Running Java Programs from a Terminal

[Kuo000]

Hi all,

I am just new to Ubuntu (installed it a few days ago alongside my Windows XP laptop). I normally just program Java in Eclipse and never use a terminal to compile and run my Java programs. I tried to compile and run a hello world Java program from a terminal using the GNU compiler for Java (gcj) with the commands:

> javac HelloWorld.java (a HelloWorld.class is created after this command)
> java HelloWorld

But after the "java HelloWorld" command, I got:

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: main/HelloWorld)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java :634)
at java.security.SecureClassLoader.defineClass(Secure ClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader .java:277)
at java.net.URLClassLoader.access$000(URLClassLoader. java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java: 212)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.j ava:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:3 21)
at sun.misc.Launcher$AppClassLoader.loadClass(Launche r.java:294)
at java.lang.ClassLoader.loadClass(ClassLoader.java:2 66)
Could not find the main class: HelloWorld. Program will exit.

What's wrong with this? I think I do have the Java runtime (JavaSE-1.6 and java-6-openjdk) installed with the installation of Eclipse. My HelloWorld program simply prints a "Hello World" in the console and runs perfect in my Eclipse. Do I need to manually set the classpath for the java runtime for the program to run from a terminal, and if so how???

Thanks very much for any help.

[trent.josephsen]

My Java is rusty, but I do believe that's caused by a faulty package line in the .java file. "package main;" perhaps? Delete it and everything should be fine.

[Barrucadu]

Is your class called "HelloWorld"? Show the source code.

[Kuo000]

My Java is rusty, but I do believe that's caused by a faulty package line in the .java file. "package main;" perhaps? Delete it and everything should be fine.

Thank you. I could run the HelloWorld using command "java HelloWorld" once I deleted the line "package main;" in the source code.

But what if I actually want to put some java files into one package and have another java file import that package? Then how should I compile and run the program in this case?

[jdonnell]

you should use either ant or maven. Ultimately you have to learn how the java class path works.

http://download.oracle.com/javase/1.5.0/docs/tooldocs/windows/classpath.html
http://ant.apache.org/
http://maven.apache.org/

[trent.josephsen]

The "package" line prepends "main." to the class name. In order to call the class with the java binary, you must call it by its full name:

java main.HelloWorld
But java won't look in the current directory for that class (./HelloWorld.class); it looks for ./main/HelloWorld.class.

The simple and quick fix is to put all the .class files that belong in the package main into the directory main.

With the "package main" line in HelloWorld.java, this works:


$ mkdir main
$ mv HelloWorld.java main
$ javac main/HelloWorld.java
$ java main.HelloWorld

For all but the simplest programs, you should consider an automated make solution like ant. This should get you started though!

[spearfish]

I had a similar problem with a new install of Java on a new laptop. My classpath was not set right.

My fix was to go into the .bashrc file and make sure the . (dot) directory was in the classpath so that java could see the file I had just compiled in my current directory.

In my .bashrc file there are a few lines that look like this:

# added for Java
JAVA_HOME=/home/username/jdk1.6.0_23
PATH=$JAVA_HOME/bin:$PATH
CLASSPATH=$JAVA_HOME/lib:.

The little addition of the :. was what made the JDK tool "java" go to the current directory. :)

Before, if I typed:
$ javac helloworld.java

and then

$ java hello
it would say there was an error loading the class, because it didn't know where to look. When you add . to the CLASSPATH it knows where to search for classes you just compiled.

[davmaz]

The "package" line prepends "main." to the class name. In order to call the class with the java binary, you must call it by its full name:

java main.HelloWorld
But java won't look in the current directory for that class (./HelloWorld.class); it looks for ./main/HelloWorld.class.

The simple and quick fix is to put all the .class files that belong in the package main into the directory main.

With the "package main" line in HelloWorld.java, this works:


$ mkdir main
$ mv HelloWorld.java main
$ javac main/HelloWorld.java
$ java main.HelloWorld

For all but the simplest programs, you should consider an automated make solution like ant. This should get you started though!

I thought I had finally seen a solution to what I thought was not having CLASSPATH set correctly here. But even this does not work on my machine. It still prints:
13:41 ~ > javac main/HelloWorld.java
13:49 ~ > java main.HelloWorld
Exception in thread "main" java.lang.NoClassDefFoundError: main/HelloWorld (wrong name: HelloWorld)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader. java:632)
at java.lang.ClassLoader.defineClass(ClassLoader.java :616)
at java.security.SecureClassLoader.defineClass(Secure ClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader .java:283)
at java.net.URLClassLoader.access$000(URLClassLoader. java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java: 197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.j ava:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:3 07)
at sun.misc.Launcher$AppClassLoader.loadClass(Launche r.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:2 48)
13:49 ~ >

This is driving me nuts!

[trent.josephsen]

That's because you deleted the "package main;" line. When you moved it to a subdirectory, you caused the opposite of your original problem: you can now find the file main/HelloWorld.class, but since it's internally labeled as just HelloWorld (not main.HelloWorld) java says "Wait, something's gone wrong here, this class's internal name doesn't match where I found it in the filesystem."

It makes a twisted amount of sense if you don't think about it too much. (This is one of the reasons I don't like Java as a first language.)

Sorry for the late response, I've been incommunicado :/