A servlet is accessible only when you map it to a particular URL in your WEB.XML file. It's very clear in the spec. Please look at the following example of Tomcat.
<servlet-mapping>
<servlet-name>invoker</servlet-name>
<url-pattern>/servlet/*</url-pattern>
</servlet-mapping>
This will map all URLs after "http://<host>/servlet" to the servlet "invoker". For your case, you have to add this portion to map your "HelloWorldExample" to an URL, such as "http://localhost:8080/servlet/hello".
<servlet-mapping>
<servlet-name>invoker</servlet-name>
<url-pattern>/servlet/*</url-pattern>
</servlet-mapping>
This will map all URLs after "http://<host>/servlet" to the servlet "invoker". For your case, you have to add this portion to map your "HelloWorldExample" to an URL, such as "http://localhost:8080/servlet/hello".