Been looking at TreeMap's implementation rather closely and observed something I don't understand. %-) TreeMap (as of JDK 6 and later) implements NavigableMap. NavigableMap requires an implementation of a navigableKeySet() method which returns a NavigableSet<K>. So, in TreeMap I see: public NavigableSet<K> navigableKeySet() Then, the NavigableSet interface has an Iterator<E> descendingIterator() and Iterator<E>iterator(). The former returns an iterator over the elements in descending order. The latter in ascending order. I also noticed in TreeMap, NavigableMap requires an implementation of a descendingKeySet(), i.e. public NavigableSet<K> descendingKeySet(). Again, the NavigableSet interface has an Iterator<E> descendingIterator() and iterator() as described above. When I populate a TreeMap with some dummy data, let's say I populate it with 15 Integer keys and String's representing those Integer keys as values, I observe something I don't understand when using the navigableKeySet().descendingIterator(). When, I try to iterate over the returned NavigableKeySet, I see the lowest key printed and that's it. Nothing else prints. From looking at the JavaDoc for TreeMap and NavigableSet, this doesn't seem right. Shouldn't it iterate over the entire set in descending order? I looked at the source and it creates an Iterator that starts at the lowest key and then tries to find the next lowest key (which obviously doesn't exist). Am I not reading something right in the JavaDoc? When iterating using navigableKeySet().iterator() I see all 15 keys printed in ascending order, (as expected). When I iterate with descendingKeySet().descendingIterator() I see the 15 keys printed in ascending order, (as expected since descending set followed by a descending iterator should give me ascending order). When iterating using descendingKeySet().iterator, I see the 15 keys printed in descending order, (as expected). I've attached a very simple program that illustrates what I'm describing. Could someone clarify this behavior? thanks, charlie ...