title: Why does ConcurrentHashMap prevent null keys and values?
body: <p>The JavaDoc of <a href="http://java.sun.com/javase/6/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="noreferrer"><code>ConcurrentHashMap</code></a> says this:</p>
  
  <blockquote>
    <p>Like <code>Hashtable</code> but unlike <code>HashMap</code>, this class does <em>not</em> allow <code>null</code> to be used as a key or value.</p>
  </blockquote>
  
  <p>My question: Why?</p>
  
  <p>2nd question: Why doesn't <code>Hashtable</code> allow null?</p>
  
  <p>I've used a lot of HashMaps for storing data. But when changing to <code>ConcurrentHashMap</code> I got several times into trouble because of NullPointerExceptions.</p>