Exibindo os valores das check boxes marcadas em um formulário

<form name="cadastro" action="EstudosServlet" method="post">
  Suas linguagens favoritas:<br>
  <input name="java" type="checkbox" id="java" 
    value="Java">Java<br>
  <input name="php" type="checkbox" id="php" 
    value="PHP">PHP<br>
  <input name="python" type="checkbox" id="python" 
    value="Python">Python<br>
  <br>
  <input type="submit" name="Submit" value="Enviar">
</form>

<input name="python" type="checkbox" id="python" 
  value="Python">Python<br>

package estudos;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class EstudosServlet extends HttpServlet{
  public void doPost(HttpServletRequest req,
    HttpServletResponse res) throws ServletException,
    IOException{

    PrintWriter pw = res.getWriter();

    // obtém os valores selecionados na lista
    String java = req.getParameter("java");
    String php = req.getParameter("php");
    String python = req.getParameter("python");
    
    pw.println("As linguagens marcadas foram:" + "<br>");    

    if(java != null)
      pw.println(java + "<br>");
    if(php != null)
      pw.println(php + "<br>");
    if(python != null)
      pw.println(python + "<br>");
  }
}