1. Using Objects
Object: an entity in your program that you can manipulate
Attributes
Methods
Method: consists of a sequence of instructions that can access the data of an object.
System.out has a println method
You do not know how it works
What is important is that it does the work you request of it

2. Classes A class describes a set of objects with the same behavior.
Some string objects:
"Hello World"
"Goodbye”
You can invoke the same methods on all strings.

3. Class vs Object
A class is a blue print of a particular classification of objects
An object belongs to a class of objects
An object is an instance of a class
Memory space
Class: not allocated
Object: allocated
Declaration
Class: once
Object: many times

4. Building Classes All Java code must be inside classes Class types
Utility classes
Used to store constants, utility methods, etc.
Driver classes
Abstract Data Types (ADTs)
“blue-prints” to build objects

5. Defining ADT classes
class attributes are normally encapsulated (i.e. private)
class methods are normally public
avoid using input/output inside the methods of a class
static attributes and methods
class constants

6. Public class BankAccount {   private double balance; private int accountNumber; private static int lastAssignedNumber; //automatically initialized with zero public static final double OVERDRAFT_FEE=5; // constant   /*overloaded constructors */ public BankAccount() { balance=0; lastAssignedNumber++; accountNumber= lastAssignedNumber; }   public BankAccount(double initialBalance) { balance= initialBalance; lastAssignedNumber++; accountNumber= lastAssignedNumber; }   public static int showLastAccountNumber(){ return lastAssignedNumber; }   public void deposit(double amount) { double newBalance=balance+amount;//newBalance and amount are local variables balance=newBalance; } public void withdraw(double amount) {// a mutator method double newBalance=balance-amount; balance=newBalance; }
Case Study

7. public double getBalance() { // an accessor method return balance; } public void transfer (double amount, BankAccount other) { // a side effect balance=balance-amount; other.balance=other.balance+amount; } } /* A class to test the BankAccount class */ public class BankAccountTest { public static void main (String[] args) { BankAccount harrysChecking=new BankAccount(); int a=100; BankAccount tomsChecking=new BankAccount(a); a=2000; harrysChecking.deposit(a); harrysChecking.withdraw(500); harrysChecking.transfer(500, tomsChecking); System.out.println(harrysChecking.getBalance()); System.out.println(tomsChecking.getBalance()); System.out.println(BankAccount. showLastAccountNumber()); System.out.println(BankAccount. OVERDRAFT_FEE); } }  

8. Scope of variables Instance attributes/variables
One for each object – dynamically allocated
Local variables
Parameter variables
Static variables and constants
Only one copy – belong to the class

9. Instance and static methods
Instance method
Belong to the object
Called from the name of the object
harrysChecking.getBalance()
Can call instance/static methods/variables
Static method
Belong to the class
Called from the name of the class
BankAccount. showLastAccountNumber()
Can only call other static methods/variables

10. Parameter Passing Pass by value Pass by reference
Variables of primitive data types
a=2000;
harrysChecking.deposit(a);
Pass by reference
Objects/arrays
Checking.transfer(500, tomsChecking);

11. Other things to consider…
Constructors
Default constructor
Used only to initialize the encapsulated variables to the default values (0, false, null)
Accessors and mutators
“Getters” and “setters”
Static methods
Utility methods or
Used to handle static variables

12. Case Study Cash Register
public class CashRegister{ public CashRegister() { purchase = 0; payment = 0; }   public void recordPurchase(double amount) { double total = purchase + amount; purchase = total; }   public void enterPayment(double amount) { payment = amount; }   public double giveChange() { double change = payment - purchase; purchase = 0; payment = 0; return change; } private double purchase; private double payment; }  
Case Study
Cash Register

13. public class CashRegisterTester{ public static void main(String[] args) { CashRegister register = new CashRegister(); register.recordPurchase(29.50); register.recordPurchase(9.25); register.enterPayment(50); double change = register.giveChange(); System.out.println(change); } }