title: add component to jpanel on runtime
body: <p>I have a <code>JPanel</code> and <code>JButton</code> on the <code>JFrame</code>.<br/>
  on runtime add <code>JLabel</code> to <code>JPanel</code> When click <code>JButton</code>.<br/></p>
  
  <p>I use of the following code:<br/></p>
  
  <pre><code> panel.setLayout(null);
  
   jLabel _lbl=new jLabel();
   _lbl.setText("Label");
   panel.add(_lbl);
   panel.validate();
  </code></pre>
  
  <p>but no display any <code>JLabel</code> in <code>JPanel</code>.</p>