一个GUI初始化的有关问题

package org.gui;import java.awt.*;import java.awt.event.*;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.Date;public class TestBiao extends Frame{    /**     * @param args     */    public TestBiao(){        this.setSize(400, 400);        this.setVisible(true);        this.addWindowListener(new WindowAdapter(){            public void windowClosing(WindowEvent e){                setVisible(false);                System.exit(0);            }        });        TestCanvas tc = new TestCanvas();        this.add(tc);    }    public static void main(String[] args) {        // TODO Auto-generated method stub        new TestBiao();    }}class TestCanvas extends Canvas{    long startTime = 0;    long endTime = 0;    Date dd,dn = null;    String show = "";    boolean bTrue =true;    SimpleDateFormat sf = null;    public TestCanvas(){        this.setVisible(true);        this.enableEvents(AWTEvent.MOUSE_EVENT_MASK);        sf = new SimpleDateFormat("HH:mm:ss");        try {            dd = (Date)sf.parseObject("00:00:00");        } catch (ParseException e) {            e.printStackTrace();        }    }    public void paint(Graphics g){        System.out.println("进入paint方法..");        if(bTrue){            g.fill3DRect(0, 0, 80, 30, true);            g.setColor(Color.WHITE);            g.drawString("00:00:00", 20, 20);        }else{            g.fill3DRect(0, 0, 80, 30, true);            g.setColor(Color.WHITE);            g.drawString(show, 20, 20);        }    }    protected void processMouseEvent(MouseEvent e){        if(e.getID()==MouseEvent.MOUSE_PRESSED){            System.out.println("鼠标点击..");            bTrue = true;            startTime = System.currentTimeMillis();        }else if(e.getID()==MouseEvent.MOUSE_RELEASED){            System.out.println("鼠标释放..");            bTrue = false;        }    }}