写一个基于java的数据库管理系统，但JFrame界面显示不出来

package DBS;
import java.sql.*;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.border.*;
import javax.swing.JOptionPane;

public class EX12_5_StudentMangement extends JFrame implements ActionListener{
    //JFrame jf=new JFrame("窗口");
    
    
    JMenuBar bar=null;
    JMenu menu1,menu2,menu3,menu4,menu5;//菜单栏
    JMenuItem item1,item2,item3,item4,item5;//项目栏
    EX12_6_StudentAdd zengjia;
    EX12_7_StudentQuery chaxun;
    EX12_8_StudentUpdate gengxin;
    EX12_9_StudentDelete shanchu;
    EX12_5_StudentMangement(){
        super("学生信息管理系统");//???1
        zengjia=new EX12_6_StudentAdd();
        chaxun=new EX12_7_StudentQuery();
        gengxin=new EX12_8_StudentUpdate();
        shanchu=new EX12_9_StudentDelete();
        bar=new JMenuBar();
        menu1=new JMenu("信息录入");
        menu2=new JMenu("信息查询");
        menu3=new JMenu("信息更新");
        menu4=new JMenu("信息删除");
        menu5=new JMenu("退出系统");
        item1=new JMenuItem("录 入");
        item2=new JMenuItem("查 询");
        item3=new JMenuItem("更 新");
        item4=new JMenuItem("删 除");
        item5=new JMenuItem("退 出");
        menu1.add(item1);
        menu2.add(item2);
        menu3.add(item3);
        menu4.add(item4);
        menu5.add(item5);
        bar.add(menu1);
        bar.add(menu2);
        bar.add(menu3);
        bar.add(menu4);
        bar.add(menu5);
        setJMenuBar(bar);
        //this.add(bar,BorderLayout.NORTH);
        //jf.setJMenuBar(bar);
        
        item1.addActionListener(this);
        item2.addActionListener(this);
        item3.addActionListener(this);
        item4.addActionListener(this);
        item5.addActionListener(this);
        
        //this.setLayout(new FlowLayout(FlowLayout.LEFT));
        
        JLabel label=new JLabel("学生信息管理系统",JLabel.CENTER);//JLabel标签组件，显示信息
        label.setFont(new Font("学生信息管理系统",0,35));
        String s=" ";
        Font f=new Font(s,Font.BOLD,60);
        label.setBackground(new Color(0,255,255));
        label.setForeground(new Color(255,128,255));
        add(label,"Center");
        setVisible(true);
        setSize(350,300);
    }
        public void actionPerformed(ActionEvent e){
            if(e.getSource()==item1){
                this.getContentPane().removeAll();
                this.getContentPane().add(zengjia,"Center");
                this.getContentPane().repaint();
                this.getContentPane().validate();
            }
            if(e.getSource()==item2){
                this.getContentPane().removeAll();
                this.getContentPane().add(chaxun,"Center");
                this.getContentPane().repaint();
                this.getContentPane().validate();
            }
            if(e.getSource()==item3){
                this.getContentPane().removeAll();
                this.getContentPane().add(gengxin,"Center");
                this.getContentPane().repaint();
                this.getContentPane().validate();
            }
            if(e.getSource()==item4){
                this.getContentPane().removeAll();
                this.getContentPane().add(shanchu,"Center");
                this.getContentPane().repaint();
                this.getContentPane().validate();
            }
            if(e.getSource()==item5){
                System.exit(0);
            }
        }
        public static void main(String args[]){
            EX12_5_StudentMangement stuM=new EX12_5_StudentMangement();
            stuM.setVisible(true);
            //stuM.setSize(300,300);
            stuM.addWindowListener(new WindowAdapter(){
                public void windowClosing(WindowEvent e){
                    System.exit(0);
                }
            });
        }
}

上面的车、注释是自己做的一些尝试，但都没有用

```c
Exception in thread "main" java.lang.IllegalArgumentException: cannot add to layout: unknown constraint: center
    at java.awt.BorderLayout.addLayoutComponent(BorderLayout.java:463)
    at java.awt.BorderLayout.addLayoutComponent(BorderLayout.java:424)
    at java.awt.Container.addImpl(Container.java:1127)
    at java.awt.Container.add(Container.java:973)
    at DBS.EX12_6_StudentAdd.<init>(EX12_6_StudentAdd.java:74)
    at DBS.EX12_5_StudentMangement.<init>(EX12_5_StudentMangement.java:22)
    at DBS.EX12_5_StudentMangement.main(EX12_5_StudentMangement.java:99)

报上面的错，不知道怎么回事，同学的是可以显示的

EX12_5_StudentMangement.java:99
99行是那一句代码？