public class Line {
    // Invariant: p1 and p2 are different points
    private Point p1, p2;

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

    public Point getP1() { return p1; }
    public Point getP2() { return p2; }

    public double slope () throws ArithmeticException {
	return ((p1.getY() - p2.getY()) / (p1.getX() - p2.getX()));
    }
}