How do JComboBoxes paint outside their bounds?

Question

I'm making a little widget That is essentially a text field, but as the user starts typing in it it does a query to a jdbc table and lists posible completions in a drop down box. Rather like a combobox really, except I tried doing this as an extension of the JComboBox class, but then the combobox feel interfered with the feel I wanted for this widget. (Focus issues, issues with the ComboBoxEditor and how it handles values as items.)
I started with an implementation where I kept the list of completions in an undecorated JDialog that positioned itself nicely under the text field whenever it was shown, however there were problems when moving the window the field was in, and if the size of the list changed, and much trouble in general.
I've now got an implementation where I've made a custom LayoutManager to position the list correctly, and it works perfectly except that the list isn't "floating". That is, I must add it to the widget component and when I want to draw it I must make sure the widget's bounds are large enough to hold the list. This doesn't really work since this leaves a large blank area under the text field when the list is hidden.
So what I really want to know is: How do JComboBoxes do their popups? How do they keep the bounds of the actual component small, and yet allow the popup to paint over other components? I tried overriding the paintComponent() method and changing the Graphics object's clipRect before painting the popup, still didn't work.
First a screenshot of what it looks like if I inflate the preferredSize of the widget in the layout manager: (Sorry about norwegian text on labels)
http://img238.imageshack.us/my.php?image=indexbrowserwidget5gw.jpg
You can see that the drop down list gets cut off where the bounds of the widget ends.
Here's the code for what I've got now:
(I just noticed while previewing that this ended up VERY long. Sorry about that. The interesting bit is the actual LayoutManager.)
package no.fldt.sdb.swing;
import java.awt.*;
import java.awt.event.*;
import java.math.BigDecimal;
import java.sql.*;
import java.util.*;
import java.util.Timer;
import javax.swing.*;
import javax.swing.event.*;
import no.fldt.sdb.sql.SVDatabaseManager;
@SuppressWarnings("serial")
public class SVIndexBrowserOriginal extends JPanel {
     private int numDesc;
     private final String stringSQL;
     private final String pkSQL;
     private PreparedStatement selectStringStatement;
     private PreparedStatement selectPKStatement;
     private SearchableCache cache = new SearchableCache();
     private Value value;
     private JTextField field = new JTextField();
     private FieldKeyListener keyListener = new FieldKeyListener();
     private JButton browseButton = new JButton();
     private JList popupList = new JList();
     private JScrollPane popupPane = new JScrollPane(popupList);
     public SVIndexBrowserOriginal(String tableName, String pkName, String ... descriptionNames) {
          super.setLayout(new BrowserLayout(field,browseButton,popupPane));
          super.add(field);
          super.add(browseButton);
          super.add(popupPane);
          popupPane.setVisible(false);
          field.addFocusListener(new FocusAdapter(){
               public void focusLost(FocusEvent e) {
                    if(e.getOppositeComponent() == popupList) return; //OK to yield to this
                    if(popupPane.isVisible()) hideHintPopup();
          browseButton.setFocusable(false);
          browseButton.setMargin(new Insets(0,1,0,0));
          browseButton.addActionListener(new ActionListener(){
               public void actionPerformed(ActionEvent e) {
                    keyListener.cancel();
                    if(populateHintList() == -1)
                         hideHintPopup();
                    else
                         displayHintPopup();
          field.addKeyListener(keyListener);
          field.addActionListener(new ActionListener(){
               public void actionPerformed(ActionEvent e) {
                    if(popupPane.isVisible()){
                         doPopupSelection();
          popupList.addKeyListener(new KeyAdapter(){
               boolean loopedThrough = false;
               public void keyPressed(KeyEvent e) {
                    switch(e.getKeyCode()){
                         case KeyEvent.VK_ESCAPE:
                              cancelPopupSelection();
                              break;
                         case KeyEvent.VK_ENTER:
                         case KeyEvent.VK_TAB:
                              doPopupSelection();
                              break;
          this.stringSQL = generateStringSQL(tableName, pkName, descriptionNames);
          this.pkSQL = generatePKSQL(tableName, pkName, descriptionNames);
          this.numDesc = descriptionNames.length;
          try {
               this.selectStringStatement = SVDatabaseManager.prepareStatement(stringSQL);
               this.selectPKStatement = SVDatabaseManager.prepareStatement(pkSQL);
               reBuildStringCache();
          } catch (SQLException ex) {
               // ex.printStackTrace();
     // The hint popup:
     private Vector<Value> displayList = new Vector<Value>();
     private int populateHintList() {
          popupList.setForeground(Color.BLACK);
          String start = field.getText();
          int sStart = field.getSelectionStart();
          start = start.substring(0,sStart);
          displayList.clear();
          if(start.length() > 0){
               try {
                    Long.parseLong(start);
                    displayList.addAll(getPossibleMatchesForPK(start));
               } catch (Exception ex) {
                    displayList.addAll(cache.getPossibleMatchesForDesc(start));
          if(displayList.size() == 1){
               setValue(displayList.get(0));
               return 1;
          if(displayList.isEmpty() && start.length() > 0){
               displayList.add(NO_VALUE);
               popupList.setForeground(Color.GRAY);
          popupList.setModel(new DefaultComboBoxModel(displayList));
          popupList.setSelectedIndex(0);
          if(displayList.size() > 0){
               Value v = displayList.get(0);
               if(!v.isNoValue()){
                    int cp = start.length();
                    field.setText(v.desc);
                    field.setCaretPosition(v.desc.length());
                    //To select the text that is suggested so if the user continues typing it will be overwritten.
                    field.moveCaretPosition(cp);
