Having trouble understanding an error code i keep getting. Please help!

[dannygirl sylvest]

I'm doing an assignment where i have to Design a SavingsAccount class that stores a savings account's annual interest rate and balance. the class constructor should accept the amount of the savings account's starting balance. The class should also have methods for subtracting the amount of withdrawal, add the amount of deposit, and adding the amount of monthly interest to the balance. The monthly interest rate is the annual interest rate divided by 12.

class SavingsAccount
{
private double balance; // Account balance
private double annualinterestrate;

// add code to the constructors
public SavingsAccount()
{
balance = 0.0;

}


public SavingsAccount(double startBalance, double AnnualInterestRate)
{
balance = startBalance;
annualinterestrate = AnnualInterestRate;
}
private double MonthlyInterestRate( double annualrate)
{
return annualrate /12;
}

public void addInterest()
{
double interest = (annualinterestrate * balance);
}

The deposit method makes a deposit into
the account.
@param str The amount to add to the
balance field, as a String.
*/

public void deposit(String str)
{
balance += Double.parseDouble(str);
}

/**
The withdraw method withdraws an amount
from the account.
@param amount The amount to subtract from
the balance field.
*/

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

/**
The withdraw method withdraws an amount
from the account.
@param str The amount to subtract from
the balance field, as a String.
*/

public void withdraw(String str)
{
balance -= Double.parseDouble(str);
}

/**
The setBalance method sets the account balance.
@param b The value to store in the balance field.
*/

public void setBalance(double b)
{
balance = b;
}

/**
The setBalance method sets the account balance.
@param str The value, as a String, to store in
the balance field.
*/

public void setBalance(String str)
{
balance = Double.parseDouble(str);
}

/**
The getBalance method returns the
account balance.
@return The value in the balance field.
*/

public double getBalance()
{
return balance;
}
}

45 minutes ago - 1 week left to answer.
Additional Details
I forgot to mention what my project consisted of: Design a SavingsAccount class that stores a savings account's annual interest rate and balance. the class constructor should accept the amount of the savings account's starting balance. The class should also have methods for subtracting the amount of withdrawal, add the amount of deposit, and adding the amount of monthly interest to the balance. The monthly interest rate is the annual interest rate divided by 12. To add the monthly interest to the balance, multiply the monthly interest rate by the balance, and add the result to the balance. ( this is compound interest ) Use DecimalFormat class to format output.

package assignment06;

import java.util.Scanner; // Needed for Scanner class.
import java.text.DecimalFormat; // Needed for DecimalFormat class.

/**
*
* @author Dsylvester
*/
public class testAccount {
public static void main(String[] args)
{
String input; // To hold user input
double balance, interestRate;
double interest, deposit, withdrawl;
double totalInterest = 0, totalDeposit = 0, totalWithdrawls = 0;

int month=0;
char repeat = 'Y';

// Create Decimal Format.
DecimalFormat dollar = new DecimalFormat ("#,###.00");

// Create a Scanner object.
Scanner keyboard = new Scanner(System.in);

//Prompt for starting Balance.
System.out.print(" What is your starting balance? ");
balance = keyboard.nextDouble();

// Prompt for Annual Interest Rate.
System.out.print("What is your annual Interest Rate? ");
interestRate = keyboard.nextDouble();

I'm new at this and i am still trying to grasp the concept of calling methods and the use of constructors. When i run my code this is what happens
Your total deposit is 0.0
Your total withdrawls were 0.0
Your total interest is 0.008333333333333333
Your total balance is $ 1,000.01
Calculate the month 2 (Y/N)
Exception in thread "main" java.lang.StringIndexOutOfBoundsExceptio… String index out of range: 0
at java.lang.String.charAt(String.java:686)
at assignment06.testAccount.main(testAccoun…
Java Result: 1
BUILD SUCCESSFUL (total time: 11 seconds)

i'm still not sure exactly what is going wrong . Thanks for your input

[fbr - added code tags]

[Paul Beckett]

Welcome to the Java Ranch!
Please can you use UseCodeTags when posting code samples.

[Paul Beckett]

first lets look at your exception stack trace.

...StringIndexOutOfBoundsException. This indicates that your String does not contain a character at the index you've asked for. For the String "abcd" which has a length of 4 characters but the maximum index you can ask for is 3 (Strings are zero indexed). Your stack trace says "String index out of range: 0". This indicates you've asked for index 0 of the String and it wasn't found so its an empty String (if it was null you would get a NullPointerException instead).

...String.charAt. This first line tells you exactly where the exception was thrown (line 686 of the String class in the chatAt method). I can't see anywhere in your code where the charAt method is called. Is your code listing complete?

...testAccount.main. This is a very useful line in your stack trace and tells you from where the String.charAt method was called. The stack trace will include the line number to help you but you haven't included the full stack trace in your post.

[fred rosenberger]

I added code tags to make your post slightly more readable.