Introduction to Object-Oriented Programming CS 21a: Introduction to Computing I First Semester,

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 are what are in these objects and how these objects behave

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 (a.k.a 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

Examples of Objects ► state/attributes ► on (true or false) ► behavior ► switch on ► switch off ► check if on LightBulb BankAccount ► state/attributes ► balance ► behavior ► deposit ► withdraw ► check balance Car ► 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 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 Example (A Preview) public class BankAccount { private double balance = 0; public double getBalance() { return balance; } public void deposit( double amount ) { balance = balance + amount; } … } BankAccount ► state/attributes ► balance ► behavior ► deposit ► withdraw ► check balance BankAccount double balance double getBalance() void deposit( double amount ) … and more BankAccount.java

Class Definition in 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 public class BankAccount { private double balance = 0; public double getBalance() { return balance; } public void deposit( double amount ) { balance = balance + amount; } … } BankAccount.java BankAccount "double" means a floating point number like

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

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

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

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…

Lab Exercise, continued public static void main( String args[] ) { BankAccount b = new BankAccount(); b.deposit( ); b.withdraw( ); System.out.println( b.getBalance() ); b.deposit( ); System.out.println( b.getBalance() ); } ► Compile and execute BankSystem.java ► The values and should be printed out

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