running java on my Ubuntu 6.06 machine

mohtasham1983 · 10-26-2006, 05:54 AM

I've just installed sun-java5-jre and sun-java5-jdk from synaptic and followed instruction in this site: https://help.ubuntu.com/community/Java

After doing all steps I just tried to run a simple Hello World program.

Code:

public class HelloWorld
{
	public static void main(String [] args)
	{
		System.out.println("Hello, World");
	}
}

I did javac HwlloWorld.java and made HelloWorld.class file.
After running "java HelloWorld.class" I got the following error:
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld/class

Does anyone know what's wrong with jave on my machine?

trevelluk · 10-26-2006, 07:27 AM

Nothing's wrong

You should run "java HelloWorld" without the .class extension.

Hope this helps.

mohtasham1983 · 10-26-2006, 03:53 PM

I got the same error after running java HelloWorld.java

demon_vox · 10-26-2006, 04:06 PM

I think that you need to put your HelloWorld.java in a directory which is in the CLASSPATH. You should check what your CLASSPATH variable says.

Cheers!

trevelluk · 10-26-2006, 04:59 PM

Ah, it's not "java HelloWorld.java" either. Just "java HelloWorld"

demon_vox is right, the file does have to be in a directory in your CLASSPATH, but the current directory (i.e. ".") should always be in this. It can't hurt to check though.

mohtasham1983 · 10-26-2006, 06:57 PM

WOW java HelloWorld works.
Do you know why it is like this? I was told that to run java files one must make a binary file which is readable by java virtual machine and then run the program. But in this case it seems to me I can run this program like an interpreted language.

Anyway thank you very much

jlliagre · 10-26-2006, 11:09 PM

Quote:

Originally Posted by mohtasham1983

WOW java HelloWorld works.
Do you know why it is like this? I was told that to run java files one must make a binary file which is readable by java virtual machine and then run the program.

It is indeed the case.
"java HelloWorld" is using HelloWorld.class, which is a binary (bytecode) file.

Quote:

But in this case it seems to me I can run this program like an interpreted language.

It's not the case. Java isn't interpreted.

trevelluk · 10-27-2006, 07:16 AM

Quote:

Do you know why it is like this?

It's the way Java organises binary files. If you're writing a more complex program, with several classes, you can organise the classes into groups, called packages, to help keep things more organised. A dot is used to separate package and class names, so what's happening when you run "java HelloWorld.class", is Java is looking for a class called "class", in a package called "HelloWorld". Which, of course, doesn't exist. By running "java HelloWorld", then Java will look for a class called "HelloWorld" in the default package, which is what you want.