Changing the size of a JComboBox

Question

Greetings,
I am trying to build a mechanism to perform zooming on a JPanel with an arbitrary collection of components (including nested JPanels). I've tried a number of things with little success. The following is my most promising contraption. It can zoom labels, textfields, checkboxes, and buttons. However, for some reason, the combo box refuses to accept a changes to its size. I'm not sure why this is the case. Its font changes size appropriately.
Anyway, I'm running in JDK 1.4, and the following program lays out a palette of components. To change the size of the components, hit F1 to zoom in, and F2 to zoom out.
import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.KeyEventPostProcessor;
import java.awt.KeyboardFocusManager;
import java.awt.event.KeyEvent;
import java.awt.geom.AffineTransform;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class Demo extends JPanel
    public class Zoomer
        private double m_zoom = 1;
        private JPanel m_panel;
        public Zoomer(JPanel panel)
            m_panel = panel;
        private AffineTransform getTransform()
            return AffineTransform.getScaleInstance(m_zoom, m_zoom);
        private Font transform(Font font)
            return font.deriveFont(getTransform());
        private Dimension transform(Dimension dimension)
            Dimension retval = new Dimension();
            retval.setSize(dimension.getWidth() * m_zoom, dimension.getHeight() * m_zoom);
            return retval;
        private void performZoom(Container container)
            Component[] components = container.getComponents();
            for(int i = 0; i < components.length; i++)
                Component component = (Component)components;
component.setFont(transform(component.getFont()));
component.setSize(transform(component.getSize()));
for(int i = 0; i < components.length; i++)
Component component = components[i];
if(component instanceof Container)
performZoom((Container)component);
public double getZoom()
return m_zoom;
public void setZoom(double zoom)
if(zoom > 8.0 || zoom < 0.125) return;
m_zoom = zoom;
performZoom(m_panel);
public void zoom(double factor)
setZoom(getZoom() * factor);
public Demo()
JPanel panel = new JPanel();
panel.add(buildPanel());
panel.add(buildPanel());
final Zoomer zoomer = new Zoomer(panel);
add(panel);
KeyboardFocusManager.getCurrentKeyboardFocusManager().addKeyEventPostProcessor(new KeyEventPostProcessor()
public boolean postProcessKeyEvent(KeyEvent e)
if(e.getID() != KeyEvent.KEY_PRESSED) return false;
if(e.getKeyCode() == KeyEvent.VK_F1)
zoomer.zoom(1.2);
if(e.getKeyCode() == KeyEvent.VK_F2)
zoomer.zoom(1/1.2);
return false;
private JPanel buildPanel()
JPanel panel = new JPanel();
panel.add(new JLabel("label: "));
panel.add(new JTextField("Hello World"));
panel.add(new JCheckBox("checkbox"));
panel.add(new JComboBox(new String[] { "Bread", "Milk", "Butter" }));
panel.add(new JButton("Hit Me!"));
return panel;
* Create the GUI and show it. For thread safety, this method should be
* invoked from the event-dispatching thread.
private static void createAndShowGUI()
// Create and set up the window.
JFrame frame = new JFrame("Demo");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// Create and set up the content pane.
Demo newContentPane = new Demo();
newContentPane.setOpaque(true); // content panes must be opaque
frame.setContentPane(newContentPane);
// Display the window.
frame.pack();
frame.setVisible(true);
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()
createAndShowGUI();

Answer 1

component.setSize(transform(component.getSize()));First of all the above line is not needed. The LayoutManager will determine the bounds (size and location) of each component in the container based on the rules of the LayoutManager. The Flow Layout is the default layout manager for a panel and it simply uses the preferred size of the component as the size of the component.
So what happens is that when you change the font of the component you are changing the preferred size of the component.
So why doesn't the combo box work? Well I took a look at the preferred size calculation of the combo box (from the BasicComboBoxUI) and it actually caches the preferred size. The combo box uses a renderer, so the value is cached for performance one would assume. The method does recalculate the size when certain properties change. Note the isDisplaySizeDirty flag used in the code below:
         else if (propertyName.equals("prototypeDisplayValue")) {
                isMinimumSizeDirty = true;
                isDisplaySizeDirty = true;
                comboBox.revalidate();
         else if (propertyName.equals("renderer")) {
                isMinimumSizeDirty = true;
                isDisplaySizeDirty = true;
                comboBox.revalidate();
            }It also handles a Font property change as well:
            else if ( propertyName.equals( "font" ) ) {
                listBox.setFont( comboBox.getFont() );
                if ( editor != null ) {
                    editor.setFont( comboBox.getFont() );
                isMinimumSizeDirty = true;
                comboBox.validate();
            }but notice that the isDisplaySizeDirty flag is missing. This would seem to be a bug (but I don't know why two flags are required).
Anyway, the following change to your code seems to work:
// component.setSize(transform(component.getSize()));
if (component instanceof JComponent)
     ((JComponent)component).updateUI();
}