Return number of index of number

Question

Input:
Int number=123;
Int index =0;
Output:
1
How can I implement a method that get number and index and return the number of the index in complexity O(1)??

Answer 1

$("li").click(function(){
var str = $(this).index();
alert(str); });
by this you can get the number by entering the index and complexity is also O(n)

Answer 2

this is my code:
public static int getNumberByIndex(int num, int index){
num = (int) (num / Math.pow(10, index));
num = num %10;
return num;

}

my question is how i can change the index = 0 to be index = len(number) in O(1)?

thanks