paintComponent的调用时机与java 中类的初始化的问题

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public class Game {

	public static void main(String[] args) {
		
		Snake snake = new Snake();
		Food food = new Food();
		Ground ground = new Ground();
		GamePanel gamePanel = new GamePanel();
		Controller controller = new Controller(snake, food, ground, gamePanel);

		JFrame frame = new JFrame();
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setSize(300, 300);
		frame.add(gamePanel, BorderLayout.CENTER);
		
		gamePanel.addKeyListener(controller);
		snake.addSnakeListener(controller);//如果把这个类屏蔽掉,就会报空指针
		
		frame.setVisible(true);
		controller.startGame();
	}
}



public class GamePanel extends JPanel {

	private Snake snake;
	private Food food;
	private Ground ground;

	public void display(Snake snake, Food food, Ground ground) {
		System.out.println("GamePanel display is done");
		System.out.println("snake ==== " + snake);
		this.food = food;
		this.snake = snake;
		this.ground = ground;
		this.repaint();
	}

	@Override
	protected void paintComponent(Graphics g) {
		// 重新显示
		this.snake.showMe();//这边会报snake为空指针
		this.food.showMe();
		this.ground.showMe();
	}

}

public class Snake {

	private Set<SnakeListener> snakeListener = new HashSet<SnakeListener>();

	public void addSnakeListener(SnakeListener l) {
		if (l != null) {
			this.snakeListener.add(l);
		}
	}

	public void move() {
		System.out.println("snake move is done");
	}

	public void eatFood() {
		System.out.println("snake eatFood is done");
	}

	public void changeDirection() {
		System.out.println("snake changeDirection is done");
	}

	public boolean isEatBody() {
		System.out.println("snake isEatBody is done");
		return false;
	}

	public void showMe() {
		System.out.println("snake showMe is done");
	}

	private class SnakeDriver implements Runnable {

		@Override
		public void run() {
			// 蛇是否活着
			boolean isSnakeLive = true;
			while (isSnakeLive) {
				move();

				for (SnakeListener l : snakeListener) {
					l.snakeMoved(Snake.this);
				}

				try {
					Thread.sleep(1000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		}

	}

	public void start() {
		new Thread(new SnakeDriver()).start();
	}

}

snake.addSnakeListener(controller);//如果把这个类屏蔽掉,就会报空指针

this.snake.showMe();//这边会报snake为空指针

也就是说我Snake snake = new Snake();并没有让GamePanel.snake被初始化

但是加上snake.addSnakeListener(controller);就会让GamePanel.snake被初始化,

对于这种现象,很迷惑.

请大神帮忙梳理一下,谢谢.

[lsy873792170]

你在new GamePanel() 的时候并没有把Snake 对象传进去啊，GamePanel里面的snake还是空的

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引用 1 楼 lsy873792170 的回复:

你在new GamePanel() 的时候并没有把Snake 对象传进去啊，GamePanel里面的snake还是空的

是呀,但是不屏蔽掉snake.addSnakeListener(controller); 这句代码,也不会报错. 所以很纠结