Principles of Object-oriented Programming

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Principles of Object-oriented Programming

• Programming Language Paradigms
• August 26, 2002

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Pre-history

• 40s/50s von Neumann architecture, stored
  program concept
• Allows the development of general-purpose
  computing machines
• Languages tend to be closely tied to the
  underlying architecture
• Structured programming emerges
• Verb and task oriented
• Difficult to scale

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History

• Simula-67 objects, early GUI elements
• Xerox PARC (70s) Smalltalk, bitmap displays,
  multiple windows, mouse, menus, icons, etc.
• Smalltalk-80 modern version
• released outside PARC.
• Used a virtual machine and a bytecode compiler.
• Spinoff companies create commercial versions.

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Squeak History

• 1995 (Apple) Alan Kay et al. create an
  open-source version of Smalltalk.
• Development of a platform that can be used by
  people of all ages and skill levels.
• Squeak compiler and VM written in Squeak.
• Support for sound, 2-D and 3-D graphics,
  networking/web, Flash, etc.

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Other OO history

• C developed at ATT labs (Stroustrup79)
• Interested in an efficient language that extended
  C
• Strong typing
• Objective C
• Used in NeXT and OS X combination of C and
  Smalltalk.
• Java developed by Sun (1991-1995)
• Initially developed as a language for
  communicating appliances/set-tops
• Modified for use with WWW.
• Smalltalk features in a typed environment

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About Objects

• An object is a way to capture both the features
  of a data structure and its behavior.
• Noun-oriented encourages program design through
  thinking about pieces
• Construction of reusable elements

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Classes and objects

• A class is a characterization of a type of
  object.
• The Muppet class refers to the set of all
  muppets.
• An object kermit is an instantiation of the
  Muppet class.
• Classes have
• Instance variables
• Class variables
• methods

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An example

• Object subclass Muppet
• instanceVariableNames name
• classVariableNames numberOfSpecies
• category muppets
• To create a muppet
• Kermit lt- Muppet new.

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Messages

• In Smalltalk, methods are invoked by sending the
  object a message.
• noun verb arg1 mod arg2 .
• Modeled on speech acts.

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Inheritance

• One of the most popular OO features is
  inheritance.
• Can define new classes in terms of existing
  classes.
• This creates a class hierarchy in which
  subclasses take on the behavior of their parent
  classes.
• A subclass can generate new behaviors by
• Defining new methods
• Overriding a parent method.

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Inheritance example
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Polymorphism

• Polymorphism is the idea that an object or method
  can perform the same operation on different data
  types.
• Addition is polymorphic in most languages.
• Allows a programmer to write code without knowing
  the types of the data involved.
• listOfStuff sort lt- dont need to know what
  this is a list of.

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Polymorphism example
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Encapsulation

• Encapsulation provides a way of packing all the
  structure and behavior of an object into a single
  module.
• Interface the public face of an object
• Implementation How methods are carried out.
• Prevents users from misusing internal structure.
• Implementation can be changed without affecting
  external programs.

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Encapsulation example