code: public int convertBoundaryToAbsolutePosition(int relativePosition) {
          if (relativePosition < 0 || relativePosition > length())
              throw new IllegalArgumentException("Relative position outside this range (" + relativePosition + ").");
  
          if (reversed)
              return upper - relativePosition;
          else
              return relativePosition + lower;
      }
docs: Converts relative boundary position to absolute position
  
  @param relativePosition relative boundary position
  @return absolute point position