public Direction hitWall() {
		if (pos_x + v_x < 0)
			return Direction.LEFT;
		else if (pos_x + v_x > max_x)
			return Direction.RIGHT;
		if (pos_y + v_y < 0)
			return Direction.UP;
		else if (pos_y + v_y > max_y)
			return Direction.DOWN;
		else return null;
	}