多线程程序中的java.lang.NullPointerException

Question

这是代码：

import java.applet.Applet;
import java.awt.Color;`
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

@SuppressWarnings("serial")
public class Pong extends Applet implements Runnable, KeyListener{

final int width = 700, height = 500;

public static int score = 0;

Thread thread;
HumanPaddle p1;
Ball b1;

public void init(){
    this.resize(width, height);

    this.addKeyListener(this);

    thread = new Thread(this);
    thread.start();

    p1 = new HumanPaddle(1);
    b1 = new Ball();
}

public void paint(Graphics g){
    g.setColor(Color.black);
    g.fillRect(0, 0, width, height);
    p1.draw(g);
    b1.draw(g);

    g.setColor(Color.red);
    g.drawString("Score: " + Integer.toString(score), width/2 - 20, 10);

}

public void update(Graphics g){
    paint(g);
}

public void run() {
    for(;;){

        p1.move();
        b1.move();

        b1.paddleCollision(p1, p1);  

        if(b1.getX() > width + 10){score++;
        }else if(b1.getX() < -10){score--;}

        repaint();

        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

public void keyPressed(KeyEvent key) {
    if(key.getKeyCode() == KeyEvent.VK_UP || key.getKeyCode() == KeyEvent.VK_W){
        p1.setUpAccel(true);
    }else if(key.getKeyCode() == KeyEvent.VK_DOWN || key.getKeyCode() == KeyEvent.VK_S){
        p1.setDownAccel(true);
    }
}

public void keyReleased(KeyEvent key) {
    if(key.getKeyCode() == KeyEvent.VK_UP || key.getKeyCode() == KeyEvent.VK_W){
        p1.setUpAccel(false);
    }else if(key.getKeyCode() == KeyEvent.VK_DOWN || key.getKeyCode() == KeyEvent.VK_S){
        p1.setDownAccel(false);
    }
}

public void keyTyped(KeyEvent key) {

}

}

关键是有时候，代码会失败并显示

“线程“ Thread-3中的异常”，java.lang.Thread.run（未知源）的Pong.run（Pong.java:48）处的java.lang.NullPointerException”

我知道这意味着第48行第48行有错误：“ p1.move（）;”

并且没有意义，因为它是一种有效的方法，有时只会失败...

请，我需要快速帮助...预先感谢您。

Answer 1

您有比赛条件。 因为您在初始化p1之前在init（）中启动了线程，所以冒着线程可能在init有机会对其进行初始化之前运行的风险。 您只需要在线程启动之前将p1和b1的初始化移到：

...
    p1 = new HumanPaddle(1);
    b1 = new Ball();

    thread = new Thread(this);
    thread.start(); 
}