想实现从数据库读出数据到label的功能，但老是在jLabel那里出问题，请大家帮忙看看吧~谢谢~

Question

public class Operate_Stu extends javax.swing.JFrame {
public static String public_must="123";
public static String public_select="0";
public static String major_must="0";
public static String major_select="0";
}
DB_nowcredit.ReadData();
jLabel16.setText(major_must);

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下面贴一下DB_nowcredit类的内容
public class DB_nowcredit {
public static void ReadData(){
String dburl="jdbc:mysql://localhost:3306/sb";
String dbuser="root";
String dbpass="123";
String sql ="select courseinf.CourseSortNo,sum(courseinf.CourseCredit)as AllCredit from selectcourse left join courseinf on selectcourse.CourseNo = courseinf.CourseNo where StuNo=2101010101 GROUP BY courseinf.CourseSortNo";
Connection con = null;
try {
con = DriverManager.getConnection(dburl,dbuser,dbpass);
Statement stat=con.createStatement();
ResultSet rs=stat.executeQuery(sql);
while(rs.next()){
String sortname = rs.getString("CourseSortNo");
String coursecredit = rs.getString("AllCredit");
if(sortname=="3"){
Operate_Stu.public_must=coursecredit;
}
else if(sortname=="4"){
Operate_Stu.public_select=coursecredit;
}
else if(sortname=="1"){
Operate_Stu.major_must=coursecredit;
}
else if(sortname=="2"){
Operate_Stu.major_select=coursecredit;
}
}

         rs.close();
         stat.close();
         con.close();

    } catch (SQLException ex) {
        Logger.getLogger(DB_nowcredit.class.getName()).log(Level.SEVERE, null, ex);
    }

}

}

Answer 1

贴出错误提示和发生错误的行，仔细调试代码。