title: How can I calculate the difference between two ArrayLists?
body: <p>I have two ArrayLists.</p>
  
  <p><strong>ArrayList A contains:</strong></p>
  
  <pre><code>['2009-05-18','2009-05-19','2009-05-21']
  </code></pre>
  
  <p><strong>ArrayList B contains:</strong></p>
  
  <pre><code>['2009-05-18','2009-05-18','2009-05-19','2009-05-19','2009-05-20','2009-05-21','2009-05-21','2009-05-22']
  </code></pre>
  
  <p>I have to compare ArrayList A and ArrayList B. The result ArrayList
   should contain the  List which does not exist in ArrayList A.</p>
  
  <p><strong>ArrayList result should be:</strong></p>
  
  <pre><code>['2009-05-20','2009-05-22']
  </code></pre>
  
  <p>how to compare ?</p>