Java – sum of substrings of numbers

What is the best solution to find the sum of digital substrings?

For example, sum (123) = 1 2 3 12 23 123 = 164

I think this is O (n ^ 2) because

sum = 0
for i in number: // O(n)
    sum += startwith(i) // O(n)
return sum

What's the best plan? What is the best way?

This is my solution, but o (n ^ 2):

public static int sumOfSubstring(int i) {
  int sum = 0;

  String s = Integer.toString(i);

  for (int j = 0,bound = s.length(); j < bound; j++) {
   for (int k = j; k < bound; k++) {
    String subString = s.subSequence(j,k + 1).toString();
    sum += Integer.valueOf(subString);
   }
  }

  return sum;
 }

Solution

Observation:

>For the number XY, you have 11x 2Y. > For the number XYZ, you have 111x 22y 3Z. > For wxyz, you have 1111w 222x 33y 4Z

This is my c# implementation, although porting to Java should be trivial:

static long SumSubtring(String s)
{
    long sum = 0,mult = 1;
    for (int i = s.Length; i > 0; i--,mult = mult * 10 + 1)
        sum += (s[i - 1] - '0') * mult * i;
    return sum;
}

Notice that it is actually o (n)