I asked a question about arrays before, but this one won't compile
- by unit
I asked about this array a little while ago, and I can't see what the problem is. Too tired.  What have I done wrong?  Basically, I am taking a string array and trying to check to see if it contains numbers or an x (ISBN number validation).  I want to take the number from a given input (bookNum), check the input, and feed any valid input into a new array (book).  At the line 
'bookNum.charAt[j]==book[i]'
I get the 'not  a statement error'.  What gives?
  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)))
      bookNum.CharAt[j]==book[i];
      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");