At first glance use the length() method to get the length of string.
int stringLength = aString.length();
Secondly, Character class is used to convert a chat into upper/lower case. Use Character class method with argument. like
for (int i = 0; i < stringLength; i++)
{
char ch = Character.toLowerCase(aString.charAt(i));
System.out.print (ch);
if (Character.isLetter(aString.charAt(i)))
numLetters++;
}