For Example:

//input.jsp
<form name="input" action="/servlet/validation" method="Post">
<table border=0>
<tr>
    <td>Input your name</td>
    <td><input type="text" name="name"></td>
    <td><input type="submit" value="Submit"></td>
</tr>
</table>
</form>

//web-inf/classes/
/* file name = "validation.java" */
import java.io.*;
import javax.servlet.http.*;
import javax.servlet.*;

public class validation  extends HttpServlet{
    public void doPost(HttpServletRequest request,HttpServletResponse response)
 throws ServletException,IOException{
  String name = request.getParameter("name");
  if (name==null){
   name="";
  }
if (name.equals(""){
 RequestDispatcher.getRequestDispatcher("error.jsp").forward(request,response); }
else{
 PrintWriter out=response.getWriter();
 out.print("Hello " +name);
}}}