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copied!<p><code>Set</code> is an interface, so no, you cannot directly instantiate it. Interfaces would be pretty useless if you couldn't have an instance of one, though! The instance returned by <code>tree.keySet()</code> is some concrete <em>implementation</em> of the <code>Set</code> interface.</p> <p>Let's get super-specific, and look at <a href="http://www.docjar.com/html/api/java/util/TreeMap.java.html#791" rel="nofollow">the <code>TreeMap#keySet()</code> source code</a>:</p> <pre><code>public Set<K> keySet() { return navigableKeySet(); } </code></pre> <p>Okay, that doesn't tell us much. We need to drill down:</p> <pre><code>public NavigableSet<K> navigableKeySet() { KeySet<K> nks = navigableKeySet; return (nks != null) ? nks : (navigableKeySet = new KeySet(this)); } </code></pre> <p>So the concrete type returned is a <code>KeySet</code>! There's your implementation of the <code>Set</code> interface. <a href="http://www.docjar.com/html/api/java/util/TreeMap.java.html#1021" rel="nofollow">http://www.docjar.com/html/api/java/util/TreeMap.java.html#1021</a></p> <p>Which explains this:</p> <pre><code>System.out.println(set instanceof Set); // prints true System.out.println(set instanceof HashSet); // prints false </code></pre> <p><code>Set</code> is an interface; <code>HashSet</code> is an implementation of that interface. <code>foo instanceof Set</code> will be <code>true</code> for every instance <code>foo</code> of <em>any</em> <code>Set</code> implementation. We already established that the concrete type of the object returned by <code>TreeMap#keySet()</code> is a <code>KeySet</code>, not a <code>HashSet</code>, so that explains why <code>set instanceof HashSet</code> is <code>false</code> – because <code>set</code> is a <code>KeySet</code>, so it cannot be a <code>HashSet</code>!</p> <p>If that still doesn't make sense to you, <a href="http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html" rel="nofollow">read up on <code>instanceof</code></a>:</p> <blockquote> <p>The <code>instanceof</code> operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.</p> </blockquote>

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