title: TreeSet constructor with Comparator<?> parameter
body: <p>In <a href="https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html#TreeSet(java.util.Comparator)" rel="noreferrer">Java’s documentation for its class TreeSet</a> one of the constructors is shown to have the following header:</p>
  <pre><code>TreeSet(Comparator&lt;? super E&gt; c)
  </code></pre>
  <p>Can someone help explain why there is a constructor for TreeSet which takes a comparator object as its argument? I have no clue why this is done.</p>