code: public double probabilityOf(E key) {
      if (counter.containsKey(key)) {
        return counter.getCount(key);
      } else {
        int remainingKeys = numberOfKeys - counter.size();
        if (remainingKeys <= 0) {
          return 0.0;
        } else {
          return (reservedMass / remainingKeys);
        }
      }
    }
docs: Returns the normalized count of the given object.
  
  @return the normalized count of the object