The Unified Modeling Language User Guide

1
The Unified Modeling Language User Guide

• Chapter 13
• Instances

2
Chapter 13 Topics

• Introduction
• Terms and concepts
• Active Objects
• Review

3
Introduction

• An instance is a concrete manifestation of an
  abstraction to which a set of operations may be
  applied and which may have a state that stores
  the effects of the operation.
• Instances are used to represent concrete things
  that live in the real world. For example
• A physical house is an instance of a blueprint.
• A cherry pie is an instance of a recipe.
• A musical performance is an instance of a score.
• An object is an instance of a class.
• A link is an instance of an association.

4
Introduction

• An unlimited number of instances can be created
  from a single abstraction.
• Any given instance can be mapped to a single
  abstraction.
• Instances are used to
• Model prototypes of a system.
• Simulate the state of a system in production at a
  particular point in time.
• Instances are graphically represented using the
  abstraction icon with the abstraction name
  underlined.

5
Terms and Concepts

• Abstractions and Instances
• An instance is something that takes up space in
  the real or conceptual world, and you can do
  things to it.
• Instances most commonly show up in instance
  diagrams (static model) or interaction diagrams
  (dynamic modeling).
• Objects are the most commonly used instances.

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Terms and Concepts

• Names
• Every instance, at a minimum, must have a name
  in the form of its abstraction type proceeded by
  a colon. This is known as an anonymous instance.
  
• AbstractionType
• A name may also optionally contain a variable
  name identifier.
• variableName AbstractionType
• Instances may have simple names.
• AbstractionType
• Instances may have path names.
• PackageNameAbstractionType

7
Terms and Concepts

• Operations
• The member functions contained within an object
  that may be invoked by other objects having
  knowledge of the existence of that object.
• State
• Every object has a state.
• The state of an object changes over its lifetime.
• The state of an object at a particular point in
  time is a composite of the values of all its
  member data.
• State diagrams are used to model the relevant
  states that an object can pass through.

8
Object State
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Terms and Concepts

• Active Objects
• Are objects that have their own independent
  thread of control.
• Are used in the context of concurrent
  programming.
• Are graphically represented as an object icon
  with its borders drawn using a thick line.
• Are useful in modeling real-world concurrency.
• Complicate communication between objects due to
  synchronization issues.

10
Review

• Remember, objects and other instances are used to
  model concrete things that exist in the real
  world.
• Instances are not abstractions, they are
  instantiations of abstractions. They exist in
  space and time.
• Instances generally appear in the context of
  instance diagrams.
• Instance diagrams provide a simulation of a
  system in production. They provide the designer
  with another prospective on the system being
  built.