public static int c2i(String s) {
return s.length() == 1 ? s.charAt(0)-'A' : c2i(s.substring(0, s.length()-1))*26 + (s.charAt(s.length()-1)-'A');
}
public static String i2c(int i) {
return i < 26 ? ("" + (char)('A' + i)) : i2c(i/26) + i2c(i%26);
}

public static String padTo2Digit(String s) {
return ("A" + s).substring(s.length()-1);
}

String input = "AA";
System.out.println(padTo2Digit(i2c(c2i(input)+1)));