Lo siento, no sabía cómo redactar bien el título. Pero estoy haciendo un juego de Java donde tienes una paleta en la parte inferior de tu pantalla y puedes moverla hacia la izquierda y hacia la derecha para evitar la caída de asteroides. Tengo la paleta funcionando, pero no sé cómo hacer que un asteroide caiga. El asteroide será solo un rectángulo por ahora. También me gustaría poder ajustar la velocidad y cuántos asteroides caen. Aquí está mi código:

public class Main extends Applet implements KeyListener, MouseListener {
private Rectangle rect;
private Rectangle asteroidrect;
private ArrayList<Integer> keysDown;
private Image dbImage;
private Graphics dbg;
Random randomGenerator = new Random();
int speed = 8;
int level = 1;     // change to 0 once start menu works
int xpos, ypos; 
int blockx = 0;
int blocky = 0;
int width = 1024;
int height = 768;
String version = "0.0.1";
public static final int START_X_POS = 160;
public static final int START_Y_POS = 160;
public static final int START_WIDTH = 256;
public static final int START_HEIGHT = 64;
boolean startClicked; 
boolean asteroid = false;


public void init() {
    setSize(width, height); 
    addKeyListener(this);
    addMouseListener(this);
    setBackground(Color.black); 
    Frame c = (Frame)this.getParent().getParent();
    c.setTitle("Asteroid Attack - Version " + version);
    keysDown = new ArrayList<Integer>();
    rect = new Rectangle(460, 700, 64, 12);
    asteroidrect = new Rectangle(0, 0, 48, 48);
    addAsteroid();
}

public void update(Graphics g) {
    dbImage = createImage(this.getSize().width, this.getSize().height);
    dbg = dbImage.getGraphics ();
    if (dbImage == null) {}
    dbg.setColor(getBackground ());
    dbg.fillRect(0, 0, this.getSize().width, this.getSize().height);
    dbg.setColor(getForeground());
    paint(dbg);
    g.drawImage(dbImage, 0, 0, this);
}


public void paint(Graphics g) {
    Graphics2D g2 = (Graphics2D)g;
    if (level != 0) {
        g2.setPaint(Color.gray);
        g2.fill(rect);
        //if (asteroid == true) {
            g2.setPaint(Color.red);
            g2.fill(asteroidrect);
        //}
    }
}

@Override
public void keyPressed(KeyEvent e) {
    if (!keysDown.contains(e.getKeyCode()))
        keysDown.add(new Integer(e.getKeyCode()));
    moveChar();
}

@Override
public void keyReleased(KeyEvent e) {
    keysDown.remove(new Integer(e.getKeyCode()));
}

public void moveChar() {
    if (level != 0) {
        int x = rect.x;
        int y = rect.y;
        if (keysDown.contains(KeyEvent.VK_LEFT)) {
            x -= speed;
        }
        if (keysDown.contains(KeyEvent.VK_RIGHT)) {
            x += speed;
        }
        rect.setLocation(x, y);
        repaint(); 
    }
}

public void addAsteroid() {
    asteroid = true;    
}

@Override
public void keyTyped(KeyEvent e) {}

@Override
public void mouseClicked(MouseEvent me) {
    if (level == 0) {
        xpos = me.getX(); 
        ypos = me.getY();
        if (xpos >= START_X_POS && ypos >= START_Y_POS && xpos <= START_X_POS + START_WIDTH && ypos <= START_X_POS + START_HEIGHT ) {
            level = 1;
        }
    }
}

@Override
public void mouseEntered(MouseEvent me) {}

@Override
public void mouseExited(MouseEvent me) {}

@Override
public void mouseReleased(MouseEvent me) {}

@Override
public void mousePressed(MouseEvent me) {}
}

Como puede ver, ya he agregado un objeto asteroide y el rectángulo.