新手求助！private static Connection Conn;

import java.sql.*; 

public class Jdbc {
    private static Connection Conn;
    //这一句起了什么作用  为什么没有这句运行就错误
    public static void main(String [] args){
      String driverName="com.microsoft.sqlserver.jdbc.SQLServerDriver";
      String dbURL="jdbc:sqlserver://localhost:1433;DatabaseName=mydate";
      String userName="sa";
      String userPwd="lzm0922";
      try{
          Class.forName(driverName);
          Connection Conn=DriverManager.getConnection(dbURL,userName,userPwd);
          System.out.println("连接数据库成功");
          Statement stmt = Conn.createStatement();
          ResultSet rs=stmt.executeQuery("SELECT * FROM BOOK");
          while(rs.next()){
              String Bno=rs.getString("Bno");
              System.out.println(Bno);
              System.out.println(rs.getString("Bname"));
          }
          rs.close();
          stmt.close();
          Conn.close();
      }
      catch(Exception e){
        e.printStackTrace();
        System.out.print("连接失败");
      }    
    }
}



把这一句的private改为public也出现  错误 为什么呢  求前辈帮助、