By Herbert J. Bernstein

© Copyright 2000,2002 Herbert J. Bernstein

Object Modeling is a general framework for designing information systems. It focus in objects, the actions they perform and the messages they send to one another to cause those actions to be taken. The major differences among object modeling, network modelling, data modeling and process modeling are that in the first we focus on the actions in response to information, objects which form the system, the actions they perform, and the ways in which they pass information to one another, while in the second we concentrate on where, when and how much information is moved, while in the third we focus on what information is moved and where it is moved, while in the last we focus on how it is moved and when it is moved.

The Object Model

• Objects
  • Any portion of a system which can respond to information (or events or messages) with changes of state (actions, methods, behaviors)
  • State -- values assigned to object attributes (tags, properties, variables)
    • Public attributes -- used to communicate with environment
    • Private attributes -- used to remember what has happened, not visible to the environment
  • Methods -- actions the object performs in response to events or messages
    • Public methods -- available to the environment to change the state of the object
    • Private methods -- used to internally by the object, not visible to the environment
  • Encapsulation -- only the object's methods are allowed to change its state
• Classes
  • Types of which particular objects are instances
  • May be organized hierarchically
  • Inheritance
    • Defining attributes and methods in terms of attributes and methods of other classes
    • Supertypes -- generalizations
    • Subtypes -- specializations
    • Multiple inheritance -- cross-breeding from multiple supertypes
  • Polymorphism -- use of the same name for multiple behaviors
• Object Oriented Analysis (OOA)
  • Identify existing objects and classes of objects within a system
  • Identify the relationships among classes
  • "Is a ... ", "contains a ...", multiplicities
  • Message passing relationships -- direct, broadcast, hierarchical
• Object Oriented Design (OOD)
  • Create appropriate classes for objects needed by a new process
  • Create appropriate relationships among classes
  • May build on analysis by reuse, inheritance
• Object Oriented Programming (OOP)
  • Implementation of OOD
  • Often template driven, esp. for GUI interfaces
• Use Case Model
  • A way to organize object modeling
  • Identify events (messages), actor(s) (originator(s)), action(s)
  • Assumptions
  • Preconditions -- messages that must be received and actions that must be completed before processing
  • Normal behavior -- sequence of actions
  • Exception handling
  • Postconditions
• Object Modeling in Practice
  • Models are written in OMG Unified Modeling Language (UML)
    (See www.uml.org)
  • UML drawings are interchanged as XML text documents
  • Object classes are specified by:
    • Name
    • State (a set of attributes eoch of which may have values)
    • Operations (actions or methods)
    • Inheritance is shown by arrows from children (subclasses) to the parent (superclass)
  • Objects (as distinct from classes) are shown by underlining the name (or other distinctive modficiation of the class symbol)
  • Modern case tools support UML
    (See, http://www.objectsbydesign.com/tools/umltools_byPlatform.html and, for free examples, www.argouml.org
  • A copy of the commercial version of argouml (poseidon) is installed on arcib in /usr/local/poseidon/.
  • see vnc.html for information on remote GUI access via ssh and vnc