1. Introduction to Object-Oriented Programming
CS 21a: Introduction to Computing I
Department of Information Systems and Computer Science
Ateneo de Manila University

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 specify what are in these objects and how these objects behave
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

3. So … What is an Object?
A “thing” that has type, identity, state, and behavior
type: belongs to a “class” of similar objects
identity: is a distinct “instance” of a class of objects
state / attributes: has a set of properties (aka fields)
each field can have different values
behavior: has “methods” (things that the object knows how to do)
we say we “call” a method on the object
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

4. Examples of Objects LightBulb Car BankAccount state/attributes
# of liters of gas in tank
total # of km run so far
efficiency (km/liter)
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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

5. BankAccount example (A Preview)
state/attributes
balance
behavior
get balance
deposit
withdraw
public class BankAccount {
private double balance = 0;
public double getBalance()
return balance; }
public void deposit( double amount )
balance = balance + amount; …
BankAccount.java
BankAccount
BankAccount
double balance
double getBalance()
void deposit( double amount )
… and more
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

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; …
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
“double” means a floating
Point number like
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

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; …
BankAccount.java
Constructor: special method that handles initialization
For now, view constructors as an alternative to initializing fields as they are declared
Later: more advanced uses for constructors
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

9. Lab Exercise: Try it in BlueJ
Create and compile a BankAccount class (BankAccount.java)
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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

10. Lab Exercise, continued
Instantiate and use a BankAccount object in a Java application (use the command prompt)
How? In the same folder containing the BankAccount class, create BankSystem.java
Inside the public static void main(String args[]) method of BankSystem.java, type the following code…
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

11. Lab Exercise, continued
public static void main( String args[] ) {
BankAccount b = new BankAccount();
b.deposit( );
b.withdraw( );
System.out.println( b.getBalance() );
b.deposit( ); }
Compile and execute BankSystem.java
The values and should be printed out
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

12. 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
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved