Object Oriented Programming

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Object Oriented Programming

• CSC 171 FALL 2001
• LECTURE 11

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History ENIAC Electronic Numerical Integrator
and Computer

• 1943 1947 Work on ENIAC at the Univ. of
  Pennsylvania
• John Mauchly
• J. Presper Eckert.
• The world's first electronic digital computer was
  developed to compute World War II ballistic
  firing tables.

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Computer Science

• What is computer science?

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Computer Science

• fundamentally, computer science is the science
  of abstraction creating the right model for a
  problem and devising the appropriate mechanizable
  technique to solve it.
• - A. Aho and J. Ullman

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Acts of mind - Locke

• The acts of the mind, wherein it exerts its
  power over simple ideas, are chiefly these three
  

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Aggregation

• 1. Combining several simple ideas into one
  compound one, and thus all complex ideas are made

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Comparision

• 2. The second is brining two ideas, where simple
  or complex, together, and setting them by one
  another so as to take aview of them at
  once,without uniting them into one, by which it
  gets all its ideas of relations.

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Abstraction

• 3. The third is separating them from all other
  ideas that accompany them in their real
  existence this is called abstraction, and thus
  all general ideas are made.
• John Locke,
• An Essay Concerning
• Human Understanding, (1690)

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Object Orientation

• Objects are the means of
• aggregation and
• abstraction,
• in an OO programming language
• Aggregation bundling data elements
• Abstraction - inheritance

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Data Encapulation

• OOP encapulates
• data
• behavior

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Interfaces

• We define systems in terms of many objects
• Each type of object has certain behaviors
• methods
• Each object has certain data
• Instance variables
• Objects communicate with other via
• well defined interfaces

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Information Hiding

• When we design objects which communicate
  exclusively via interfaces -
• we insulate (abstract) the outward functionality
• from the inward implementation.
• This insulation is the principle of information
  hiding the inward details of the implementation
  are hidden from the outside objects

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Benefits of Information Hiding

• Information hiding promotes program
  modifiability.
• Clients are not required to know the internals of
  a class in order to use it.
• So, if the class changes inside, then the client
  need not be changed
• The client and the object are said to be loosely
  coupled

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Example A bank account
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A specific instance
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Multiple instances
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Bank Account - behaviors

• public class BankAccount
• private double balance
• public BankAccount() balance 0
• public BankAccount(double initialBalance)
• balance initialBalance
• public void deposit(double amount)
• balance balance amount
• public void withdraw(double amount)
• balance balance - amount

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this

• Java conserves storage by maintaining only one
  copy of each method per class
• The same method is invoked by every object
• Every object has its own copy of its instance
  variables
• Every object, by default has a reference to
  itself
• this is the name of every objects reference to
  itself

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Bank Account - this

• public class BankAccount
• private double balance
• public BankAccount() this.balance 0
• public BankAccount(double balance)
• this.balance balance
• public void deposit(double amount)
• this.balance this.balance amount
• public void withdraw(double amount)
• this.balance this.balance - amount

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Sometimes, classes share

• Consider a variation of the BankAccount
• public class BankAccount
• . . . .
• private double balance
• private int accountNumber
• // we want to assign sequential numbers

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Sharing data between objects of the same class

• We want to set the account number automatically
• public class BankAccount
• private double balance
• private int accountNumber
• private int lastAssignedNumber 0 //Will
  this work?
• public BankAccount()
• lastAssignedNumber
• accountNumber lastAssignedNumber //???????

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Static/class variables

• We dont want each instance of the class to have
  its own value for lastAssignedNumber
• We need to have a single value that is the same
  for the entire class.
• These are called class or static variables

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Sharing data between objects of the same class

• We want to set the account number automatically
• public class BankAccount
• private double balance
• private int accountNumber
• private static int lastAssignedNumber 0
• public BankAccount()
• lastAssignedNumber
• accountNumber lastAssignedNumber

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Finally

• Some types of variables are fixed constants, that
  we do not want to change
• Like conversion factors
• We can use the keyword final to prevent changes
• So we have a constant variable
• Sort of like
• jumbo shrimp
• freezer burn

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Classes share constants

• Consider a variation of the BankAccount
• public class Converter
• private final static double miles2km 0.6