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
}