title: Whats the benefit of allowing multiple null keys in hashmap?
body: <p>Just for experimenting, I added multiple <code>null</code> keys in a <code>Hashmap</code> instance. And it didn't complain. What's the benefit of doing that?</p>
  
  <p>The code is,</p>
  
  <pre><code>Map hmap = new HashMap();
  hmap.put("sushil","sushil11" );
  hmap.put(null,null);
  hmap.put(null,"king");
  hmap.put(null,"nagasaki");
  hmap.put(null,null);
  </code></pre>
  
  <p>How many keys are there in the map?</p>