启动Applet未初始化小行星游戏

Question

我正在学习Java，并且最好的方法是通过手机上的QnA，练习书，游戏编程和应用程序的混合。到目前为止它已经成功但......

在其中一本游戏编程书籍中，我制作了一个创建小行星游戏的java小程序。我正在使用NetBeans，当我运行该文件时，applet窗口打开，空白，并在底部显示Start：Applet未初始化。

在书中，它将此作为初始化事件：

//Primary class for the game
public abstract class Asteroids extends Applet implements Runnable, KeyListener 
{

    //The Main thread becomes the game loop

    Thread gameloop;

    //Use this as a double buffer
    BufferedImage backbuffer;

    //The Main drawing object for the back buffer
    Graphics2D g2d;

    //toggle for drawing bounding boxes
    boolean showBounds = false;

    //create the asteroid array
    int ASTEROIDS = 20;
    Asteroid[] ast = new Asteroid[ASTEROIDS];

    //Create the bullet Array
    int BULLETS = 10;
    Bullet[] bullet = new Bullet[BULLETS];
    int currentBullet = 0;

    //The Players Ship
    Ship ship = new Ship();

    //Create the Identity transform(0,0)
    AffineTransform identity = new AffineTransform();

    //Create a random number generator
    Random rand = new Random();

    //Applet init event
    public void init() 
    {
        //Create the back buffer for smooth graphics
        backbuffer = new BufferedImage(640, 480, BufferedImage.TYPE_INT_RGB);
        g2d = backbuffer.createGraphics();

        //Set Up the Ship!
        ship.setX(320);
        ship.setY(240);

        //Set the Bullets up!
        for (int n = 0; n < BULLETS; n++) 
        {
            bullet[n] = new Bullet();
        }

        //Creating Asteroids!
        for (int n = 0; n < ASTEROIDS; n++) 
        {
            ast[n] = new Asteroid();
            ast[n].setRotationVelocity(rand.nextInt(3) + 1);
            ast[n].setX((double) rand.nextInt(600) + 20);
            ast[n].setY((double) rand.nextInt(440) + 20);
            ast[n].setMoveAngle(rand.nextInt(360));
            double ang = ast[n].getMoveAngle() - 90;
            ast[n].setVelX(calcAngleMoveX(ang));
            ast[n].setVelY(calcAngleMoveY(ang));
        }
        //Start User Input Listener
        addKeyListener(this);

这就是它所说的应该启动applet。

我不知道这是否只需要，但还有更多。我已检查，双重检查并三重检查代码，这是正确的。