Overview An Object diagram falls under the structural diagramming family. An Object diagram is an instance of a Class diagram that shows a complete or a partial view of the structure. This type of diagram displays instances of classifiers and links (instances of associations) between them.
Purpose
An Object diagram is usually used in the early phases of a project to model examples that show a possible snapshot of the system execution, for example, what the system can look like at some point in time. Additionally, it focuses on the attributes of a set of objects and helps to analyze how these objects relate to each other.
Usage
An Object diagram can be used to:
represent a class instance
describe the static aspect of the system
understand the behavior of a particular object
analyze object relations
perform forward and reverse engineering on chosen systems.
Summary
Object diagrams are valuable because they:
can be used to model test cases in order to validate the Class diagram
support the investigation of requirements by modeling examples from the problem domain
examine a specific iteration of a general system
later in the project, model test cases to see whether the created Class diagram is correct and complete
allows users to get a high-level overview of the system.