JComobox is not showing in the JDialog

I have 2 classes. when I put bold 3 lines in the method addCourses() the dialog does not show combobox in the Panel but when I remove from addCourses and put those bold lines in the constructor, JComboBox are shown in the Panel.

But data will not show because data items updates to ComboBox will happen after Constru开发者_如何学编程ctor is created.

How can I solve this problem.

this.mainPanel.add(courseCombo, BorderLayout.NORTH);

this.mainPanel.add(sessionCombo, BorderLayout.CENTER);

this.mainPanel.add(courseButton, BorderLayout.SOUTH);

public class Updator {

CourseListFrame clf = new CourseListFrame();

for(...){
      clf.addContentsToBox(displayName, className);
}

clf.addCourses();
}

and second class is

public class CourseListFrame extends JDialog implements ActionListener {

    public JPanel mainPanel = new JPanel(new BorderLayout(2, 2));
    public JButton courseButton = new JButton(("Submit"));
    public JComboBox courseCombo;
    public JComboBox sessionCombo;
    public Multimap<String, String> map;   // = HashMultimap.create();
    public static CourseListFrame courseListDialog;

    public CourseListFrame() {
        super(this.getMainFrame());
        this.getContentPane().add(mainPanel);

        map = HashMultimap.create();
        courseCombo = new JComboBox();
        courseCombo.addItem("Select Courses");
        courseCombo.addActionListener(this);
        sessionCombo = new JComboBox();
    }

    public void addContentsToBox(String course, String session) {
        map.put(course, session);
        courseCombo.addItem(course);
    }

    public void actionPerformed(ActionEvent e) {
        JComboBox cb = (JComboBox) e.getSource();
        String str = (String) cb.getSelectedItem();
        setSessionCombo(str);
    }



    public void setSessionCombo(String course) {
        if (map.containsKey(course)) {
            sessionCombo.removeAllItems();
            Iterator it = map.get(course).iterator();
            while (it.hasNext()) {
                sessionCombo.addItem(it.next());
            }
        }
    }

    public void addCourses() {
        this.mainPanel.add(courseCombo, BorderLayout.NORTH);
        this.mainPanel.add(sessionCombo, BorderLayout.CENTER);
        this.mainPanel.add(courseButton, BorderLayout.SOUTH);

    }

    public static void showCourseListDialog() {
        if (courseListDialog == null) {
            courseListDialog = new CourseListFrame();
        }
        courseListDialog.pack();
        courseListDialog.setVisible(true);
        courseListDialog.setSize(260, 180);
    }
}

The reason why they arent showing is because you are probably calling the static showCourseListDialog() to show your dialog. This method will test whether your static courseListDialog is null, and if so, create one and set that dialog visible, not the clf that you instantiated.

If in your showCourseListDialog() you call the addCourses() method after instantiating your 'singleton', you should be OK:

public static void showCourseListDialog() {
    if (courseListDialog == null) {
        courseListDialog = new CourseListFrame();
        courseListDialog.addCourses();// <<---- this is key!
    }
    courseListDialog.pack();
    courseListDialog.setVisible(true);
    courseListDialog.setSize(260, 180);
 }

That said, by having the static courseListDialog, it is apparent that you want that dialog to be a singleton. If that is the case, I would at least make your constructor private. You want to proactively avoid the situation that you are getting into where you can construct multiple instances of a singleton. You still would have a race condition to deal with in your showCourseListDialog, but as you will only be calling this method in the EDT, you should be safe.

Take a look at this and other topics on Singleton development in Java (and dont forget to read the con arguments where it is described as an anti-pattern)