public class BadLineThree {
    // Invariant: p1 and p2 are different points
    private final Point[] points = new Point[2];

    public BadLineThree(Point p1, Point p2) throws IllegalArgumentException {
         if (!p1.equals(p2)) {
	     points[0] = p1;
	     points[1] = p2;
	 }
	 else {
	     throw new IllegalArgumentException (
						 "Points to Line Constructor must be different:  " +
						 p1.toString() + "given twice.");
	 }
    }

    public Point[] getPoints() { return points; }

    public double slope () throws ArithmeticException {
	return ((points[0].getY() - points[1].getY()) / (points[0].getX() - points[1].getX()));
    }
}