拜个早年：祝工作顺利！麻烦大家帮我看下我写的这个java连接数据库类哪错了——？

这个是接口：
package com.hy;

import java.sql.Connection;
import java.sql.ResultSet;

public interface IDb {
public Connection createCon();
public void createStatement();
public ResultSet createResultSet();
}


这个是实现接口的类
package com.hy;

import java.sql.*;


public class DbOperate implements IDb{
private Connection con=null;
private Statement stmt=null;
private ResultSet rs=null;

private String driver=null;
private String url;
private String username;
private String password;

public DbOperate(String driver,String url,String username,String password){
this.driver=driver;
this.url=url;
this.username=username;
this.password=password;
}
public Connection createCon() {

Class.forName(driver);
con=DriverManager.getDriver(url,username,password);
return con;
}

public ResultSet createResultSet(String sql) {

try {
rs=stmt.executeQuery(sql);
} catch (SQLException e) {

e.printStackTrace();
}
return rs;
}

public void createStatement() {
// TODO Auto-generated method stub

try {
stmt=con.createStatement();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}



}

}
不知道这么写哪里错了，我想把代码写规范了，也就是说该有的接口都有，可是我不知道该怎么写了，麻烦前辈帮我指点一下，谢谢了，顺便拜个早年，祝工作顺利！