Wednesday, January 17, 2007

Lecture 2

Class Definition and Syntax

  • from last time we had the class BankAccount
  • with methods
    • withdraw, deposit, getbalance
  • the bank account has a hidden balance that is only accessable via the methods.

Method Definition Syntax

public void deposit (double amount)
public void withdraw(double amount)
public double getbalance()

  • these are seperated by : access specifiers (public), return type (void,double), method names (deposit, withdraw, getbalance) and method parameters [double amount, ()]
  • every class representing an object requires a special method called a constructor
  • the role of the constructor is to properly initialize any hidden data fields (eg. sets balance to zero as default)
  • this is always executed when new instances of the class are created
  • the name of the constructor method is the same as the name of the class
  • constructors have no return type, not even void
  • you cannot have a method w/ the same name of the class unless it is a constuctor
  • Object classes contain fields of private data. Not accessible to the user or Application Programmer(AP)
  • example: (Balance, account number, account holder, etc.)

Class Declaration Syntax

public class BankAccount {

//constructor declataion

// methods

//private data


  • we can fill in the method declarations (including the constructor methods) as follows:

Class Declaration Syntax

public class BankAccount {

public BankAccount() {

//constructor statements

public void deposit(double parameter){

// deposit statement


public void withdraw(double amount){
//withdraw statement
public double getbalance()
//getbalance statements

//private declatarations


  • what was written is the public interface of the class.
  • these methods are the only way for the AP to interact w/ the class
  • this forms the basis of the API
  • filling in the details
  • consider the private data
    • private data we called Instance Fields
    • fields of data for every instance of the object
  • access specifier, type and name

private double balance;
private int accountnumber;

  • private:
    • not accessable outside the object
    • not part of the public interface
    • accessible to methods in the class

Public class BankAccount {
private double balance; //instance field
public BankAccount() {balance = 0;} //constructor
public void deposit (double amount) {

balance = balance + amount; // method

public void withdraw (double amount) {
balance = balance - amount; //method

No comments: