请教为什么感觉代码木有有关问题，而且数据库中也有数据，可是却查不出来啊

import java.util.*;import java.sql.*;public class InputStudent  {    public static void main(String args[]) {              Connection con;        String sql;        Statement stmt;        int num;       String name,sex;       int age;//一、java直接连接Oracle10数据库               try {                    Class.forName("oracle.jdbc.driver.OracleDriver");                } catch(java.lang.ClassNotFoundException e) {           System.err.print("ClassNotFoundException: ");        }        try {             con = DriverManager.getConnection("jdbc:oracle:thin:@127.0.0.1:1521:ORCL","system","system");               stmt = con.createStatement();       //写 select语句            sql = " SELECT * FROM S where sex=?";             PreparedStatement stmt0=con.prepareStatement(sql); //   stmt0=con.prepareStatement(sql);    stmt0.setString(1,"女");            //执行select SQL语句,返回结果集rs,可以看成是游标               ResultSet rs = stmt0.executeQuery();          //    ResultSet rs = stmt.executeQuery(sql);    //按格式要求打印输出表头部分                   System.out.println("\n\n\n\n");            System.out.println("      全体学生信息表");            System.out.println("--------------------------------");            System.out.println("学号    姓名      性别       年龄");              System.out.println("--------------------------------");            while(rs.next()){//取当前行数据项的值赋给变量                 num = rs.getInt("sno");                 name = rs.getString("sname");                 sex = rs.getString("sex");                 age = rs.getInt("age");     //java将变量的 值按要求的格式输出                          System.out.println(num+"    "+name+"        "+sex+"        "+age);            }                       System.out.println("---------------------------------");                         stmt.close();            //事务提交, 事务回滚方法是 rollbck()            con.commit();            con.close();         } catch(SQLException ex) {            System.err.println("**SQLException: " + ex.getMessage());        }              }}