Error: Could not find or load main class(Java)

You have to add the following entry to your eclipse.ini file:

-vm
C:/Prog/Java/jdk1.6.0_21/jre/bin/server/jvm.dll

Make sure you replace the version number with yours and backup the original eclipse.ini file in case of trouble and copy the lines above into the file BEFORE the --vmargs argument. If not, Eclipse will crash.

Follow this link for more information: http://www.eclipse.org/forums/index.php?t=msg&goto=559997&S=a4836b4add35f1ac144df946cb1ef64d

If the class is in a package:

package thepackagename;

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

Then calling it from the wrong directory results in:

Error: Could not find or load main class thepackagename.TheClassName

It must be called with its fully-qualified name:

java -classpath . thepackagename.TheClassName

And this command must be called from the same directory in which the thepackagename directory exists. This means you have to create a new directory called thepackagename and move TheClassName.class into it, then run the above command from the parent directory of the thepackagename directory.