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

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

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

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

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

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 like1234.25 Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

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

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

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

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

Lab Exercise, continued public static void main( String args[] ) { BankAccount b = new BankAccount(); b.deposit( 1000.00 ); b.withdraw( 100.00 ); System.out.println( b.getBalance() ); b.deposit( 2000.00 ); } Compile and execute BankSystem.java The values 900.0 and 2900.0 should be printed out Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

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