j0k3r@j0k3r:~$ javac helloservlet.java -classpath "/usr/local/apache-tomcat-7.0.47/lib/servlet-api.jar"
helloservlet.java:4: ']' expected
public class HelloServlet extends GenericServlet [  // HelloServlet is a subclass GenericServlet. GenericServlet class provides functionality that simplifies the creation of servlet. It provides versions of init() and destroy(). You need to supply only the services() methods //
                                                  ^
1 error
j0k3r@j0k3r:~$

My Java Programe is:
import java.io.*;
import javax.servlet.*;  // This Packages contains the classess and interfaces required to build servlets //

public class HelloServlet extends GenericServlet [  // HelloServlet is a subclass GenericServlet. GenericServlet class provides functionality that simplifies the creation of servlet. It provides versions of init() and destroy(). You need to supply only the services() methods //
	public void service (ServletRequest request, ServletResponse response)
throws ServletException, IOException {
	response.setContentType("text/html");
	printWriter pw = response.getWriter();
	pw.println ("<B>Hello!");
	pw.close();
	}
}