Java 几个对象详细汉语解释？
 在包下创建类：BaseDao 代码如下：
                protected Connection con = null;
                protected PreparedStatement pst = null;
                protected ResultSet rs = null;
                protected String sql = null;
                编写打开方法：
                public void open(){
                        try {
                                Class.forName("oracle.jdbc.driver.OracleDriver");
con=DriverManager.getConnection("jdbc：oracle:thin：@localhost:1521：orcl","tiger","tiger");                        
                        } catch (Exception e) {
                                e.printStackTrace();
                        }
                }
        编写关闭方法：(倒着关闭)
        public void close(){
                try {
                        if (rs!=null) {
                                rs.close();
                                rs = null;
                        }
                        if (pst!=null) {
                                pst.close();
                                pst = null;
                        }
                        if (con!=null) {