Inheritance and
is-a relationship
“Is-a'' also implies, that we can use a circle everywhere
where a point is expected. For example, write a function or
method, say move(), which should move a point in x
direction can be written as follows.
move(Point apoint, int deltax) {
    apoint.setX(apoint.getX() + deltax)
  }
As a circle inherits from a point, you can use this function
with a circle argument to move its center point and, hence,
the whole circle: