title: Why is it useful to have null values or null keys in hash maps?
body: <p><code>Hashtable</code> does not allow null keys or values, while <code>HashMap</code> allows null values and 1 null key.</p>
  <h3>Questions:</h3>
  <ol>
  <li>Why is this so?</li>
  <li>How is it useful to have such a key and values in HashMap?</li>
  </ol>