title: why we should use "null" in dialog boxes?
body: <p>I want to know why we should write "null" in dialog boxes ?</p>
  
  <pre><code>str= JOptioPane.showMessageDialog(
                                   parentComponent(null), // &lt;--
                                   messageStringExpression,
                                   boxTitleString,
                                   meeageType);
  </code></pre>
  
  <p>and when we have to write it ?
  can I write something else ?</p>
  
  <p>thanks all</p>