          int retVal = displayList.size() == 1 ? 0 :
               displayList.size() == 0 ? -1 : displayList.size();
          return retVal;
     public void displayHintPopup() {
          if(popupPane.isVisible()) return;
          popupPane.setVisible(true);
          field.requestFocus();
     public void hideHintPopup(){
          popupPane.setVisible(false);
     public void doPopupSelection() {
          Value nv = null;
          int i = popupList.getSelectedIndex();
          if(i != -1)
               nv = displayList.get(i);
          if(nv != null && ! nv.isNoValue()){
               setValue(nv);
          hideHintPopup();
          field.requestFocusInWindow();
     public void cancelPopupSelection(){
          hideHintPopup();
          field.requestFocusInWindow();
     // The layout
     private class BrowserLayout implements LayoutManager {
          private JTextField field;
          private JButton button;
          private JScrollPane listPane;
          BrowserLayout(JTextField field, JButton button, JScrollPane listPane){
               this.field = field;
               this.button = button;
               this.listPane = listPane;
          public void addLayoutComponent(String name, Component comp) {
          public void removeLayoutComponent(Component comp) {
          public Dimension preferredLayoutSize(Container parent) {
               return new Dimension(
                         button.getPreferredSize().width+field.getPreferredSize().width,
                         field.getPreferredSize().height
          public Dimension minimumLayoutSize(Container parent) {
               return new Dimension(
                         button.getMinimumSize().width+field.getMinimumSize().width,
                         field.getMinimumSize().height
          public void layoutContainer(Container parent) {
               Dimension size = parent.getSize();
               Rectangle bounds = new Rectangle();
               bounds.setSize(button.getPreferredSize().width,size.height);
               bounds.setLocation(size.width,0);
               bounds.translate((-1 * bounds.width),0);
               button.setBounds(bounds);
               bounds.width = size.width - bounds.width;
               bounds.setLocation(0,0);
               field.setBounds(bounds);
               bounds.translate(0,bounds.height);
               bounds.width = size.width-1;
               bounds.height = listPane.getPreferredSize().height;
               listPane.setBounds(bounds);
     // SQL stuff:
     private void reBuildStringCache() throws SQLException {
          //Not so important for the forum
     private Vector<Value> getPossibleMatchesForPK(String start)
               throws SQLException {
          //Not so important for the forum
     private String generateStringSQL(String tableName, String pkName,
               String[] descriptionNames) {
          //Not so important for the forum
     private String generatePKSQL(String tableName, String pkName,
               String[] descriptionNames) {
          //Not so important for the forum
     // Keylistener:
     private class FieldKeyListener extends KeyAdapter {
          private TimerTask popupTask = null;
          private Timer popupTimer = new Timer();
          public void cancel() {
               if (popupTask != null) {
                    popupTask.cancel();
                    popupTask = null;
          public void keyTyped(KeyEvent e) {
               cancel();
               if(e.getKeyChar() == 27) return; //VK_ESCAPE
               popupTask = new TimerTask() {
                    public void run() {
                         if(Math.abs(populateHintList()) == 1){
                              hideHintPopup();
                              return;
                         displayHintPopup();
               popupTimer.schedule(popupTask, 1000);
          public void keyPressed(KeyEvent e) {
               if (e.getKeyCode() == KeyEvent.VK_DOWN) {
                    cancel();
                    if(Math.abs(populateHintList()) == 1){
                         hideHintPopup();
                         return;
                    displayHintPopup();
                    popupList.requestFocus();
               }else if(e.getKeyCode() == KeyEvent.VK_ESCAPE) {
                    cancel();
                    hideHintPopup();
     // Cache:
     private class SearchableCache {
          public void put(String key, BigDecimal value) {
               //Not too important for the forum
          public BigDecimal get(String key) {
               //Not too important for the forum
          public Vector<Value> getPossibleMatchesForDesc(String start) {
               //Not too important for the forum
     public static final Value NO_VALUE = new Value(){{desc = "<No matches>"; pk = null;}};
     public static class Value {
          public BigDecimal pk = null;
          public String desc;
          public String toString(){
               return desc;
          public boolean equals(Object other){
               if (other != null && other instanceof Value) {
                    return this.pk.equals(((Value)other).pk);
               return false;
          public boolean isNoValue(){
               return this.pk == null;
     public void clearValue(){
          this.value = null;
          this.field.setText("");
     public Value getValue() {
          return this.value;
     public void setValue(Value value) {
          this.value = value;
          field.setText(value.desc);
     @Override
     public void setBackground(Color bg) {
          if(field != null)
               field.setBackground(bg);
}Any help with getting the popup to paint correctly or any hints towards a different approach woule be greatly appreciated.

Answer 1

I can think of three possible choices for a solution:
1) Use JWindow. But that is very like your first try with an undecorated JDialog.
I would assume that you will get similar problems as you describe
2) Use the PopupFactory (in javax.swing). With this you get real popups.
When the bounds of the popup fits in the parent window it will show as
a medium-weight component (AWT-component!). When the bounds
doesn't fit it will show as a heavy-weight component (own window).
3) Another possibility is JPopupMenu! You can add any Swing-Component
in a JPopupMenu! :-)
And the answer for your initial question: how is it done in JComboBox: if you have a look in javax.swing.plaf.basic.BasicComboBoxUI and the BasicComboPopup-class you will see, that it is 3) -> the JPopupMenu that is used here!