title: Change preferredSize of a JLabel after setVisible the JFrame
body: <p>I have a problem when I try to resize a JLabel. In my application appears the next strucutre. Understand every list item like something inside the previous list item. </p>
  
  <ul>
  <li><p>JFrame (Layout null, fixed size, not resizeable, used by different people).</p></li>
  <li><p>JPanel (Layout null, with a size of all the window, the place were I put my work).</p></li>
  <li><p>various JPanel with different Layouts (the areas of content inside the main panel, you can think about it like html divs...).</p></li>
  <li><p>Inside one of this "divs" with Layout null there are extended Classes of JPanel with Flow Layout.</p></li>
  <li><p>Inside every one of this extedend Classes are labels with preferredSizes.</p></li>
  </ul>
  
  <p>The thing is that when after create all of this i call theFrame.setVisible(true); and all works propertly.</p>
  
  <p>But in a moment of the flow of my application I have to change the size of one of the labels. Then, I simply call label.setPreferredSize(d) and the change doesn't change. The function works propertly if I call it before set visible the frame, but not after.</p>
  
  <p>I have the feeling that the problem is that I don't use nothing like pack(), validate(), repaint(), etc. Because I don't know what it works. I try call repaint and validate to the jlabel, and repaint the main panel, but doesn't works.</p>
  
  <p>I'm relative new with awt and swing, and this is for homework. Sorry for my bad use of English language, and thank you for your help.</p>