retreive integer data from database

i made a table named result having two fields 1>rollno 2>marks i creates a gui where i use a textfield and two buttons "ok" and "cancel" i want to take input from textfield as an integer(rollno) and the marks associated with that must be displayed.i write the following code but the error appears is "no specific value for parameter 1" so please suggest me where t am wrong. coding written in "ok" button

public void actionPerformed(ActionEvent e){ if(e.getSource()==b1){ try{

       Class.forName("com.mysql.jdbc.Driver");
      Connection cn=DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb" +

"?user=root&password=lalit&database=mydb");

      String url="select marks from student where rollno=?";
      PreparedStatement stmt=cn.prepareStatement(url);
      ResultSet rs=stmt.executeQuery();
 stmt.setString(1,tf1.getText().toString());

int n=Integer.parseInt(rs.getString(2));

System.out.println("Your marks is "+ n); stmt.close(); cn.close(); } catch(Exception ex){ JOptionPane.showMessageDialog(null, ex);

                      }

                                         }

if(e.getSource()==b2){ this.dispose();

              }

}

}