Interfaces - The Java Programming Language - page 174

The compareTo method takes a single object argument of type T and com-
pares it to the current object (expected to also be of type T) , returning
a negative, zero, or positive integer if the current object is less than,
equal to, or greater than the argument, respectively.
Consider a variation of the Point class we introduced in Chapter 1 . The
natural ordering for points could be their distance from the origin. We
could then make Point objects Comparable :
class Point implements Comparable<Point> {
/** Origin reference that never changes */
private static final Point ORIGIN = new Point();
private int x, y;
// ... definition of constructors, setters and accessors
public double distance(Point p) {
int xdiff = x - p.x;
int ydiff = y - p.y;
return Math.sqrt(xdiff * xdiff + ydiff * ydiff);
}
public int compareTo(Point p) {
double pDist = p.distance(ORIGIN);
double dist = this.distance(ORIGIN);
if (dist > pDist)
return 1;
else if (dist == pDist)
return 0;
else
return -1;
}
}