Objects can be Hit

procedure Wall.hitBy (aBall : Ball);
begin
		(* bounce the ball off the wall *)
	aBall.setDirection(convertFactor - aBall.direction);
end;

procedure Hole.hitBy (aBall : Ball);
begin
		(* drain energy from ball, remove it *)
	aBall.energy := 0.0;
	aBall.erase;
		(* move ball *)
	if aBall = CueBall then
		aBall.setCenter(50, 100)
	else begin
		saveRack := saveRack + 1;
		aBall.setCenter (10 + saveRack * 15, 250);
	end;
	(* redraw ball *) aBall.draw;
end;

procedure Ball.hitBy (aBall : Ball);
var
	da : real;
begin
		(* cut the energy of the hitting ball in half *)
	aBall.energy := aBall.energy / 2;

		(* and add it to our own *)
	energy := energy + aBall.energy;

		(* set our new direction *)
	setDirection(hitAngle(self.x - aBall.x, self.y - aBall.y));

		(* and set the hitting balls direction *)
	da := aBall.direction - direction;
	aBall.setDirection (aBall.direction + da);
end;

Intro OOP, Chapter 6, Slide 7