Objects

An object is an entity within a system that has identity, state, and behavior. An object can be a real world such as a person or thing. It can also be conceptual such as a transaction or a business process. An object has a well-defined purpose with boundaries.

The Identity of an object means that each object is unique. The object may share the characteristics of another object but it is unique in that it is a particular instance of something. A video titled Gladiators is different from one titled Halloween. Both are videos and found in the catalog but each have a unique identity.

The State of an object is one of the possible conditions that it may have at any point in time. The state is defined by a set of attributes with particular values and the relationships that the object has with other objects. The state usually changes over time. As an example, a video may be checked-out or may be available.

The Behavior of an object defines how the object responds to other objects and how it performs. Behavior is defined by the set of operations that an object can perform. In the video system, the video catalog object could have the behaviors Add a Video and Remove a Video.

The UML represents an object as a rectangle with the name underlined as shown.