JETT Workshop - Java Basics

Computer Science (CSCI) Department
College of Engineering, Computer Science, & Technology (ECST)

JETT Workshops
Java Basics

Fall 2004

More on Methods
(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

Designing the Public Interface

Behavior of bank account:
- deposit money
- withdraw money
- get balance
Methods of BankAccount class:
- deposit
- withdraw
- getBalance

BankAccount Public Interface

public BankAccount() 
public BankAccount(double initialBalance) 
public void deposit(double amount) 
public void withdraw(double amount) 
public double getBalance()

Using the Public Interface

Transfer balance

double amt = 500;
momsSavings.withdraw(amt);
harrysChecking.deposit(amt);

Add interest

double rate = 5; // 5%
double amt = acct.getBalance()
   * rate / 100;
acct.deposit(amt);

BankAccount Class Implementation
- Determine instance variables to hold object state
  private double balance
- Implement the methods and constructors

Variables
- Instance variables (fields)
  - example: balance in BankAccount
  - default initialization
    - int and double are set to 0
    - object reference is set to null
  - lifetime - object creation to object destruction
- Local variables
  - example: newBalance in deposit method
  - default initialization - none - compilation error if you try to use it
  - lifetime - variable definition to end of block
- Parameter variables
  - example: amount in deposit method
  - initilization - initialized to value of parameters passed in
  - lifetime - entry to method until method is exited
- Any of these can be either a primitive type or an object variable

Explicit and Implicit Parameters

public void withdraw(double amount)
{  
   double newBalance = balance - amount;
   balance = newBalance;
}

balance is the balance belonging to the object to the left of the dot when the method is called:
momsSavings.withdraw(500)

momsSavings is the implicit parameter

it is passed implicitly to the withdraw method
the balance that is affected in the withdraw method is the one belonging to the object called momsSavings

Java has a reserved word that acts as a variable name for the implicit parameter inside the method - this

public void withdraw(double amount)
{  
   double newBalance = this.balance - amount;
   this.balance = newBalance;
}

500 is an explicit parameter
- it is passed explicitly to the withdraw method
- it is copied into the parameter variable called amount
there can be several explicit parameters
there is only one implicit parameter

Exercise - add more methods to the BankAccount project

edit the BankAccount class
- add a new instance variable for an interest rate
- add a new constructor that will accept the balance and the interest rate
- add a method to set a new interest rate
- add a method to calculate interest on the current balance and add it to the balance
edit the BankAccountTest class
- create a variety of BankAccount objects
- call methods on them
- print out results or use the debugger