how to create sharing rule on activity object in salesforce

hello friends,
   I m using following code to ganarate 6 digit random number.but it creates 4-6 digit random number(eg. like 4567,98765,341235,...).But i need 6 digit random numbers only.how to solve this.
Integer index=0;  
    public Integer getindex()
   {
      double y = Math.random() ;//something between 0 and 999999  
      y = 999999 * y;
      index = math.round(y); 
    return index;
    }