By Herbert J. Bernstein

© Copyright 2000 Herbert J. Bernstein

Objects are a combined abstraction of data and processes. We focus on the actions they perform and the messages they send to one another to cause those actions to be taken.

• 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 attributes -- used to internally by the object, not visible to the environment
  • Encapsulation -- only the object's methods are allowed to change its state
• Abstract data types, modules, packages, classes
  • Abstract data type
    • Definition of required memory allocations and operations accessible via single syntactic unit accessible to programs (clients) to define objects
    • The only operations on objects of the specified type are those given in the definition, with details of the object hidden

  • Language	Approach to ADT
    Simula 67	Class, no hiding
    Smalltalk	Class, objects, no other types
    Ada	Package (specification, body) -- multiple ADTs with optional hiding
    Modula-2	Module -- all hidden types are pointers
    C++	Class with hiding, but non-class types permitted
    Java	Package and class with hiding, the only user-defined types are classes

• 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
• Smalltalk
  • Pure object-oriented language
  • Literals are objects
  • Classes
    • Class name
    • Superclass name
    • Declaration of instance variables (not visible)
    • Declaration of instance and class methods
  • Variables
    • Typeless references to objects of any class
    • Implicitly dereferenced
    • Private variable -- local to an object, begin with lower case letter
    • Shared variable -- visible outside declaring object, begin with upper case letter
    • Named variable -- pointer
    • Indexed variable -- array reference
  • Message expression -- target-object message
  • Message
    • Unary -- method
    • Binary -- method object
    • Keyword -- keyword1: param keyword2: param ...
    • Arbitrary combinations of unary, binary and keyword messages
  • Method -- message_pattern | temporary variables | statements
    • Symbol "^" flags expression to return
    • If no return specified, return reference to receiver object
  • Assignment -- variable <- object-reference
• Java
  • Descendant of C++
  • Some primitive non-object types
  • All arrays amd user-defined types are objects
  • No subprocedures or functions other than object methods
  • One root class for all objects
  • Single inheritance
  • Quasi-multiple inheritance through interfaces and extra objects
  • Defaults to dynamic binding
  • Encapsulation through classes and packages
  • New namespace for each package
• 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
• Message passing