JPanels没有在JFrame中绘制

Question

我正在开发垂直滚动游戏，并且正在使用线程每2秒生成新的敌人。每个敌人都是JPanel中的图像。由于某些原因，生成的敌人没有出现在JFrame中，但它们存在。当玩家与其中一个敌人碰撞时，所有敌人都会出现。 这是主要的类：

package asteroidblaster;


import javax.swing.*;
import javax.swing.Timer;
import java.awt.event.*;
import java.awt.*;

public class Main extends JFrame {

/**
 * 
 */
private static final long serialVersionUID = 1L;

public Main() {
    initUI();
}

private void initUI() {
    add(new Board());
    setResizable(false);
    pack();
    setTitle("Alien Blaster");
    setLocationRelativeTo(null);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public static void main(String[] args) {

      EventQueue.invokeLater(() -> {
          JFrame ex = new Main();
          ex.setVisible(true);
      });
}

}

这是主要的游戏逻辑：

package asteroidblaster;

import javax.swing.*;
import javax.swing.Timer;
import java.awt.event.*;
import java.awt.*;
import java.util.*;

public class Board extends JPanel {

private static final long serialVersionUID = 1L;
private final int B_WIDTH = 1500;
private final int B_HEIGHT = 700;
private final int REFRESH_TIME = 150;

private AlienShip alien;
private PlayerShip player;
private ArrayList<AlienShip> enemies = new ArrayList<AlienShip>();

private Timer alienScrollTimer, mainTimer;

public Board() {
    initBoard();
}

private void initBoard() {

    setBackground(Color.BLACK);
    setFocusable(true);

    setPreferredSize(new Dimension(B_WIDTH, B_HEIGHT));
    setPlayer();
    initAlienTimer();
    gameLoop();
}

private void initAlienTimer() {
    alienScrollTimer = new Timer(25, new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            for(AlienShip as : enemies) {
                as.scrollShip();
            }
            repaint();
        }
    });
    alienScrollTimer.start();
}

private void setPlayer() {

    player = new PlayerShip();
    add(player);
    addMouseMotionListener(new MouseMotionListener() {
        @Override
        public void mouseMoved(MouseEvent e) {
            player.followMouse(e.getX(), e.getY());
            //checkCollision();
            repaint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {

        }
    });
}

private void checkCollision() {
    for(AlienShip as : enemies) {
        if(player.getBounds().intersects(as.getBounds()))
            player.setVisible(false);
    }
}

private Thread alienGenerator() {
    for(int i = 0; i < 3; i++) { //these two are being drawn
        alien = new AlienShip();
        add(alien);
        enemies.add(alien);
    }

    return new Thread(new Runnable() {
        @Override
        public void run() {
            int sleepTime = 2000;
            while(true) {
                try {
                    Thread.sleep(sleepTime);
                } catch(InterruptedException e) {
                    System.out.println(e);
                }
                alien = new AlienShip();
                add(alien);
                enemies.add(alien);
                System.out.println("Enemies: " + enemies.size());
            }
        }
    });
}


private void gameLoop() {
    alienGenerator().start();
    mainTimer = new Timer(REFRESH_TIME, new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            checkCollision();
            repaint();
        }
    });
    mainTimer.start();
}

@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);
}
}

这是AlienShip类：

package asteroidblaster;

import java.util.Random;
import java.awt.*;
import javax.swing.*;

public class AlienShip extends JPanel {
/**
 * 
 */
private static final long serialVersionUID = 1L;
private final String ALIEN_SHIP_FILE = "images/alienShip.jpg";
private final int PANEL_WIDTH = 224, PANEL_HEIGHT = 250;
private int panelX, panelY;
private Image alienShipImage;

public AlienShip() {
    initAlienShip();
}

private void loadImage() {
    alienShipImage = Toolkit.getDefaultToolkit().getImage(ALIEN_SHIP_FILE);
}

public int getX() {
    return panelX;
}

public int getY() {
    return panelY;
}

private void setY(int yCoord) {
    panelY = yCoord;
}

private int setX() {
    Random r = new Random();
    int rand = r.nextInt(14) + 1;
    return panelX = rand * 100;
}

private void initAlienShip() {
    panelX = setX();
    panelY = 0;
    setPreferredSize(new Dimension(PANEL_WIDTH, PANEL_HEIGHT));
    setBackground(Color.BLUE);
    loadImage();
}

@Override
public Rectangle getBounds() {
    return new Rectangle(panelX, panelY, PANEL_WIDTH, PANEL_HEIGHT);
}

public void scrollShip() {
    setY(getY() + 1);
}

@Override
public void paint(Graphics g) {
    super.paint(g);
    g.drawImage(alienShipImage, 0, 0, null);
}

@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);
}
}