code: public Vector3D getCorner(int index) {
          Vector3D v = new Vector3D();
          if (index <= 0)
              v.set(x1, y1, z1);
          else
              v.set(x2, y2, z2);
          return v;
      }
docs: Gets coordinates of the point.
  
  @param index The index of the point; if the index is 0, the point is first.
  @return The coordinates of the point.