请问JAVA中出现的这个异常如何解决。

Exception in thread "main" java.lang.IllegalArgumentException: adding a window t
o a container
    at java.awt.Container.addImpl(Container.java:1010)
    at java.awt.Container.add(Container.java:351)
    at ClockFrame.<init>(ClockFrame.java:18)
    at ClockFrame.main(ClockFrame.java:26)
Press any key to continue...两个类，ClockPanel.java,ClockFrame.java(调用ClockPanel实例)
ClockPanelimport java.awt.*;
import javax.swing.*;
import java.util.*;public class ClockPanel extends JFrame
{
     public ClockPanel()
     {
           super();
           String currentTime=this.getTime();
           JLabel time=new JLabel("Time:");
           JLabel current=new JLabel(currentTime);
           add(time);
           add(current);
           
     }
     
     String getTime()
     {
           String time;
           Calendar now =Calendar.getInstance();
           int hour=now.get(Calendar.HOUR_OF_DAY);
           int minute=now.get(Calendar.MINUTE);
           int month=now.get(Calendar.MONTH)+1;
           int day=now.get(Calendar.DAY_OF_MONTH);
           int year=now.get(Calendar.YEAR);
           String monthName="";
           switch (month)
           {
                 
           case(1):
           monthName="January";
           break;
           
           case(2):
           monthName="February";
           break;
           case(3):
           monthName="March";
           break;
           case(4):
           monthName="April";
           break;
           case(5):
           monthName="May";
           break;
           case(6):
           monthName="June";
           break;
                 case(7):
           monthName="July";
           break;
           
           case(8):
           monthName="August";
           break;
           case(9):
           monthName="September";
           break;
           case(10):
           monthName="October";
           break;
           case(11):
           monthName="November";
           break;
           case(12):
           monthName="December";
           break;      
           }
           time = monthName + " " + day + ", " + year + " "+hour+":"+minute;
                 return time;
           
     }
           }
ClockFrameimport java.awt.*;
import javax.swing.*;public class ClockFrame extends JFrame
{
     
     public ClockFrame()
     
     {
     
           super("clock");
           setSize(225,125);
           this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
           FlowLayout flo=new FlowLayout();
           setLayout(flo);
           ClockPanel time=new ClockPanel();
                 
           
           setVisible(true);
           this.add(p);
           add(time);
     }
     
     public static void main(String[] args)
      {
           ClockFrame clock=new ClockFrame();
           
      }
}