java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:

I got an exception when running the Server class(using java server command) as shown here (I have already start rmiregistry, using start rmiregistry command). What could be the problem?

C:\Users\william\Desktop\HelloWorld\Server>java Server
java.rmi.ServerException: RemoteException occurred in server thread; nested exce
ption is:
        java.rmi.UnmarshalException: error unmarshalling arguments; nested excep
tion is:
        java.lang.ClassNotFoundException: Hello
        at sun.rmi.server.UnicastServerRef.oldDispatch(UnicastServerRef.java:419
)
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:267)
        at sun.rmi.transport.Transport$1.run(Transport.java:177)
        at sun.rmi.transport.Transport$1.run(Transport.java:174)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:173)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:5
53)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTranspor
t.java:808)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport
.java:667)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.
java:1110)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor
.java:603)
        at java.lang.Thread.run(Thread.java:722)
        at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(Unknow
n Source)
        at sun.rmi.transport.StreamRemoteCall.executeCall(Unknown Source)
        at sun.rmi.server.UnicastRef.invoke(Unknown Source)
        at sun.rmi.registry.RegistryImpl_Stub.bind(Unknown Source)
        at Server.main(Server.java:61)
Caused by: java.rmi.UnmarshalException: error unmarshalling arguments; nested ex
ception is:
        java.lang.ClassNotFoundException: Hello
        at sun.rmi.registry.RegistryImpl_Skel.dispatch(Unknown Source)
        at sun.rmi.server.UnicastServerRef.oldDispatch(UnicastServerRef.java:409
)
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:267)
        at sun.rmi.transport.Transport$1.run(Transport.java:177)
        at sun.rmi.transport.Transport$1.run(Transport.java:174)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:173)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:5
53)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTranspor
t.java:808)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport
.java:667)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.
java:1110)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor
.java:603)
        at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.ClassNotFoundException: Hello
        at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:264)
        at sun.rmi.server.LoaderHandler.loadProxyInterfaces(LoaderHandler.java:7
30)
        at sun.rmi.server.LoaderHandler.loadProxyClass(LoaderHandler.java:674)
        at sun.rmi.server.LoaderHandler.loadProxyClass(LoaderHandler.java:611)
        at java.rmi.server.RMIClassLoader$2.loadProxyClass(RMIClassLoader.java:6
46)
        at java.rmi.server.RMIClassLoader.loadProxyClass(RMIClassLoader.java:311
)
        at sun.rmi.server.MarshalInputStream.resolveProxyClass(MarshalInputStrea
m.java:257)
        at java.io.ObjectInputStream.readProxyDesc(ObjectInputStream.java:1549)
        at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1511)
        at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1
750)
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1347)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:369)
        ... 13 more