Cardlayout and Nullpointer exception

hi!

I am trying to implement cardlayout and I have Nullpointerexception.
I don't understand why....

------------------------------------------------
public class HW5
    extends JApplet
    implements ActionListener {

  private Container container;
  private GridBagLayout layout;
  private GridBagConstraints constraints;

  private JFrame wizardFrame;

  private JScrollPane scrollPane;
  private JTextArea textArea;
  private JButton backBtn;
  private JButton nextBtn;
  private JButton finishBtn;
  private JButton cancelBtn;

  private CardLayout cardManager;
  private JPanel screen;
  private JButton controls[];
  private String namesBtn[] = {
      "Back", "Next", "Finish", "Cancel"};
//----------------
//private FlowLayout layout = new FlowLayout();

  public HW5() {
  }

  public void init() {
    wizardFrame = new JFrame("Configure Plotpad");
    layout = new GridBagLayout();
    //container.setLayout(layout);
    container = wizardFrame.getContentPane();
    constraints = new GridBagConstraints();
    // backBtn= new JButton("Back");
    // nextBtn = new JButton("Next");
    // finishBtn= new JButton("Finish");
    // cancelBtn= new JButton("Cancel");

    textArea = new JTextArea(5, 30);

    scrollPane = new JScrollPane(textArea);
    container.add(scrollPane, BorderLayout.NORTH);
    // constraints.fill = GridBagConstraints.BOTH;
    //arrangeComponent(textArea, 0,0,1,1);


    //PSizes pSizes = new PSizes(ppc);
    //container.add(pSizes, BorderLayout.CENTER);

//--------------------------------

   // controls = new JButton[namesBtn.length];

    for (int count = 0; count < 4; count++) {
      controls[count] = new JButton(namesBtn[count]);          //<--------------------Nullpointerexception!!!
    }
    controls[0].addActionListener(this);//back btn
    controls[1].addActionListener(this);//next btn

    JPanel southPanel = new JPanel();
    southPanel.add(backBtn);
    southPanel.add(nextBtn);
    southPanel.add(finishBtn);
    southPanel.add(cancelBtn);
    container.add(southPanel, BorderLayout.SOUTH);

    wizardFrame.setSize(500, 400);
    wizardFrame.setVisible(true);

//------------------------------------------------------
    screen = new JPanel();
    cardManager = new CardLayout();
    screen.setLayout(cardManager);

    PlotPadConfig ppc = new PlotPadConfig();

    PDataSin pDataSin = new PDataSin(ppc);
    screen.add(pDataSin);

    PDataCos pDataCos = new PDataCos(ppc);
    screen.add(pDataCos);

    PSizes pSizes = new PSizes(ppc);
    screen.add(pSizes);

    PExtent pExtent = new PExtent(ppc);
    screen.add(pExtent);

    PAxisFormat pAxisFormat = new PAxisFormat(ppc);
    screen.add(pAxisFormat);

    PTitles pTitles = new PTitles(ppc);
    screen.add(pTitles);

    container.add(screen, BorderLayout.CENTER);
...
...
...

  public void actionPerformed(ActionEvent event) {
    if (event.getSource() == controls[0]) {
      cardManager.previous(screen);
    }
    if (event.getSource() == controls[1]) {
      cardManager.next(screen);
    }
  }
...
...
...
dkim18Asked:
Who is Participating?
 
CEHJConnect With a Mentor Commented:
That should be

southPanel.add(controls[0]);

but i wouldn't hard-code the index, nor would i call the array of buttons 'controls' (not specific enough)
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CEHJCommented:
controls = new JButton[namesBtn.length];

should not be commented out
0
 
dkim18Author Commented:
Actually, it wan't the cause of the problem. I uncommented out and still have same problem, but this time complier complains on this line.

southPanel.add(backBtn);

I put buttons an array for cardlayout's behavior. Does it has anything to do with this problem?
0
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dkim18Author Commented:
Thank you CHEJ!
Now,  my code doen't displays these panels.
why is that?(I raised 50 points more b/c I am asking another question..)

   screen = new JPanel();
    cardManager = new CardLayout();
    screen.setLayout(cardManager);

    PlotPadConfig ppc = new PlotPadConfig();

    PDataSin pDataSin = new PDataSin(ppc);
    screen.add(pDataSin);

    PDataCos pDataCos = new PDataCos(ppc);
    screen.add(pDataCos);

    PSizes pSizes = new PSizes(ppc);
    screen.add(pSizes);

    PExtent pExtent = new PExtent(ppc);
    screen.add(pExtent);

    PAxisFormat pAxisFormat = new PAxisFormat(ppc);
    screen.add(pAxisFormat);

    PTitles pTitles = new PTitles(ppc);
    screen.add(pTitles);

    container.add(screen, BorderLayout.CENTER);
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CEHJCommented:
You're not using the layout properly. You should be adding a String to identify which card to show. See how to use at:

http://java.sun.com/docs/books/tutorial/uiswing/layout/card.html
0
 
dkim18Author Commented:
Thanks!!!
0
 
CEHJCommented:
8-)
0
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