// if you alread have a Comparator<Thing> called SortByDate, a nested static class of Thing.
Comparator<Thing> sortByDate = new Thing.SortByDate();

Comparator<Thing> sortByDescendingDate = Collections.reverseOrder( sortByDate );
// You can now use sortByDate wherever you need a Comparator for reverse date order.

// if you already have aComparable, e.g. Thing implements Comparable and has a compareTo method.
Comparator<thing> sortInDescendingNaturalOrder = Collections<Thing>.reverseOrder();
// You can now use sortInDescendingNaturalOrder wherever you need a Comparator for reverse natural order.