TreeMap.navigableKeySet().descendingIterator() sequence ?

[kevin bourrillion]

Wow.  TreeMap.java line 1024:

    return new DescendingKeyIterator(getFirstEntry());

Sure does look fishy.



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



-- 
Kevin Bourrillion @ Google
internal: go/javalibraries
google-collections.googlecode.com
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