This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates 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.