This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
As the error indicates, String#charAt returns a char, which you are trying to put in an array, designed for Strings
You need to convert the char to a String. Check out the String API for a convenient method.