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copied!<ul> <li><p><a href="http://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html#higher%28E%29" rel="nofollow"><code>set.higher(T)</code></a>: function returns the least element in this set strictly greater than the given element, or <code>null</code> if there is no such element.</p></li> <li><p><a href="http://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html#descendingSet%28%29" rel="nofollow"><code>set.descendingSet()</code></a>: returns a reverse order <code>view</code> of the elements contained in this <code>set</code>. </p></li> </ul> <p><strong>What really happens ?</strong></p> <p><code>TreeSet</code> inherently uses <code>TreeMap</code> to implements its functionality. The call to <code>descendingSet()</code> eventually call the <code>descendingMap()</code> function on the <code>TreeMap</code> instance, as is evident from the following source code: </p> <pre><code>public NavigableSet<E> descendingSet() { return new TreeSet<>(m.descendingMap()); } </code></pre> <p>Every <code>TreeMap</code> generally maintains two view: </p> <ul> <li><em>normal sorted view</em>: uses general comparator to order its element </li> <li><em>descendant Map view</em>: uses comparator which imposes reverse ordering of the ascend-ordering comparator. It uses <code>Collections.reverseOrder(m.comparator())</code> to return this descend-ordering comparator. </li> </ul> <p>I am calling these <code>view</code> because <code>TreeMap</code> doesn't actually create another descendent Map with it's <em>entries(key, value)</em>, rather it maintains two comparator, imposing opposite ordering to each other. The descendant view gets created first time when <code>descendantMap()</code> is called. Any subsequent call to this function will return the same descendent Map view.</p> <p><strong>Note:</strong> <code>set.descendingSet().descendingSet()</code> returns a view of <code>set</code> essentially equivalent to <code>set</code>. Because the resulted comparator from first call got reversed <em>again</em> by the second call of <code>descendingSet()</code>(which is actually executing <code>map.descendingMap()</code> inside).</p> <p><strong>Continuing with your Example:</strong> </p> <pre><code>System.out.printf("%n Higher than 40 : %s", set.higher(40)); // prints 45 set = set.descendingSet(); // create a reverse ordering //comparator as described above System.out.printf("%n Higher than 40 : %s", set.higher(40)); // prints 34 set = set.descendingSet(); // again trying to get descending set! System.out.printf("%n Higher than 40 : %s", set.higher(40)) // prints 45 </code></pre>

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