Help with NullPointerException

[Daniel Hollenbeck]

Hey All,

I keep getting a NullPointerException when I try to load
an image in my application.

C:\classes>java ImageApplet
java.lang.NullPointerException
at java.applet.Applet.getDocumentBase(Applet.java:59)
at ImageApplet.init(ImageApplet.java:14)
at ImageApplet.main(ImageApplet.java:44)

My simple test code is below.

Any ideas what I am doing wrong?

Thanks in advance.

Dan
holle...@mailbag.com

import java.awt.*;
import java.applet.*;

public class ImageApplet extends Applet
{
Image logo;
private MediaTracker tracker;

public void init()
{
tracker = new MediaTracker (this);
logo = getImage (getDocumentBase (), "logo.gif");

try
{
tracker.waitForAll ();
}
catch (InterruptedException e)
{
return;
}
setBackground (Color.black);
}

public void paint(Graphics g)
{
int width = logo.getWidth(this);
int height = logo.getHeight(this);

g.drawRect(52, 52, width+10, height + 10);
g.drawImage(logo, 57, 57, width, height, this);
}

public static void main(String args[])
{
ImageApplet app = new ImageApplet();
Frame frame = new Frame("Hello world!");

app.init();
app.start();

frame.add("Center", app);
frame.resize(400,400);
frame.show();
}
}

[Mark Wutka]

Daniel Hollenbeck (holle...@mailbag.com) wrote:

: I keep getting a NullPointerException when I try to load

: an image in my application.

: C:\classes>java ImageApplet
: java.lang.NullPointerException
: at java.applet.Applet.getDocumentBase(Applet.java:59)
: at ImageApplet.init(ImageApplet.java:14)
: at ImageApplet.main(ImageApplet.java:44)

: public void init()

: {
: tracker = new MediaTracker (this);
: logo = getImage (getDocumentBase (), "logo.gif");

: try
: {
: tracker.waitForAll ();
: }

It may be upset because you forgot to add the image to the
media tracker. You need a call like:
tracker.addImage(logo, 0);
after your getImage.

Mark
--
_________________________________________________________
Mark Wutka wu...@netcom.com
Author of "Hacking Java: The Java Professional's Resource Kit"
ISBN: 0-7897-0935-X (formerly titled "Java Expert Solutions")

[Doug Bell]

holle...@mailbag.com wrote:

> I keep getting a NullPointerException when I try to load
> an image in my application.

[snip]

>
> import java.awt.*;
> import java.applet.*;
>
> public class ImageApplet extends Applet
> {
> Image logo;
> private MediaTracker tracker;
>

> public void init()
> {
> tracker = new MediaTracker (this);
> logo = getImage (getDocumentBase (), "logo.gif");
>
> try
> {
> tracker.waitForAll ();
> }

> catch (InterruptedException e)
> {
> return;
> }
> setBackground (Color.black);
> }
>
> public void paint(Graphics g)
> {
> int width = logo.getWidth(this);
> int height = logo.getHeight(this);
>
> g.drawRect(52, 52, width+10, height + 10);
> g.drawImage(logo, 57, 57, width, height, this);
> }
>
> public static void main(String args[])
> {
> ImageApplet app = new ImageApplet();
> Frame frame = new Frame("Hello world!");
>
> app.init();
> app.start();
>
> frame.add("Center", app);
> frame.resize(400,400);
> frame.show();
> }
> }

The NullPointerException is caused by a reference to the AppletStub, which
hasn't been set. The AppletStub is normally setup by the browser or
AppletViewer, along with placing the Applet in a Frame. If your
instantiate your own Applets, you must do both of these steps.

Doug Bell
db...@shvn.com

[Shawn Bertrand]

Daniel Hollenbeck wrote:
>
> Hey All,

>
> I keep getting a NullPointerException when I try to load
> an image in my application.
>

> C:\classes>java ImageApplet
> java.lang.NullPointerException
> at java.applet.Applet.getDocumentBase(Applet.java:59)
> at ImageApplet.init(ImageApplet.java:14)
> at ImageApplet.main(ImageApplet.java:44)
>

> My simple test code is below.
>
> Any ideas what I am doing wrong?
>
> Thanks in advance.
>
> Dan
> holle...@mailbag.com
>

Daniel,

I wasn't aware that an applet written as an application (i.e. with a
main() function) needed to have its init() and start() functions
called. I would try taking those calls out to see what happens.

I have received a NullPointerException on a call to getDocumentBase (or
getCodeBase) before, but it was happening because I was calling it in
the constructor of the applet: when the document base and code base had
not yet been defined.

Therefore, I can only assume that by calling init() before the applet is
shown will cause the NullPointerException to occur because the
underlying applet stub code did not have a chance to initialize the
document base.

Good luck,

Shawn Bertrand
Software Engineer, Oracle Corporation
E-mail: sber...@us.oracle.com

Disclaimer: The statements and opinions expressed here are my own and
do not necessarily represent those of Oracle Corporation.