What's wrong with this Applet?

[lsaffre]

Hi,

I wrote the following Applet. When I run it in a browser, it shows the "Start" button,
but when I click on it, I get a java.lang.NullPointerException.
Even the "Hello?" doesn't get displayed. What am I doing wrong?
Thanks for any hint!

Luc

Here's the Applet code:

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;
import java.awt.Desktop;
import java.io.File;
import java.io.IOException;

public class StartFile extends Applet implements ActionListener {
  public Desktop desktop;
  public Button btn;
  public void init() {
    btn = new Button("Start");
    add(btn);
    btn.addActionListener(this);
  }
  public void actionPerformed(ActionEvent event) {
    System.out.println("Hello?"); 
    //~ if (Desktop.isDesktopSupported()) {}
    if (event.getSource() == btn) {
      String fileName = "c:\\temp\\foo.txt"; // txtFile.getText();
      System.out.println(fileName); 
      File file = new File(fileName);
      try {
        desktop = Desktop.getDesktop();
        desktop.open(file);
        //~ desktop.edit(file);
        //~ desktop.print(file);
      } catch (IOException ioe) {
        ioe.printStackTrace();
      }
    }
  }
}

Here's the Java Console output:

Exception in thread "AWT-EventQueue-11" java.lang.NullPointerException
	at StartFile.actionPerformed(StartFile.java:27)
	at java.awt.Button.processActionEvent(Unknown Source)
	at java.awt.Button.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
	at java.awt.EventQueue.access$000(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)

[Fubarable]

Which line is line 27 in your StartFile code as noted by: "StartFile.java:27"?
And how do you know that "Hello" isn't displayed? Have you inspected a Java console?

[pbrockway2]

So, which is line 27?

------

In general the NullPointerException occurs when you dereference (use) variable as if it had a non null value when, in fact, it was null.

arr[bar] = baz; // but arr is null
foo.meth(arg); // but foo is null

Once you have identified the null variable (or expression) you have to look at where you assigned it a value that you thought - or hoped!- was non null. Then figure out why that didn't happen. The javadocs are your friend here as some methods do return null.

[Norm]

You have a println first in the actionPerformed method that should have printed in the Java console.
What isn't that printout shown in your post of the java console contents?

[lsaffre]

Oops, the problem wasn't in the code at all!

It seems that I must restart my browser each time I recompiled a Java applet. I didn't notice this yesterday evening. This morning the applet worked as expected, displaying "Hello?" and "c:\temp\foo.txt" in the Java Console.

I guess I should rather use applet viewer when testing applets.

Thanks for your replies which helped me indirectly to find the explanation.

Luc