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copied!<p>What about it isn't working? Either way, I believe what you're missing is that you're continually getting the same substring, which will be the first number of the string: <code>int nextDigit = ninedigitNum.substring(0,1);</code>. In addition, you're going to want to use an <code>int</code>, not a <code>String</code>; you can technically convert from String to int if desired, but the problem itself calls for an int.</p> <p>There are two ways to do this that jump to mind. I would do this by realizing that mod in powers of 10 will give you the respective digit of an integer, but the easier way is to convert to a char array and then access directly. Note that there's no error checking here; you'll have to add that yourself. In addition, there are a LOT of 'magic numbers' here: good code typically has very, very few. I would recommend learning more data structures before attempting problems like these; to be honest there's very few things you can do without at least arrays and linked lists.</p> <pre><code>char[] ISBN = ninedigitNum.toCharArray(); //Process each number int total = 0; for(int i=0; i<9; i++){ int current_int = Integer.parseInt(ISBN[i]); total += current_int * (10 - i) } //Find value of d1 for(int i=0; i<9; i++){ if(((total + i) % 11) == 0){ total += i*100000000; break; } } return total; </code></pre> <p>In general: Use print outs with <code>System.out.println(x);</code> or use your compiler's debugger to see what's going on during processing.</p>

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