Java Exception help

[Daren_Z]

Hi there, I am trying to write a program for a class and I ran into this bug:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -8
at java.lang.String.substring(String.java:1937)
at java.lang.String.substring(String.java:1904)
at CreditCard.main(CreditCard.java:15)
Java Result: 1


My code here:

import java.util.*;
public class CreditCard {

public static void main(String[] args) {
Scanner console = new Scanner (System.in);

System.out.print("Please enter an 8-digit credit card number: ");
String cardNum = console.nextLine(); //43589795 or 73652180
int sum = Integer.parseInt(cardNum.substring(1 + 3 + 5 + 7));

System.out.print(sum); //a simple check to see if code is working


}
}

Any help would be greatly appreciated!

Thanks,

Daren

[pbrockway2]

So, which is line 15 of CreditCard.java?

According to the stack trace that's the line which caused the ArrayIndexOutOfBoundsException. Such exception occurs whenever you do an operation on a string using character positions which are either negative or past the end of the string.

[Daren_Z]

Line 15 is: int sum = Integer.parseInt(cardNum.substring(1 + 3 + 5 + 7));

the odd thing is none of these are negetive or past the boundaries of the string. That is, unless I screwed up somewhere. The string is exactly 8 characters

[Daren_Z]

I figured it out. The compiler was adding all of the values together in order to find one value, which was in turn 8 over the amount of integers available to choose from. Is there a way to get all 4 digits in one line of code? or do I need 4 separate lines?

[pbrockway2]

No, you'll need to extract the digits of interest one at a time and add their corresponding int values together.

As always the API docs for substring() will help. Notice how it comes in two flavours.