Javac Error Message Interpretation

[Java_Lover]

Hello All.

I am reading the book Java in easy steps and I am getting a javac error when compiling the code in the book. Here's the error:

The serializable class HelloWorldApplet does not declare a static final serialVersionUID field of type long
public class HelloWorldApplet extends JApplet

The code:

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

public class HelloWorldApplet extends JApplet
{
	String message;
	
	public void init()
	{
		message = "Hello World2";
	}

	public void paint(Graphics artist)
	{
	
		artist.drawString(message, 20,30);
	}

}

This is placed in a BARE BONES html document. My question is what does the error mean?

[pbrockway2]

First observe that this is not an error, nor is it reported by the javac compiler.

C:\Documents and Settings\pbrockway2\Desktop>type HelloWorldApplet.java
import javax.swing.*;
import java.awt.*;

public class HelloWorldApplet extends JApplet
{
        String message;

        public void init()
        {
                message = "Hello World2";
        }

        public void paint(Graphics artist)
        {

                artist.drawString(message, 20,30);
        }

}
C:\Documents and Settings\pbrockway2\Desktop>javac -cp . HelloWorldApplet
.java

C:\Documents and Settings\pbrockway2\Desktop>

You can get the compiler to report this message by using one of the -X options.

C:\Documents and Settings\pbrockway2\Desktop>javac -Xlint -cp . HelloWorl
dApplet.java
HelloWorldApplet.java:4: warning: [serial] serializable class HelloWorldApplet h
as no definition of serialVersionUID
public class HelloWorldApplet extends JApplet
       ^
1 warning

C:\Documents and Settings\pbrockway2\Desktop>

So, it's a warning not an error. And you can basically ignore it. (Or compile from the command line in which case ... you never see it).

--------------

There is a description of what this (not really "New to Java") problem is at stackoverflow. Where the recommend telling your IDE to ignore the lack of id, or to autogenerate one.

[Java_Lover]

Thanks for your reply. I do see now where it's just a warning and not an error. My code did run as expected. I did look at the link and attempt to decipher the thread - I've still got the training wheels on so I am not grasping everything but do want to immerse myself.

[pbrockway2]

That's a good approach. You should be curious about messages as they always mean *something* even if, as in this case, you can ignore the warning. Your book should really have anticipated this situation (especially if you are using an IDE it recommends).