为什么我的java applet不可见？

Question

我将一个简单的java应用程序转换为java applet，但它只显示了一个黑色灰色方块而不是我的应用程序。该应用程序由poolframe.java，ball.java，poolapplet.java组成。

package pool;

import java.awt.Graphics;
import java.util.Timer;
import javax.swing.JApplet;
import javax.swing.JFrame;

public class PoolApplet extends JApplet
{
    private JFrame appletFrame;

    @Override
    public void init()
    {
        appletFrame = new PoolFrame(this);
        appletFrame.setVisible (true);
        appletFrame.setEnabled (true);
    }//End of main
    public void close()
    {
        appletFrame.dispose();
        appletFrame=null;
    }
}

// pooframe.java

package pool;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.GridBagLayout;
import java.awt.GridBagConstraints;
import java.awt.Insets;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;
import javax.swing.JLabel;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.util.Timer;
import java.util.TimerTask;
import javax.swing.JTextField;

public class PoolFrame extends JFrame
{
    //variables
    double angle = 0;

    //objects
    public ball b;
    private JButton newGame, go, pause, exit;
    private JLabel NewGame, Speed, Direction, Table;
    private JTextField speed, direction;
    private Timer timer;

    //layout managers
    private GridBagLayout grid;
    private FlowLayout field;
    private FlowLayout global;
    //panels
    private JPanel GameField, input,spacer;
    private ActionListener actionlistener;
    private PoolApplet myapplet;

    public PoolFrame(PoolApplet mainapp)
    {
        super("Pool by William Murmann");
        myapplet = mainapp;  
        grid = new GridBagLayout();
        global = new FlowLayout(FlowLayout.CENTER,0,0);
        setLayout(global);

        timer = new Timer();

        GridBagConstraints c = new GridBagConstraints();
        Font gen = new Font("Sans-Serif", Font.PLAIN, 18);

        //Input fields
        b = new ball();
        b.setPreferredSize(new Dimension(600,300));
        b.setSize(600,300);
        b.setBackground(Color.black);


        spacer = new JPanel();
        spacer.setPreferredSize(new Dimension(1200,5));


        JPanel input = new JPanel();
        input.setLayout(grid);

        Speed = new JLabel("Speed");
        c.gridx = 1;
        c.gridy = 0;
        c.insets = new Insets(0,10,20,0);
        input.add(Speed, c);

        speed = new JTextField();
        speed.setPreferredSize(new Dimension(75,30));
        c.gridx = 1;
        c.gridy = 1;
        c.insets = new Insets(0,10,20,0);
        input.add(speed, c);

        Direction = new JLabel("Direction");
        c.gridx = 2;
        c.gridy = 0;
        c.insets = new Insets(0,10,20,0);
        input.add(Direction, c);

        direction = new JTextField();
        direction.setPreferredSize(new Dimension(75,30));
        c.gridx = 2;
        c.gridy = 1;
        c.insets = new Insets(0,10,20,0);
        input.add(direction, c);

        newGame = new JButton("New");
        c.gridx = 0;
        c.gridy = 3;
        input.add(newGame, c);

        go = new JButton("Go");
        c.gridx = 1;
        c.gridy = 3;
        input.add(go, c);

        pause = new JButton("Pause");
        c.gridx = 2;
        c.gridy = 3;
        input.add(pause, c);

        exit = new JButton("Exit");
        c.gridx = 3;
        c.gridy = 3;
        input.add(exit, c);

        buttonhandler action = new buttonhandler();
        newGame.addActionListener(action);
        go.addActionListener(action);
        pause.addActionListener(action);
        exit.addActionListener(action);

        add(b);
        add(spacer);
        add(input);
        setSize(600,900);
        setDefaultCloseOperation (JFrame.DISPOSE_ON_CLOSE);
    }
    private class buttonhandler implements ActionListener
    {
        public void actionPerformed(ActionEvent event)
        {
            if(event.getSource() == newGame)
            {
               angle = 0;
               b.changePosition(300, 130);
               timer.cancel();
               speed.setText("");
               direction.setText("");
            }
            else if(event.getSource() == go)
            {
                timer.cancel();
                if(!"".equals(speed.getText()) && !"".equals(direction.getText()))
                {
                    angle = Double.parseDouble(direction.getText());
                    startTimer(Integer.parseInt(speed.getText()));
                }
            }
            else if(event.getSource() == pause)
            {
                timer.cancel();
            }
            else if(event.getSource() == exit)
            {
                myapplet.close();
            }
        }
        public void startTimer(int HSeconds)
        {
            timer = new Timer();
            timer.schedule(new TimerTask() {
                @Override
                public void run() {
                    move();
                }
            }, 0, HSeconds);
        }
        public void move()
        {
            if(b.x < 0 || b.x > 600)
            {
                timer.cancel();
                b.changePosition(300, 130);
                speed.setText("");
                direction.setText("");
            }
            if(b.y <= 0 || b.y >= 280)
            {
                angle = 180 - angle;
            }
            double X = Math.sin(Math.toRadians(angle));
            double Y = Math.cos(Math.toRadians(angle));
            b.changePosition(b.x + X,b.y + Y);

        }
    }
}//end  

// HTML

<html>
  <body>
    <br>
    </br>
    <applet code = "pool.PoolApplet.class" width = "1000" height = "900">
    </applet>
  </body>
</html>

有一次，我在一个单独的窗口中打开了应用程序，但我无法回到那一点。

编辑:: 这是一个github页面https://github.com/wmurmann/applet 与https://rawgithub.com/wmurmann/applet/master/runapplet.html一起运行应用程序。