Question

I have 2 Jcombo Boxs: which is combo1 and combo2

I choose combo1 and I can get information for combo2 but The problem is I can get informatiob for combo2 but it is not updated. I also try to use updata.UI() but it doesn’t help.

This is the code in side

public void actionPerformed(ActionEvent e) { 
    JComboBox cb = (JComboBox)e.getSource();
    String uname1 = (String)cb.getSelectedItem();

        combo2 = update(uname1);
        combo2.updateUI();

}

This is code inside update

protected JComboBox update(String name) {
    JComboBox tmp = new JComboBox();
    //Read Content from XML file (University is bigger than Year)
    NodeList nList = doc.getElementsByTagName("University");
    System.out.println("Inside Fn " + name);
    for(int i = 0 ; i < nList.getLength();i++) {
        Element el = (Element)nList.item(i);
        if(name.contentEquals(el.getAttributeNode("name").getNodeValue()))
        {
            NodeList tmpyList = el.getElementsByTagName("Year");
            for(int j = 0 ; j < tmpyList.getLength();j++) 
            {
                Element yl = (Element)tmpyList.item(j);
                System.out.println(yl.getAttribute("yr"));
                tmp.addItem(yl.getAttribute("yr"));
            }
        }
    }
    return tmp; //Return ComboBox to combo2
}

---

Thank you for your kindness, I try to use your code but it is not work (It still not update), please help me

This is my constructor

public JFrameExample() {
    String[] comboboxdefault = { "Select" };

    JComboBox combo1 = Universitylist();
    JComboBox combo2 = new JComboBox(comboboxdefault);
    JComboBox combo3 = new JComboBox(comboboxdefault);

    uList.addActionListener(this);
    yList.addActionListener(this);
    dList.addActionListener(this);


    JPanel student_information = new JPanel(new GridLayout(0,1));
    uList.setName("University List");
    yList.setName("Year List");

    // University List

    student_information.add(combo1);
    // Database Year List

    student_information.add(combo2);
    // Programme List

    student_information.add(combo3);
    //Add Components to this container, using the default FlowLayout.
    add(student_information);

}

This is the combo2 Update it is return String Array

protected String[] updateyList(String name)  
{
    String[] tmp = null;

           //Read from XML file
    for(int i = 0 ; i < nList.getLength();i++) {
        Element el = (Element)nList.item(i);
        if(name.contentEquals(el.getAttributeNode("name").getNodeValue()))
        {
            NodeList tmpyList = el.getElementsByTagName("Year");
            tmp = new String[tmpyList.getLength()];
            for(int j = 0 ; j < tmpyList.getLength();j++) 
            {
                Element yl = (Element)tmpyList.item(j);
                //Add to String Array
                tmp[j] = yl.getAttribute("yr");
            }
        }
    }

    return tmp;
}

In the Action Perform

public void actionPerformed(ActionEvent e) {
    JComboBox cb = (JComboBox)e.getSource();
    String uname1 = (String)cb.getSelectedItem();
    System.out.println(cb.getName()); // To make sure I got the combo1. 
    try {
        //I change to the model method 
        DefaultComboBoxModel model = new DefaultComboBoxModel(  updateyList(uname1) );
        System.out.println(model.getSize()); 
        combo2 = new JComboBox(); // If I don't have this line it will throw  error Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        combo2.setModel(model);

    } catch (ParserConfigurationException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    } catch (SAXException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    } catch (IOException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }

}

And this is for creating GUI function

private static void createAndShowLoginGUI() {

   //Create and set up the window.
    JFrame frame = new JFrame("Login");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Create and set up the content pane.
    JFrameExample newContentPane = new JFrameExample();
    newContentPane.setOpaque(true); //content panes must be opaque
    frame.setContentPane(newContentPane);

    //Display the window.
    frame.pack();
    frame.setVisible(true);
}

This is main function

public static void main(String[] args)
{
    //Schedule a job for the event-dispatching thread:

    //creating and showing this application's GUI.

    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowLoginGUI(); 
        }
    });
}

I think I did something wrong but I don’t know where

Answer 1

As you do now, you recreate the combo box every time (by returning the tmp in update method). This as it seems is not reflecting in the UI and maybe others will post the reason. But if you can change to update the combo values (changing the model or deleting the current values and adding the new ones) instead of recreating, then the following post can help you Dynamically change JComboBox