Ok, credo che tu compili e fai partire i file java dal Prompt dei Comandi (cmd)... Io ho appena provato a farlo e mi da un errore, questo:

Exception in thread "main" java.lang.NoClassDefFoundError: boxE (wrong name: Box
DemoE/main/boxE)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknow n Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.launcher.LauncherHelper.checkAndLoadMain(Unkno wn Source)

Immagino sia lo stesso che da a te...
Mi confermi che usi il cmd?