<p>Private methods that are never executed are dead code. Dead code
  means unnecessary, inoperative code that should be removed. This helps
  in maintenance by decreasing the maintained code size, making it easier
  to understand the program and preventing bugs from being introduced.</p>
<p>In the following two cases, private methods are not considered as
  dead code by SonarQube :</p>
<ul>
  <li>Private empty constructors that are intentionally used to
    prevent any direct instantiation of a class.</li>
  <li>Private methods : readObject(...), writeObject(...),
    writeReplace(...), readResolve(...) which can contractually be used
    when implementing the Serializable interface.</li>
</ul>