validate isbn number java

1    public boolean validateIsbn10( String isbn )
2    {
3        if ( isbn == null )
4        {
5            return false;
6        }
7
8        //remove any hyphens
9        isbn = isbn.replaceAll( "-", "" );
10
11        //must be a 10 digit ISBN
12        if ( isbn.length() != 10 )
13        {
14            return false;
15        }
16
17        try
18        {
19            int tot = 0;
20            for ( int i = 0; i < 9; i++ )
21            {
22                int digit = Integer.parseInt( isbn.substring( i, i + 1 ) );
23                tot += ((10 - i) * digit);
24            }
25
26            String checksum = Integer.toString( (11 - (tot % 11)) % 11 );
27            if ( "10".equals( checksum ) )
28            {
29                checksum = "X";
30            }
31
32            return checksum.equals( isbn.substring( 9 ) );
33        }
34        catch ( NumberFormatException nfe )
35        {
36            //to catch invalid ISBNs that have non-numeric characters in them
37            return false;
38        }
39    }
40