The comparison contract states that sgn(compare(x, y)) == -sgn(compare(y, x)).
(An immediate corollary is that compare(x, x) == 0.) This comparison
implementation either a) cannot return 0, b) cannot return a negative value but
may return a positive value, or c) cannot return a positive value but may return
a negative value.
The results of violating this contract can include TreeSet.contains never
returning true or Collections.sort failing with an IllegalArgumentException
arbitrarily.
In the long term, essentially all Comparators should be rewritten to use the Java 8 Comparator factory methods, but our automated migration tools will, of course, only work for correctly implemented Comparators.