title: Get a HashSet out of the keys of a HashMap?
body: <p>I have a pretty big (100'000s of entries) <code>HashMap</code>. Now, I need a <code>HashSet</code> containing all the keys from this <code>HashMap</code>. Unfortunately, <code>HashMap</code> only has a <code>keySet()</code> method which returns a <code>Set</code> but not a <code>HashSet</code>.</p>
  
  <p>What would be an efficient way to generate such a <code>HashSet</code> using Java?</p>