title: Java DecimalFormat returns a "?"
body: <p>My <code>DecimalFormat</code> is sometimes returning a '?' when trying to <code>format()</code>. Is there an input that would create this scenario?</p>
  
  <p>For example:</p>
  
  <pre><code>DecimalFormat df = new DecimalFormat("#.####");
  df.format(X); // output : '?'
  </code></pre>
  
  <p>What could <code>X</code> possibly be?</p>