1. Introduction to Object-Oriented Programming
Notes Supplement
CS 1054 Introduction to Programming in Java
Spring 2008

2. Object-Oriented Programming
The traditional definition of a program is: a sequence of instructions to be executed on 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
Introduction to Object-Oriented Programming

3. Introduction to Object-Oriented Programming
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)
Introduction to Object-Oriented Programming

4. Introduction to Object-Oriented Programming
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
Note
each object is an “instance” of that “type” of object
each instance has its own values for its attributes
e.g., different accounts can have different balances
BankAccount
Introduction to Object-Oriented Programming

5. BankAccount example (A Preview)
public class BankAccount {
private double balance = 0;
public double getBalance()
return balance; }
public void deposit( double amount )
balance = balance + amount;
// … more code follows
BankAccount.java
BankAccount
state/attributes
balance
behavior
get balance
deposit
withdraw
Introduction to Object-Oriented Programming

6. Class Definition in Java
BankAccount
public class BankAccount {
private double balance = 0;
public double getBalance()
return balance; }
public void deposit( double amount )
balance = balance + amount;
// … more code follows
BankAccount.java
type (or class)
state/attributes (fields)
behavior (methods)
may have input parameters in parenthesis
may have output (or “return”) type
has “body” with code
Introduction to Object-Oriented Programming

7. A Class with a Constructor
public class BankAccount {
private double balance;
public BankAccount()
balance = 0; }
public double getBalance()
return balance;
public void deposit( double amount )
balance = balance + amount;
// … more code follows
BankAccount.java
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
Introduction to Object-Oriented Programming

8. 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
Introduction to Object-Oriented Programming

9. Introduction to Object-Oriented Programming
BlueJ Demo
Create and compile a BankAccount class (BankAccount.java => 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
Introduction to Object-Oriented Programming

10. 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 a public static void main(String args[]) method, containing the following code…
Introduction to Object-Oriented Programming

11. 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 */
Introduction to Object-Oriented Programming

12. Introduction to Object-Oriented Programming
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
Introduction to Object-Oriented Programming