Object Oriented Programming

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Object Oriented Programming
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Mapping the world to software

• Objects in the problem domain are mapped to
  objects in software

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Object Oriented

• Data and operations are grouped together

WombatWorld
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Data Encapsulation
class Wombat private int direction
private int leavesEaten public Wombat()
public move()
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Advantages of Encapsulation

• Protection
• Consistency
• Allows change

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Objects and Classes

• Classes reflect concepts, objects reflect
  instances that embody those concepts.

object
class
woman
Not Fred
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Objects and Classes contd
Operations MakeDesposit Transfer WithDraw GetBalan
ce
Class BankAccount Balance InterestYTD Owner Accoun
t_number
Balance500 InterestYTD 10 Owner
Mark Account_number 12345
Balance 10,000 InterestYTD 25 Owner
Martha Account_number 76543
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Objects as instances of Classes

• The world conceptually consists of objects
• Many objects can be said to be of the same type
  or class
• My bank account, your bank account, Bill Gates
  bank account
• We call the object type a class

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Instantiation

• An Object is instantiated from a Class

BankAccount myAccount myAccount new
BankAccount()
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Objects and Classes

• Class
• A template for objects
• Visible in source code

• Object
• Own copy of data
• Active in running program
• Occupies memory
• Has the set of operations (i.e., functions)
  defined in the class

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Classification
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Inheritance

• A class which is a subtype of a more general
  class is said to be inherited from it.
• The sub-class inherits the base class data
  members and member functions

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Inheritance contd

• A sub-class has all the data members of its
  base-class plus those that it declares
• A sub-class has all member functions of its base
  class (with changes) plus those that it defines
• More on this later.

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What is a good class ?

• A class abstracts real-world objects
• A class should be non-trivial in the context of
  the program (it has data and operations different
  from other classes)

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Class Design

• Use is a or has a to determine whether
  something should be a subclass or be a data item.
• Ex
• A dog has a name. Not A dog is a name.
• A dog is a mammal. Not A dog has a mammal.
  (unless its in predator-mode which isnt a step
  in oo design).

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Summary

• What is Object Oriented Programming?
• Object-oriented programming is a method of
  implementation in which programs are organized as
  collections of objects, each of which represents
  an instance of some class, and whose classes are
  all members of one or more hierarchy of classes
  united via inheritance relationships