Component - validate, invalidate

Question

What's the proper way to use the Component validate and invalidate methods?
Quick background: I've extended JPanel to draw an image as it's background. The image can be scaled and what i want to do is set the preferred size of the panel to reflect that of the scaled image size.
I've placed some zoom in/out buttons in a frame along with the panel but i can't get them to work as i would expect. The code below does what i want it to do but i don't understand why i need to set both the preferred size and the size, then why i need to call invalidate and then validate on the parent.
Any ideas/guidance on the proper way of resizing components and getting the parents to re-layout with the new sizes?
Thanks
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import javax.swing.JPanel;
public class ImagePanel extends JPanel {
     /** Original image */
     private Image image;
     /** Scaled version */
     private Image zoomedImage;
     /** Scaled width */
     private int imageZoomWidth;
     /** Scaled height */
     private int imageZoomHeight;
     @Override
     protected void paintComponent(Graphics g) {
          g.clearRect(0, 0, getWidth(), getHeight());
          g.drawImage(zoomedImage, 0, 0, this);
     public void setImage(Image image) {
          this.image = image;
          imageZoomWidth = image.getWidth(this);
          zoom(0);
     public void zoom(int delta) {
          imageZoomWidth += delta;
          zoomedImage = image.getScaledInstance(imageZoomWidth, -1,
                    Image.SCALE_FAST);
          imageZoomHeight = zoomedImage.getHeight(this);
          // Both these are needed?
          setPreferredSize(new Dimension(imageZoomWidth, imageZoomHeight));
          setSize(getPreferredSize());
          // invalidate to force re-layout of parents
          invalidate();
          // Why is this is needed too ?
          if (getParent() != null) {
               getParent().validate();
import java.awt.Color;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
public class ImagePanelTest {
     public static void main(String[] args) throws FileNotFoundException,
               IOException {
          JFrame frame = new JFrame("Image Panel Test");
          JButton zoomInButton = new JButton("Larger");
          JButton zoomOutButton = new JButton("Smaller");
          final ImagePanel imgPanel = new ImagePanel();
          imgPanel.setImage(ImageIO.read(new FileInputStream(
                    "image09-07-14_12-34-57-99.jpg")));
          imgPanel.setBorder(BorderFactory.createLineBorder(Color.black));
          zoomInButton.addActionListener(new ActionListener() {
               @Override
               public void actionPerformed(ActionEvent e) {
                    imgPanel.zoom(50);
          zoomOutButton.addActionListener(new ActionListener() {
               @Override
               public void actionPerformed(ActionEvent e) {
                    imgPanel.zoom(-50);
          Container container = frame.getContentPane();
          JPanel panel = new JPanel();
          panel.setLayout(new FlowLayout());
          panel.add(zoomOutButton);
          panel.add(imgPanel);
          panel.add(zoomInButton);
          container.add(new JScrollPane(panel));
          frame.setSize(800, 600);
          frame.addWindowListener(new WindowAdapter() {
               public void windowClosing(WindowEvent we) {
                    System.exit(0);
          frame.setVisible(true);
}

Answer 1

You're playing the role of the RepaintManager with that code. Just change your zoom method to this,
public void zoom(int delta) {
    imageZoomWidth += delta;
    zoomedImage = image.getScaledInstance(imageZoomWidth, -1,
            Image.SCALE_FAST);
    imageZoomHeight = zoomedImage.getHeight(this);
    setPreferredSize(new Dimension(imageZoomWidth, imageZoomHeight));
    revalidate();
    repaint();
}and all will be well.