I'm trying to get this ISBN checker to work. I've used the formula from this site and also this site and it seems to work fine **except** for the ISBN of my Java book. When I compute my book's ISBN (0131496980), it is apparently invalid. I have tried the other codes on both of those sites as well as other books I have at home and there is no problem. Here is my code:

```
public class DigitExtractor
{
public DigitExtractor(String code)
{
isbn = code;
}
public int nextDigit()
{
// convert to int
digit = isbn.charAt(count) - 48;
count += 1;
return digit;
}
public int getLength()
{
return isbn.length();
}
public String getISBN()
{
return isbn;
}
private String isbn;
private int digit;
private int count;
}
```

```
public class ISBNChecker
{
public static void main(String[] args)
{
DigitExtractor myDigit = new DigitExtractor("123456789X");
int sum = 0; // holds the sum
int rem = 0; // holds the remainder
int checkSum = 0; // holds the checkSum value
if (myDigit.getLength() == 10)
{
// extract the next digit and add it to the sum
for (int cnt = 0; cnt < myDigit.getLength() - 1; cnt++)
{
int temp = myDigit.nextDigit() * (myDigit.getLength() - cnt);
sum += temp;
System.out.println(temp);
}
System.out.println("sum: " + sum);
// get the remainder of sum / 11
rem = sum % 11;
System.out.println("remainder: " + rem);
// subtract remainder from 11 to get checkSum
checkSum = 11 - rem;
System.out.println("checksum: " + checkSum);
}
else if (myDigit.getLength() == 13)
{
// extract the next digit and add it to the sum
for (int cnt = 0; cnt < myDigit.getLength() - 1; cnt++)
{
int temp = myDigit.nextDigit();
if ((cnt + 1) % 2 == 0)
{
sum += temp * 3;
System.out.println(temp * 3);
}
else
{
sum += temp;
System.out.println(temp);
}
}
System.out.println("sum: " + sum);
// get the remainder of sum/10
rem = sum % 10;
System.out.println("rem: " + rem);
// subtract remainder from 10 to get checkSum
checkSum = 10 - rem;
System.out.println("checksum: " + checkSum);
}
else
System.out.println("Invalid ISBN");
// check the sum to see if it is valid
if (checkSum < 10 && (myDigit.getISBN().charAt(myDigit.getLength() - 1) == (checkSum + 48)))
System.out.println("Valid ISBN - num");
else if (checkSum >= 10 && (myDigit.getISBN().charAt(myDigit.getLength() - 1) == 'X'))
System.out.println("Valid ISBN - X");
else
System.out.println("Invalid ISBN");
}
}
```