Java Applet question

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yingjian.ma1955@gmail.com - 25 Apr 2006 01:06 GMT

I have a Java applet called TestColor that does not work. Here is the
code.

TestColor.java code:

import java.awt.*;
import java.applet.*;
public class TestColor extends Applet {
    String s1;
public void init() {
    Color c;
    s1 = getParameter("mycolor");
    if (s1.equals("blue"))
        c = Color.blue;
    else if (s1.equals("red"))
        c = Color.red;
    else if (s1.equals("green"))
        c = Color.green;
    else
        c = Color.cyan;
    setBackground(c); }}

Html code:

<HTML>
<HEAD>
<TITLE>A Simple Program</TITLE>
</HEAD>
<BODY>
<h2>Here is the applet:</h2><br>
<APPLET CODE="TestColor.class" WIDTH=550 HEIGHT=500 alt="white">
<param name="mycolor" value="blue">
Sorry, you aren't running a Java-capable browser.
</APPLET>
</BODY>
</HTML>

If I change mycolor to adjective1 in both files. It works. Could you
kindly try it and tell me why?

When I run it in a debugger, I got this msg:

java.lang.NullPointerException
    at TestColor.init(TestColor.java:9)
    at sun.applet.AppletPanel.run(AppletPanel.java:378)
    at java.lang.Thread.run(Thread.java:595)
Warning: classic VM not supported; client VM will be used.

What is the bug?

Thanks a lot.

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Amfur Kilnem - 25 Apr 2006 02:20 GMT

>I have a Java applet called TestColor that does not work. Here is the
> code.
[quoted text clipped - 45 lines]
>
> What is the bug?

You are not checking the return value of getParameter(). If "mycolor" isn't
found, the returned value will be null, and the very next line will cause
the null pointer exception.

As to why it appears not to be working with "mycolor", maybe you have a
stale applet and/or HTML in your browser...?

Reply to this Message

yingjian.ma1955@gmail.com - 25 Apr 2006 18:29 GMT

Thanks.

I tried it on another PC. It works. So it is my PC's problem. The
two PCs have XP with IE6 and Java 1.5. But the working one has AMD
cpu. The other has Intel cpu. Here is the problem with this "bad" PC.

If I use the word such as mycolor, it displays a gray box. When I move
the vertical scroll bar, the blue color flashes.
If I use the word adjective1, it displays a blue box, which is correct.

I cleaned the catch already. What can I do to fix it?

Reply to this Message

yingjian.ma1955@gmail.com - 25 Apr 2006 18:59 GMT

Sorry, it is not the word causing the problem. The code is different.
The original code works. Here is the code that has problem:

import javax.swing.*;
import java.awt.*;
public class TestColor extends JApplet {
    String s1;
public void init() {
    Color c;
    s1 = getParameter("mycolor");
    if (s1.equals("blue"))
        c = Color.blue;
    else if (s1.equals("red"))
        c = Color.red;
    else if (s1.equals("green"))
        c = Color.green;
    else
        c = Color.cyan;
    setBackground(c); }}

So it looks like JApplet does not work in this code.

Reply to this Message

Dag Sunde - 25 Apr 2006 20:12 GMT

> Sorry, it is not the word causing the problem. The code is different.
> The original code works. Here is the code that has problem:
[quoted text clipped - 17 lines]
>
> So it looks like JApplet does not work in this code.

You say both PC's have Java 1.5, but do both have the Java *plugin*
installed for IE?

And if so, is "Use Sun Java plugin" enabled in IE on both PS's?

Signature

Dag.

Reply to this Message

yingjian.ma1955@gmail.com - 26 Apr 2006 19:11 GMT

The 3rd post was not correct. Both PCs have the same behavior. The
Applet class works and the JApplet class does not work. Could you try
it to tell me how to fix it?

Thanks.

Reply to this Message

Amfur Kilnem - 26 Apr 2006 20:43 GMT

> The 3rd post was not correct. Both PCs have the same behavior. The
> Applet class works and the JApplet class does not work. Could you try
> it to tell me how to fix it?
>
> Thanks.

Try this:

import java.awt.*;
import javax.swing.*;
public class TestColor extends JApplet {
String s1;
public void init() {

Container contentPane = getContentPane();
Color c;
s1 = getParameter("mycolor");
if (s1.equals("blue"))
c = Color.blue;
else if (s1.equals("red"))
c = Color.red;
else if (s1.equals("green"))
c = Color.green;
else
c = Color.cyan;
contentPane.setBackground(c);
}
}

Reply to this Message

yingjian.ma1955@gmail.com - 26 Apr 2006 23:28 GMT

It works. Thank you very much.

Could you tell me why my JApplet did not work?

Reply to this Message

Amfur Kilnem - 26 Apr 2006 23:43 GMT

> It works. Thank you very much.
>
> Could you tell me why my JApplet did not work?

In a JApplet, everything must be put into the contentPane, which obscures
the underlying Applet.

Reply to this Message

Oliver Wong - 27 Apr 2006 19:00 GMT

> The 3rd post was not correct. Both PCs have the same behavior. The
> Applet class works and the JApplet class does not work. Could you try
> it to tell me how to fix it?

It sounds like you're misunderstanding how parameters work. Try reading
through http://java.sun.com/docs/books/tutorial/deployment/applet/param.html

- Oliver

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