Java on Ubuntu 11.10 ( Error )

[Ahmad El-Komey]

Hello,

I have installed JDK 7:

sudo apt-get install openjdk-7-jdk

also, I have installed jre:

sudo apt-get install openjdk-7-jre

I opened my text editor ( Geany ) and created a new file, named it "test.java" and started to salute the world:

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

I saved the file on the desktop, so:

cd Desktop
javac test.java
java test.class

and now here is the result:

Exception in thread "main" java.lang.NoClassDefFoundError: test/class
Caused by: java.lang.ClassNotFoundException: test.class
	at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: test.class. Program will exit.

any help?
thanks in advance ..

[Vijay Kalkundri]

Try this

cd Desktop  
javac test.java  
java test

Regards,
Vijay

[Campbell Ritchie]

Welcome to the Ranch, both of you

I would suggest the desktop is not a good place to keep programs. Least of all if you use Unity with those big icons! I think there is a better place to put your work, as I suggested here.

[Ahmad El-Komey]

@ Vijay Kalkundri: I did try that too. Still the same error shows up

@ Campbell Ritchie: Thanks for the greeting

I have uninstalled both JDK7 and JRE7:

sudo apt-get purge openjdk-7-jdk

sudo apt-get purge openjdk-7-jre

and re-installed them, but nothing happened .. the same error shows up

one of my friends suggested to install netbeans and things will turn to be OK. And for my surprise, he was right!!
I still don't know what happened in the underlying OS, and that means I need to read more and more about linux and Ubuntu.

Thanks guys for your concern

[Jeff Verdegan]

Ahmad El-Komey wrote:@ Vijay Kalkundri: I did try that too. Still the same error shows up

No, it can't be the same error.

The next step would be to try this:

cd Desktop    
javac test.java    
java -cp . test  

one of my friends suggested to install netbeans and things will turn to be OK. And for my surprise, he was right!!

The only way that could have made a difference in the above suggested command is if the installation set your CLASSPATH environment variable to include . ("dot", the pwd).

Note that you shouldn't rely on the CLASSPATH env var, but instead should specify the classpath on the command line.

[Campbell Ritchie]

It is worth trying the following instructions at a terminal (root or sudo access not required):

which java
which javac
echo $PATH
echo $CLASSPATH

If you get no response to the which instructions, you will have to search for the java tool. You should see what the PATH points to, and it is best to have an empty system CLASSPATH.

[Campbell Ritchie]

Ahmad El-Komey wrote: . . . one of my friends suggested to install netbeans . . .

That is not usually good advice. Learning NetBeans is difficult and may distract you from learning Java™.

[Ahmad El-Komey]

Campbell Ritchie wrote:It is worth trying the following instructions at a terminal (root or sudo access not required):

which java
which javac
echo $PATH
echo $CLASSPATH

If you get no response to the which instructions, you will have to search for the java tool. You should see what the PATH points to, and it is best to have an empty system CLASSPATH.

here's the output - after netbeans installation -

~$ which java
/usr/bin/java
~$ which javac
/usr/bin/javac
~$ echo $PATH
/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
~$ echo $CLASSPATH

the last echo command has no output.

[Jesper de Jong]

Ahmad El-Komey wrote:
I saved the file on the desktop, so:

cd Desktop
javac test.java
java test.class

and now here is the result:
...

You get that error because to run the program, you must pass the class name, which is test, not the filename test.class to the java command.

So it must be: java test
and not: java test.class

[Campbell Ritchie]

That PATH and the results of the which instructions suggest you are using the Java™ installation which comes with Ubuntu. If you look in our FAQ, it explains about OpenJDK and the Sun/Oracle version. I don’t know how to use apt to install the Sun/Oracle JDK7, but you can use the instructions here. If you do that, remember you will need to set a PATH, as desribed in that FAQ I showed you.