title: List of objects. Operations with Integer entries
body: <p>The List might contain both Integers and String values. In this case, should I create the List of Objects, right?</p>
  
  <pre><code>List&lt;Object&gt; list = new ArrayList&lt;Object&gt;();
  </code></pre>
  
  <p>How to perform simple arithmetic operations with the Integer entries of the List?</p>
  
  <pre><code>list.add(1);
  list.add("ok");
  list.add(2);
  Integer a = list.get(0) - list.get(2); // does not work
  </code></pre>