Objects

In UML models, objects are model elements that represent instances of a class or of classes. You can add objects to your model to represent concrete and prototypical instances. A concrete instance represents an actual person or thing in the real world. For example, a concrete instance of a Customer class represents an actual customer. A prototypical instance of a Customer class contains data that represents a typical customer.

A class represents an abstraction of a concept or of a physical thing, whereas an object represents a concrete entity. An object has a well-defined boundary and is meaningful in the application. Objects have the characteristics that are listed in the following table:

Characteristic	Description
State	The state is the condition in which an object can exist. An object’s state is implemented with a set of attributes and usually changes over time.
Behavior	Behavior determines how an object responds to requests from other objects. Behavior is implemented by a set of operations.
Identity	The identity of an object makes it unique. You can use the unique identity of an object to differentiate between multiple instances of a class if each instance has the same state.

Each object must have a unique name. A complete object name has three parts: object name, role name, and class name. You can use any combination of the parts when you name an object. The following table shows several object name variations for an online shopping system.

Syntax	Example	Description
object/role:class	cart100/storage:cart	A named instance (cart100) of the cart class performs the role of storage during an interaction.
object:class	cart100:cart	A named instance (cart100) of the cart class
/role:class	/storage:cart	Anonymous instance of the cart class that performs the role of storage in an interaction.
object/role	cart/storage	An object named cart that plays the storage role. This object is either an object that hides the name of the class or an instance that is not associated with a class.
object	cart100	An object named cart100. This object is either an instance that hides the name of the class or an instance that is not associated with a class.
/role	/storage	An anonymous instance performs the role of storage. This object is either an instance that hides the name of the object and class or an instance that is not associated with an object or class.
:class	:cart	Anonymous instance of the customer class.

Types of objects

The following table lists the three types of object.

Types of objects	Description
Active	A active object owns a thread of control and can initiate control activity. Processes and tasks are types of active objects.
Passive	A passive object holds data, but does not initiate control.
Multiple instance	Is a collection of objects or multiple instances of the same class. Multiple instances are commonly used to show that a set of objects interacts with a single message (or stimulus).

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