This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.