JAVA问题请教。“不是抽象的，并且未覆盖 javax”
源程序如下：

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.swing.event.*;

public class TextTest
{
  public static void main(String[] args)
  {
  TextTestFrame frame=new TextTestFrame();
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setVisible(true);
  }
  }

  /**
  A frame with two text fields to set a clock.
  */
  class TextTestFrame extends JFrame
{
  public TextTestFrame()
  {
  setTitle("TextTest");

  DocumentListener listener =new ClockFieldListener();

  JPanel panel=new JPanel();

  panel.add(new JLabel("Hours:"));
  hourField=new JTextField("12",3);
  panel.add(hourField);
  hourField.getDocument().addDocumentListener(listener);

  panel.add(new JLabel("Minutes:"));
  minuteField=new JTextField("00",3);
  panel.add(minuteField);
  minuteField.getDocument().addDocumentListener(listener);

  add(panel,BorderLayout.SOUTH);

  clock=new ClockPanel();
  add(clock,BorderLayout.CENTER);
  pack();
  }

  public void setClock()
{
try
{
int hours=Integer.parseInt(hourField.getText().trim());
int minutes=Integer.parseInt(minuteField.getText().trim());
clock.setTime(hours,minutes);
}
catch (NumberFormatException e)
{
}
}

public static final int DEFAULT_WIDTH=300;
public static final int DEFAULT_HEIGHT=300;

private JTextField hourField;
private JTextField minuteField;
private ClockPanel clock;

private class ClockFieldListener implements DocumentListener
{
public void insertUpdate(DocumentEvent event) { setClock( );}
public void removetUpdate(DocumentEvent event) { setClock( );}
public void changedUpdate(DocumentEvent event) {}
}
}

class ClockPanel extends JPanel
{
public ClockPanel()
{
setPreferredSize(new Dimension(2*RADIUS+1,2*RADIUS+1));
}

public void paintComponent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2=(Graphics2D) g;
Ellipse2D circle=new Ellipse2D.Double(0,0,2*RADIUS,2*RADIUS);
g2.draw(circle);

double hourAngle=Math.toRadians(90-360*minutes/(12*60));
drawHand(g2,hourAngle,HOUR_HAND_LENGTH);

double minuteAngle=Math.toRadians(90-360*minutes/(60));
drawHand(g2,minuteAngle,MINUTE_HAND_LENGTH);
}

public void drawHand(Graphics2D g2,double angle,double handLength)
{
Point2D end =new Point2D.Double(RADIUS+handLength*Math.cos(angle),RADIUS-

handLength*Math.sin(angle));
  Point2D center=new Point2D.Double(RADIUS,RADIUS);
  g2.draw(new Line2D.Double(center,end));
}

public void setTime(int h,int m)
{
minutes=h*60+m;
repaint();
}

private double minutes=0;
private int RADIUS=100;
private double MINUTE_HAND_LENGTH=0.8*RADIUS;
private double HOUR_HAND_LENGTH=0.6*RADIUS;
}

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这个是一个定义时钟的程序，即输入一个时刻，时钟就会转成该时刻的样子：

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错误描述如下：

D:\java\java1>javac TextTest.java
TextTest.java:67: TextTestFrame.ClockFieldListener 不是抽象的，并且未覆盖 javax.
swing.event.DocumentListener 中的抽象方法 changedUpdate(javax.swing.event.Docume
ntEvent)
  private class ClockFieldListener implements DocumentListener
  ^
1 错误

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