Additional Graphs

Figure 4 shows the inheritance graph which I had in mind when I wrote the excercise. It makes no use of multiple inheritance and just covers the fact, that I want each object to have a reference point.

 
Figure 4:  Fourth version of inheritance graph.

When I discussed this with one of my colleagues (who is mathematically oriented) he stated that he would divide the set of classes/objects into 2D and 3D-objects. This is because both types require a different ``space''. This would lead to an inheritance graph as shown in Figure 5.

 
Figure 5:  Fifth version of inheritance graph.

In the other hand, if we talk about object-orientation and want to reuse as much as possible, we could argue for all versions which let 3D-Point inherit from Point. For example, assume that Point defines a method to move around:

  class Point {
  attributes
    int x, y

  methods
    move(int dx, dy) {
      x <- x + dx
      y <- y + dy
    }
    ...
  }

If we let 3D-Point inherit from Point we could formulate a move method in 3D as:

  class 3D-Point inherits from Point {
  attributes
    int z

  methods
    move(int dx, dy, dz) {
      Point::move(dx, dy)
      z <- z + dz
    }
    ...
  }

Here, 3D-Point::move() reuses the move method of Point. Graphically interpreted, we first move a 3D object in its xy-plane and then move it in z-direction.