Introduction to Object-Oriented Programming

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Introduction toObject-Oriented Programming

• Notes Supplement
• CS 1054 Introduction to Programming in Java
• Spring 2008

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Object-Oriented Programming

• The traditional definition of a program isa
  sequence of instructions to be executedon a
  computer
• In the Object-Oriented Programming (OOP)
  paradigm, a program that executes is a collection
  of interacting objects
• In this paradigm, the programs we write/specify
  what are in these objects and how these objects
  behave

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What is an Object?

• Definition A thing that has type, identity,
  state, and behavior
• type belongs to a class of similar objects
• identity is a distinct instance of that class
• state / attributes has a set of properties (or
  fields)
• each field has a value, that may be different per
  object of the same class
• behavior what the object knows how to do
• objects of the same class can carry out the same
  behavior (methods)

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Examples of Objects

• state/attributes
• of gallons of gas in tank
• total of miles run so far
• efficiency (mpg)
• behavior
• drive
• load gas
• change efficiency
• check gas
• check odometer reading

• state/attributes
• on (true or false)
• behavior
• switch on
• switch off
• check if on

LightBulb
Car

• state/attributes
• balance
• behavior
• deposit
• withdraw
• check balance

BankAccount
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BankAccount example(A Preview)
BankAccount

• state/attributes
• balance
• behavior
• get balance
• deposit
• withdraw

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Class Definition in Java
BankAccount

• type (or class)
• state/attributes (fields)
• behavior (methods)
• may have input parameters in parenthesis
• may have output (or return) type
• has body with code

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A Class with a Constructor

• Constructor special method that initializes the
  object
• For now, view constructors as an alternative to
  initializing fields as they are declared
• Later more advanced uses for constructors

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A Class and Its Instances

• A single class can have multiple instances
• Each instance is a separate object
• Each instance can have different values for its
  fields
• The definition of methods is the same for all
  instances of the same type
• Thus, there is only one class definition
• Written as the .java file for that class

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BlueJ Demo

• Create and compile a BankAccount class
  (BankAccount.java gt BankAccount.class)
• Create BankAccount objects(instances of the
  class)
• Right-click on the class
• Carry out operations on the BankAccount objects
  (invoke the deposit and getBalance methods)
• Right-click on the instances

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Using BankAccount objects in a separate Java
program

• Instantiate and use a BankAccount object in a
  separate Java application
• How? Within the same Java project, create,
  compile and execute BankAccountTester.java
• with apublic static void main(String args)
  method, containing the following code

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Java code using a BankAccount object

• BankAccount myAccount // declare variable
• myAccount new BankAccount() // create instance
• myAccount.deposit( 1000 ) // call a method
• myAccount.deposit( 500 ) // call it again
• System.out.print( My balance is now )
• System.out.println( myAccount.getBalance() )
• / We expect 1500 to be printed by the last
  statement /

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Summary

• In Java, we write programs for objects
• These programs are called classes
• A class consists of fields and methods to specify
  the state and behavior for its objects
• Once a class has been defined, objects of that
  class can be created (instantiated)
• Methods are invoked on an object, and may cause
  the state of the object to change