval process where one of the steps invokes a credit card approval web serviceusing an easy graphical notation such as activity diagrams.