I asked a question about arrays before, but this one won't compile

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). A开发者_如何学运维t 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");

== is java is used for equivalence comparison. If you want to assign it, use a single =.

The first issue here is that charAt is a function, and thus needs parenthesis even though you are accessing with an index like an array.

The other issue is that the line is a boolean expression, which just by itself does not mean anything. A lot of people are suggestion that you mean to make an assignment to that character, but just changing to a single equals causes other problems. The left side of an equals sign needs to be a variable, and the result of a function is not a variable.

Strings are immutable, so you can not simply change one of the characters in the string. Earlier in your code, you have a call to replaceAll(), that returns a new string with the alterations. As written, this altered string is being lost.

There are few odd problems here. For starters, did you mean for book to be an array of Strings, as opposed to just one string? You're trying (assuming CharAt was written properly and the assignment was proper) to assign a character to a string.

Second, instead of copying character by character, why not check the whole string, and copy the whole thing at the end if it is a proper ISBN? Depending on what you do with Exceptions (if you continue regardless), you could add a boolean as a flag that gets set if there is an error. At the end, if there is no error, then make book = to booknumber.replace(etc...)

  bookNum.CharAt[j]==book[i];

Should be

  bookNum.CharAt[j]=book[i];

You are using an equality boolean operator, not an assignment one.

Looks like you're using .charAt(i) wrong! Assuming that "bookNum" is a String, you should use:

bookNum.charAt(i)==book[i];

Instead. Note that this is a boolean expression, and not "=".

The line bookNum.CharAt[j]==book[i]; isn't a statement. It's a comparison. Perhaps you want bookNum.CharAt[j]=book[i]; (single = instead of ==).

Edit: That's not going to fix things, though, since you can't assign to bookNum.CharAt[j].