public class Quadrilateral extends GeometricObject {
    protected Point p1, p2, p3, p4;
    public Quadrilateral (Point p1, Point p2, Point p3, Point p4) {
	// should check these points for a convex quadrillateral in this order
	this.p1 = p1;
	this.p2 = p2;
	this.p3 = p3;
	this.p4 = p4;
    }
    public double area() { // diagonal gives 2 triangles; use semiperimeter formula
	double diag  = p1.distTo(p3);
	double s12   = p1.distTo(p2);
	double s23   = p2.distTo(p3);
	double s34   = p3.distTo(p4);
	double s41   = p4.distTo(p1);
	double semi1 = (s12 + s23 + diag) / 2;
	double semi2 = (diag + s34 + s41) / 2;
	return Math.sqrt(semi1 * (semi1-s12) * (semi1-s23) * (semi1-diag))  +
	    Math.sqrt(semi2 * (semi2-s34) * (semi2-s41) * (semi2-diag));
    }
}
// Local Variables:
// c-basic-offset:  4
// compile-command: "javac Quadrilateral.java; java TestQuad"
// End: