Resize JComboBox dropdown doesn't work without customized ListCellRenderer

Question

Based on the forum thread Horizontal scrollbar for JComboBox across multiple look and feel , the following code will work, if only I provide a customized ListCellRenderer (A JPanel with several JLabels).
FYI, here is my ListCellRenderer code [http://jstock.cvs.sourceforge.net/viewvc/jstock/jstock/src/org/yccheok/jstock/gui/ResultSetCellRenderer.java?view=markup]
Here is the code which adjust the drop down list width. The setup instruction is exactly same as the one mentioned in forum by Kleopatra
    private void adjustPopupWidth() {
        if (this.getItemCount() == 0) return;
        Object comp = this.getUI().getAccessibleChild(this, 0);
        if (!(comp instanceof JPopupMenu)) {
            return;
        JPopupMenu popup = (JPopupMenu) comp;
        JScrollPane scrollPane = (JScrollPane) popup.getComponent(0);
        Object value = this.getItemAt(0);
        Component rendererComp = this.getRenderer().getListCellRendererComponent(null, value, 0, false, false);       
        if (rendererComp instanceof JXTable) {
            scrollPane.setColumnHeaderView(((JTable) rendererComp).getTableHeader());
        Dimension prefSize = rendererComp.getPreferredSize();
        Dimension size = scrollPane.getPreferredSize();
        size.width = Math.max(size.width, prefSize.width);
        scrollPane.setPreferredSize(size);
        scrollPane.setMaximumSize(size);
        scrollPane.revalidate();
    }However, when come to a JComboBox, without explicitly provided it a list cell renderer, the above code will have NPE being thrown at line
       Component rendererComp = this.getRenderer().getListCellRendererComponent(null, value, 0, false, false);
       // Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
       // at javax.swing.plaf.basic.BasicComboBoxRenderer.getListCellRendererComponent(BasicComboBoxRenderer.java:94)Hence, I modify the code as follow and hoping it will work.
    private void adjustPopupWidth() {
        if (this.getItemCount() == 0) return;
        Object comp = this.getUI().getAccessibleChild(this, 0);
        if (!(comp instanceof JPopupMenu)) {
            return;
        JPopupMenu popup = (JPopupMenu) comp;
        JScrollPane scrollPane = (JScrollPane) popup.getComponent(0);
        Object value = this.getItemAt(0);
        //Component rendererComp = this.getRenderer().getListCellRendererComponent(null, value, 0, false, false);
        Component rendererComp = this.getRenderer().getListCellRendererComponent((JList)scrollPane.getViewport().getView(), value, 0, false, false);       
        if (rendererComp instanceof JXTable) {
            scrollPane.setColumnHeaderView(((JTable) rendererComp).getTableHeader());
        Dimension prefSize = rendererComp.getPreferredSize();
        Dimension size = scrollPane.getPreferredSize();
        size.width = Math.max(size.width, prefSize.width);
        scrollPane.setPreferredSize(size);
        scrollPane.setMaximumSize(size);
        scrollPane.revalidate();
    }No more exception being thrown this time. Just that my dropdown list doesn't resize at all when I have a long String. It remains normal size as usual, with horizontal scrollbar being shown to catter the long String.
Is there anything I had missed out?
Thanks
Edited by: yccheok on Oct 23, 2010 9:40 PM
Edited by: yccheok on Oct 23, 2010 9:41 PM

Answer 1

Yes. The problem solved. Out of curiosity, is it necessary to have statement? As I remove it, it just work as well.
scrollPane.revalidate();I include SSCCE for this problem.
package sandbox;
import java.awt.Component;
import java.awt.Dimension;
import javax.swing.JList;
import javax.swing.JPopupMenu;
import javax.swing.JScrollPane;
import javax.swing.plaf.basic.BasicComboPopup;
* @author yccheok
public class NewJFrame extends javax.swing.JFrame {
    /** Creates new form NewJFrame */
    public NewJFrame() {
        initComponents();
    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {
        jComboBox1 = new javax.swing.JComboBox();
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        getContentPane().setLayout(new java.awt.FlowLayout());
        jComboBox1.setEditable(true);
        jComboBox1.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Long Long Long Long Item 4" }));
        jComboBox1.setPreferredSize(new java.awt.Dimension(80, 20));
        jComboBox1.addPopupMenuListener(new javax.swing.event.PopupMenuListener() {
            public void popupMenuCanceled(javax.swing.event.PopupMenuEvent evt) {
            public void popupMenuWillBecomeInvisible(javax.swing.event.PopupMenuEvent evt) {
            public void popupMenuWillBecomeVisible(javax.swing.event.PopupMenuEvent evt) {
                jComboBox1PopupMenuWillBecomeVisible(evt);
        getContentPane().add(jComboBox1);
        pack();
    }// </editor-fold>
    private void jComboBox1PopupMenuWillBecomeVisible(javax.swing.event.PopupMenuEvent evt) {
        adjustPopupWidth();
    * @param args the command line arguments
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NewJFrame().setVisible(true);
    private void adjustPopupWidth() {
        if (jComboBox1.getItemCount() == 0) return;
        Object comp = jComboBox1.getUI().getAccessibleChild(jComboBox1, 0);
        if (!(comp instanceof JPopupMenu)) {
            return;
        JPopupMenu popup = (JPopupMenu) comp;
        JScrollPane scrollPane = (JScrollPane) popup.getComponent(0);
        Object value = jComboBox1.getItemAt(0);
        Component rendererComp = jComboBox1.getRenderer().getListCellRendererComponent((JList)scrollPane.getViewport().getView(), value, 0, false, false);
        //if (rendererComp instanceof JXTable) {
        //    scrollPane.setColumnHeaderView(((JTable) rendererComp).getTableHeader());
        //Dimension prefSize = rendererComp.getPreferredSize();
        BasicComboPopup basic = (BasicComboPopup)comp;
        Dimension prefSize = basic.getList().getPreferredSize();
        Dimension size = scrollPane.getPreferredSize();
        size.width = Math.max(size.width, prefSize.width);
        scrollPane.setPreferredSize(size);
        scrollPane.setMaximumSize(size);
        //scrollPane.revalidate();
    // Variables declaration - do not modify
    private javax.swing.JComboBox jComboBox1;
    // End of variables declaration
}