How do I traverse a map backwards?

Just keep getting the last key and the head map before it:

if (!map.isEmpty()) {
  Object last = map.lastKey();
  boolean first = true;
  do {
    if (!first) {
      System.out.print(", ");
    }
    System.out.print(last);
    last=map.headMap(last).lastKey();
    first=false;
  } while (last != map.firstKey());
  System.out.println();
}