Exception in thread "main" java.lang.NoClassDefFoundError

A friend of mine wanted to start to learn java, and I wanted to help him, so we created a simple Hello World example.

public class Hello {

public static void main(String argv[]) {

System.out.println("Hello world !");

}

}

then, when you compile it:

> javac Hello.java

and want to execute it:

> java Hello

Exception in thread "main" java.lang.NoClassDefFoundError: Hello

We set the CLASSPATH and JAVA_HOME enviroment variable correctly, and the problem was still the same..

When we copied the Hello.class into a lib directory of JAVA_HOME, it worked, but not from the current directory !

We tried different versions of java.. and do you know where this stupid problem is?

in the CLASSPATH there should be:
CLASSPATH=.;c:\programme\jboss\lib;c:\programme\
j2sdk1.4.2_08\lib;c:\programme\j2sdk1.4.2_08\jre\lib; ...

yes.. a dot .. it means, that it will search for classes bytecode in the current directory !

you can also tell java to search the current directory with the -classpath parameter

> java -classpath . Hello

Problems like this can really discourage java newbies, and that moment was very embarassing :)

Problem:
Basically, the Exception in thread "main" java.lang.NoClassDefFoundError:

means, that the class which you are trying to run was not found in the classpath.

Solution: you need to add the class or .jar file which contains this class into the java classpath. When you are running a java class from the command line, you need to add the dot (.)

java YourSingleClass -cp . 

into the classpath which tells the JVM to search for classes in actual directory.

If you are running a class from a .jar file, you need to add this jar file into the classpath:

java org.somepackage.SomeClass -cp myJarWithSomeClass.jar