The specified character of the sequence currently represented by the string buffer, as indicated by the
index argument, is returned. The first character of a string buffer is at index
0, the next at index
1, and so on, for array indexing.
The index argument must be greater than or equal to 0, and less than the length of this string buffer.
@param index the index of the desired character.
@return the character at the specified index of this string buffer.
@exception IndexOutOfBoundsException if index is negative or greater than or equal to length().
@see java.lang.StringBuffer#length()