ScrollPane not validating correctly when scaling a JPanel

Question

I am drawing on a JPanel which I would like to be able to zoom on, im using the 2D graphics for zooming. When I scale the scaling seems to work correctly, but when the JPanel gets bigger then the size it had when I added it to the ScrollPane, the graphics starts to "leave" the viewing area. I would like to know what I can do about this. I am currently using the revalidate method before my paint, and I can se that ssize of the JPanel is changing, but not totally to what i set the PreferedSize to, why not is beyond my understanding.
Im making the JPanel, which is a class extending JPanel , then the ScrollPane, adding the scrollPane to my frame and im not using layoutmanagers AFAIK anyway.
Code to edit the JPanel and create scrollPane in the frame class
        //Editing the scrollable JPanel
        workSpace.setPreferredSize(new Dimension(800, 800));
        workSpace.setOriginalHeight(800);
        workSpace.setOriginalWidth(800);
        workSpace.setLayout(null);
        //Creating and adding the scrollPane
        scrollPane = new JScrollPane(workSpace);
        scrollPane.setBounds(new Rectangle(285, 155, 685, 455));
        contentPane.add(scrollPane, null);
The paint method
    public void paintComponent(Graphics _g) {
        super.paintComponent(_g);
        //do rendering as offscreen, to avoid "flicker".
        Image offscreen = createImage(this.getWidth(), this.getHeight()); //make an image
        Graphics2D g2d = (Graphics2D) offscreen.getGraphics(); //get the graphics object from the image
        g2d.scale(scale, scale);
        //clear contents
        g2d.setPaint(DS_Data.getBGColor());
        g2d.fillRect(0, 0, this.getWidth(), this.getHeight());
        String[] dxfList = EntityListHandler.getListNames(); //get a list of names
        if (dxfList.length > 0) {
            //System.out.println("<WorkSpace.paint> Drawing - restraints x: " + this.getWidth() + "  y: " + this.getHeight());
            Block drawBlock; //block object
            for (int i = 0; i < dxfList.length; i++) {
                drawBlock = EntityListHandler.getTopLevelBlock(dxfList);
if (drawBlock != null) {
drawBlock.doDraw(g2d, this.getWidth(), this.getHeight(), 0,
0, 0, -3, false); //brug ALTID -3 til at kalde doDraw p� blokke, det fort�ller det er f�rste block
g2d.drawLine(this.getWidth(), 0, this.getWidth(), this.getHeight());
g2d.drawLine(0, this.getHeight(), this.getWidth(), this.getHeight());
g2d.dispose(); //dispose of the graphics2d context when were done with it.
_g.drawImage(offscreen, 0, 0, this);
} else {
System.out.println("<WorkSpace.paint> Listen er ikke klar .");
g2d.drawString("No content loaded.", 200, 200);
The zoom method controlled by the mouse wheel
    //Zoom method reacting on a mousewheel event
    public void this_mouseWheelMoved(MouseWheelEvent e) {
        if (e.getWheelRotation() == 1) {
            scale -= ZOOM_FACTOR;
        } else if (e.getWheelRotation() == -1) {
            scale += ZOOM_FACTOR;
        height = (int) (orig_height * scale);
        width = (int) (orig_width * scale);
        setPreferredSize(new Dimension(width, height));
        revalidate();
        repaint();
Some set methods just to show that I have them.
    public void setOriginalHeight(int _orig_height) {
        orig_height = _orig_height;
    public void setOriginalWidth(int _orig_width) {
        orig_width = _orig_width;
    }TIA hope somebody is able to clarify some things for me.
Kenneth

Answer 1

Swing gives you doble buffering for free. Try this
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
public class ZoomScale extends JPanel implements MouseWheelListener {
    Block[] blocks;
    int origWidth, origHeight;
    double scale;
    final double ZOOM_FACTOR = 0.1;
    public ZoomScale() {
        scale = 1.0;
        addMouseWheelListener(this);
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        if(blocks == null)
            initGraphics();
        int w = getWidth(), h = getHeight();
        g2.scale(scale, scale);
        for(int j = 0; j < blocks.length; j++)
            blocks[j].draw(g2);
        g2.setPaint(Color.red);
        g2.drawLine(w, 0, w, h);
        g2.setPaint(Color.blue);
        g2.drawLine(0, h, w, h);
    public void mouseWheelMoved(MouseWheelEvent e) {
        if (e.getWheelRotation() == 1) {
            scale -= ZOOM_FACTOR;
        } else if (e.getWheelRotation() == -1) {
            scale += ZOOM_FACTOR;
        System.out.println("scale = " + scale);
        revalidate();
        repaint();
    public Dimension getPreferredSize() {
        return new Dimension((int)(scale * origWidth), (int)(scale * origHeight));
    private void initGraphics() {
        origWidth  = getWidth();
        origHeight = getHeight();
        double w = origWidth;
        double h = origHeight;
        double r = Math.min(w,h)/4;
        blocks = new Block[4];
        blocks[0] = new Block(new Line2D.Double(w/16, h/16, w*15/16, h*15/16),
                              Color.green.darker());
        blocks[1] = new Block(new Line2D.Double(w*15/16, h/16, w/16, h*15/16),
                              Color.orange);
        blocks[2] = new Block(new Rectangle2D.Double(w/16, h/16, w*7/8, h*7/8),
                              Color.blue);
        blocks[3] = new Block(new Ellipse2D.Double(w/2-r, h/2-r, 2*r, 2*r),
                              Color.red);
    public static void main(String[] args) {
        ZoomScale zs = new ZoomScale();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new JScrollPane(zs));
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
class Block {
    Shape s;
    Color color;
    Block(Shape s, Color color) {
        this.s = s;
        this.color = color;
    public void draw(Graphics2D g2) {
        g2.setPaint(color);
        g2.draw(s);
}