On Thu, Apr 17, 2008 at 2:54 PM, charlie hunt <
charlie.hunt@sun.com> wrote:
> 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 ...
>
>