an instance of any class
 and <code>d1.diff(null)</code> should throw a <code>NullPointerException</code>.</p>
 
 <p>
 <code>Diffable</code> classes lend themselves well to unit testing, in which a
 easily readable description of the differences between an anticipated result and
 an actual result can be retrieved. For example:
 </p>
 <pre>
 Assert.assertEquals(expected.diff(result), expected, result);
 </pre></div>
<dl><dt><span class="strong">Since:</span></dt>
  <dd>3.3</dd>
<dt><span class="strong">Versi