为什么我的程序拒绝输出？什么是“applet未初始化”

Question

我的代码尽管没有任何缺陷并且完全编译，但由于applet“未初始化”而拒绝显示任何内容。为什么我的程序拒绝输出！ 我要求在回答这个问题时，你认为我是业余爱好者。

提前致谢!!!

import java.util.*; 

 import java.awt.*;

 import javax.swing.*;
 import java.awt.event.KeyEvent.*;
 import java.awt.event.KeyListener.*;
import java.applet.*;
 import java.awt.Graphics; 


public class Tester extends JApplet implements Runnable
{
// instance variables - replace the example below with your own
private int xCoord;
private int yCoord;
Thing t;
Graphics g;
boolean isRunning = true;
Thread th;
public void init()
{
   xCoord = 100;
   yCoord = 100;
   t = new Thing(xCoord,yCoord);
   th = new Thread(this);
   setSize(500,500);
   t.draw(g,xCoord,yCoord);
}
public void run()
{
    while(isRunning)
    {
        t.update(this);
        repaint();
        try
        {
           Thread.sleep(20); 
        }
        catch(Exception e)
        {

        }
    }
}
/**
 * Called by the browser or applet viewer to inform this JApplet that it 
 * should start its execution. It is called after the init method and 
 * each time the JApplet is revisited in a Web page. 
 */
public void start()
{
   th.start();
    // provide any code requred to run each time 
    // web page is visited
}

/** 
 * Called by the browser or applet viewer to inform this JApplet that
 * it should stop its execution. It is called when the Web page that
 * contains this JApplet has been replaced by another page, and also
 * just before the JApplet is to be destroyed. 
 */
public void stop()
{
    isRunning = false;// provide any code that needs to be run when page
    // is replaced by another page or before JApplet is destroyed 
}

/**
 * Paint method for applet.
 * 
 * @param  g   the Graphics object for this applet
 */
public void paint(Graphics g)
{
    t.paint(g);
    g.fillRect(100,100,100,100);
}

/**
 * Called by the browser or applet viewer to inform this JApplet that it
 * is being reclaimed and that it should destroy any resources that it
 * has allocated. The stop method will always be called before destroy. 
 */
public void destroy()
{
   isRunning=false;// provide code to be run when JApplet is about to be destroyed.
}

}

class Thing
{
private int x;
private int y;
public Thing(int x, int y)
{
    this.x = x;
    this.y = y;
}
public void draw(Graphics g, int x, int y)
{
    g.setColor(Color.black);
    g.fillOval(x,y,50,50);
}
public void paint(Graphics g)
{
    draw(g,x,y);
}
public void update(Tester te)
{
    x+=1;
    y-=1;
}

}