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null Pointer Exception from applet viewer  RSS feed

Siegfried Heintze
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After Craig Wood (in the Other API's forum) graciously provided me with a nice swing Java2D example, I wanted to run it is as an applet. I have succeeded in converting it -- sort of.

I'm getting the stack trace below when I run it in the applet viewer. (The stack trace is from the appletviewer, not my code).
Below that, I have posted my modification of Craig's swing example.

I suspect there is something wrong with the way I converted his swing example to an applet. Craig uses a JScrollPane in his main program and I wanted JScrollPane within the Applet. So I created an JApplet within the JScrollPain within a second JApplet. Was JApplet overkill for the inner applet: could I have used JPanel?

Thank you,

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: component argument pData

at Method)



at sun.awt.Win32GraphicsConfig.createSurfaceData(




at java.awt.event.InvocationEvent.dispatch(

at java.awt.EventQueue.dispatchEvent(

at java.awt.EventDispatchThread.pumpOneEventForHierarchy(

at java.awt.EventDispatchThread.pumpEventsForHierarchy(

at java.awt.EventDispatchThread.pumpEvents(

at java.awt.EventDispatchThread.pumpEvents(


Compilation finished at Sun Oct 08 10:33:09

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.swing.event.*;

* See
* From Craig Wood
* $Log:,v $
* Revision 1.2 2006/10/07 21:14:45 siegfried
* Add applet compatibility.
* Revision 1.1 2006/10/07 19:54:31 siegfried
* initial from Craig Wood
* Begin commands to execute this file using Java with bash
* export CLASSPATH=.
* javac -g
* javac -g
* cat <<EOF >PanAndZoomTmp.html
* <HTML>
* <HEAD>
* <TITLE>Pan And Zoom</TITLE>
* </HEAD>
* <H1>Graphics Only</H1>
* <HR>
* <H2>Example:</H2>
* <APPLET CODE="GraphicsOnly.class" WIDTH=500 HEIGHT=400 >
* <B>Error! You must use a Java enabled browser.</B>
* </BODY>
* </HTML>
* appletviewer PanAndZoomTmp.html
* rm PanAndZoomTmp.html
* # IEXPLORE file://PanAndZoom.html
* # java GraphicsOnly
* rm GraphicsOnly.class
* End commands to execute this file using Java with bash
public class GraphicsOnly extends JApplet {
class InnerGraphicsOnly extends JPanel implements ChangeListener {
Shape[] shapes;
Dimension size;
double scale = 1.0;
public InnerGraphicsOnly() {
size = new Dimension(10, 10);
setBackground(new Color(240, 200, 200));
public void stateChanged(ChangeEvent e) {
int value = ((JSlider) e.getSource()).getValue();
scale = value / 100.0;
protected void paintComponent(Graphics g) {
Graphics2D g2 = (Graphics2D) g;
if (shapes == null)
// Keep shapes centered on panel.
double x = (getWidth() - scale * size.width) / 2;
double y = (getHeight() - scale * size.height) / 2;
AffineTransform at = AffineTransform.getTranslateInstance(x, y);
at.scale(scale, scale);
g2.setPaint(new Color(240, 240, 200));
public Dimension getPreferredSize() {
int w = (int) (scale * size.width);
int h = (int) (scale * size.height);
return new Dimension(w, h);
private void initShapes() {
shapes = new Shape[3];
int w = getWidth();
int h = getHeight();
shapes[0] = new Rectangle2D.Double(w / 16, h / 16, w * 7 / 8, h * 7 / 8);
shapes[1] = new Line2D.Double(w / 16, h * 15 / 16, w * 15 / 16, h / 16);
shapes[2] = new Ellipse2D.Double(w / 4, h / 4, w / 2, h / 2);
size.width = w;
size.height = h;
private JSlider getControl() {
JSlider slider = new JSlider(JSlider.HORIZONTAL, 10, 400, 100);
return slider;

public void init(){
GraphicsOnly.InnerGraphicsOnly app = new InnerGraphicsOnly();
getContentPane().add(new JScrollPane(app));
getContentPane().add(app.getControl(), "Last");
public static void main(String[] args) {
GraphicsOnly outer = new GraphicsOnly();
GraphicsOnly.InnerGraphicsOnly app = InnerGraphicsOnly();
JFrame f = new JFrame();
f.getContentPane().add(new JScrollPane(app));
f.getContentPane().add(app.getControl(), "Last");
f.setSize(400, 400);
f.setLocation(200, 200);
Craig Wood
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I'm getting the stack trace below when I run it in the applet viewer.
I get the same exception although it doesn't occur everytime I close the appletviewer. This started in j2se 1.5 and did not happen in j2se 1.4. I never researched it and have not been able to find a way to avoid it; just figured someone would get it worked out eventually.
Was JApplet overkill for the inner applet: could I have used JPanel?
I haven't tried your code but probably, yes. An Applet is an extension of Panel (AWT) and a JApplet is and extension of Applet (Swing). So a JApplet can host a JPanel. What makes an Applet/JApplet special is that they are top–level containers.
Here is how I would create a JApplet to host the GraphicsOnly component:

The only change required in the GraphicsOnly file is relaxing the access modifier of the getControl method from private to protected so the applet can call/access it.
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