I have asked a couple of questions about this for loop:

  String[] book = new String [ISBN_NUM];
  bookNum.replaceAll("-","");
  if (bookNum.length()!=ISBN_NUM)
    throw new ISBNException ("ISBN "+ bookNum + " must be 10 characters");
  for (int i=0;i<bookNum.length();i++)
  {
      if (Character.isDigit(bookNum.charAt(i)))
      book[j]=bookNum.charAt(i);  //this is the problem right here
      j++;
      if (book[9].isNotDigit()|| 
          book[9]!="x"        ||
          book[9]!="X")
      throw new ISBNException ("ISBN " + bookNum + " must contain all digits" + 
                               "or 'X' in the last position");
  }

which will not compile. An answer I had from the other question I asked told me that the line where the error occurs is wrong in that bookNum.charAt(i) is an (immutable) string, and I can't get the values into a book array that way. What I need to do on my assignment is check an ISBN number (bookNum) to see that it is all numbers, except the last digit can be an 'x' (valid ISBN). Is this the best way to do it? If so, what the hell am I doing wrong? If not, what method would be a better one to use?