Exception : Failed to load application class: com.example.hellovaadin.Hell

hi i am beginner in vaadin framework , in first step i have done all the configuration with apachi tomcat ..after that i run the helloworld vaadin program and got this error
...................................................................................

Mar 13, 2012 1:44:49 AM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Allocate exception for servlet Hellovaadin Application
javax.servlet.ServletException: Failed to load application class: com.example.hellovaadin.HellovaadinApplication
at com.vaadin.terminal.gwt.server.ApplicationServlet.init(ApplicationServlet.java:71)
at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1206)
at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:827)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:129)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:602)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Unknown Source)

.........................................my code is....................................................

package com.example.hellovaadin;

import com.vaadin.Application;
import com.vaadin.ui.*;

public class HellovaadinApplication extends Application {
@Override
public void init() {
Window mainWindow = new Window("Hellovaadin Application");
Label label = new Label("Hello Vaadin user");
mainWindow.addComponent(label);
setMainWindow(mainWindow);
}

}

..............................................my web.xml file is ...........................................................

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<display-name>HelloVaadin</display-name>
<context-param>
<description>Vaadin production mode</description>
<param-name>productionMode</param-name>
<param-value>false</param-value>
</context-param>
<servlet>
<servlet-name>Hellovaadin Application</servlet-name>
<servlet-class>com.vaadin.terminal.gwt.server.ApplicationServlet</servlet-class>
<init-param>
<description> Vaadin application class to start</description>
<param-name>application</param-name>
<param-value>com.example.hellovaadin.HellovaadinApplication</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>Hellovaadin Application</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file>
</welcome-file-list>
</web-app>