JComboBox causes a runtime error

Question

I'm trying some GUI development in Java and when I added the following code, I got a runtime error during initialization of the class:

public class Search_Album_Main_Tab extends JPanel{
    JComboBox   search_list;
    JTextArea   searched_data;
    JButton     search_button;
    Results_Main_Tab rmt;
    Search_Action_Listener listener;

    public Search_Album_Main_Tab(Results_Main_Tab results_main_tab)
    {
        String[] search_options = {"Album", "Artist", "Genre", "ID", "Year"};
        setLayout(new GridLayout(3,1));
        rmt = results_main_tab;
        listener = new Search_Action_Listener();

        /*  Searched data   */
        searched_data = new JTextArea();

        /*  Search button   */
        search_button = new JButton("Search Album");
        search_button.addActionListener(listener);

        /*  Drop down menu  */
        search_list = new JComboBox(search_options);

        add(search_list);
        add(searched_data);
        add(search_button);     
    }

    private class Search_Action_Listener implements ActionListener
    {

        public void actionPerformed(ActionEvent event) 
        {
            if (event.getSource() == search_button)
            {

            }

        }

    }

}

The error I get is:

dcm_gui.GUI at localhost:53806  
    Thread [main] (Suspended (exception NullPointerException))  
        CUIAquaComboBox.applySizeFor(JComponent, CoreUIConstants$Size) line: 454    
        CUIAquaUtilControlSize.applyUISizing(JComponent, CoreUIConstants$Size) line: 99 
        CUIAquaUtilControlSize.access$200(JComponent, CoreUIConstants$Size) line: 13    
        CUIAquaUtilControlSize$PropertySizeListener.applyComponentSize(JComponent, Object) line: 121    
        CUIAquaUtilControlSize.addSizePropertyListener(JComponent) line: 25 
        CUIAquaComboBox.installListeners() line: 47 
        CUIAquaComboBox(BasicComboBoxUI).installUI(JComponent) line: 229    
        CUIAquaComboBox.installUI(JComponent) line: 30  
        JComboBox(JComponent).setUI(ComponentUI) line: 653  
        JComboBox.setUI(ComboBoxUI) line: 238   
        JComboBox.updateUI() line: 247  
        JComboBox.init() line: 212  
        JComboBox.<init>(Object[]) line: 178    
        Search_Album_Main_Tab.<init>(Results_Main_Tab) line: 36 
        GUI.main(String[]) line: 28 
    Daemon Thread [AWT-AppKit] (Running)    
/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/bin/java (Oct 10, 2010 11:55:27 PM) 

Does anyone has any idea why?

Binyamin

Answer 1

I think Erica is right about the non-standard look & feel. CUIAquaComboBox doesn't sound like a standard element.

If you're not sure what look & feel (LAF) you are using, call System.out.println(UIManager.getLookAndFeel()); to check.

More importantly, try calling this in your main method before your GUI starts and see if it makes a difference:

UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

This will use the native LAF for your current operating system (if one can be found - and that should work fine if you're using Windows, Mac, Linux, Solaris etc.)

Answer 2

This looks like an issue with your custom look and feel. I encountered a similar problem once before, when using custom looks and feels. It is described here:

https://bugs.java.com/bugdatabase/view_bug?bug_id=4711700

That was for the JFileChooser widget, not a combo box, but it might be related.

The first thing you should try is reverting to the default "metal" look and feel. If that works, then it might be a problem with that specific L&F. I haven't used the CUIAqua L&F before, but I think it's the official Apple Java L&F. You might also like to try switching it out for QuaQua. It's an alternative Mac-like L&F, which might (depending on the actual bug) avoid the problem.