Java substring code

The question is Given 2 strings, a and b, return the number of the positions where they contain the same length 2 substring. So "xxcaazz" and "xxbaaz" yields 3, since the "xx", "aa", and "az" substrings appear in the same place in both strings. What is wrong with my solution?

int count=0;
for(int i=0;i<a.length();i++)
{
   for(int u=i; u<b.length(); u++)
       {
        String aSub=a.substring(i,i+1);
        String bSub=b.substring(u,u+1);
        if(aSub.equals(bSub))
        count++;
    }
}
return count;
}