Applet 上的 Java NullPointerException

Question

I am trying to embed an applet into an HTML for demonstration in my Uni class.我正在尝试将小程序嵌入 HTML 以在我的 Uni 类中进行演示。 Every time I try to run the execute the applet, I get a NullPointerException dialog.每次我尝试运行执行小程序时，我都会收到一个 NullPointerException 对话框。 What am I doing wrong?我究竟做错了什么？

<html>
  <head>
    <title> This is my Pacman! </title>
      <body bgcolor = black text= yellow>
        <center>
          <applet code="pacman.InitialClass.class" archive="pacSimq.jar"
            width=300 height=350>
          </applet>
         </center>
         <hr>
           <a href="https://github.com/awernick/PacSim"> The repository. </a>
      </body>
</html>

This is the main class for the applet.这是小程序的主类。 The jar contains multiple classes also. jar 也包含多个类。

package pacman;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.image.BufferStrategy;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.net.URL;
import java.util.Random;

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

    private final int WIDTH = 224;
    private final int HEIGHT = 288;
    private Map map;
    private Hero pacman;
    private Blinky blinky;
    private Graphics second;
    private Image image, bg, current,current_bl, pm1, pm2, pm3, pm4, pm5, pm6, pm7, pm8, pm9,bl1,bl2,bl3,bl4,bl5,bl6,bl7,bl8;
    private URL base;
    private int counter;

    public void init() {
        setSize(WIDTH, HEIGHT);
        setBackground(Color.BLACK);
        setFocusable(true);
        addKeyListener(this);
        Frame frame = (Frame) this.getParent().getParent();
        frame.setTitle("Pac-Man");
        frame.setResizable(false);
        try {
            base = getDocumentBase();
            System.out.println(base);
        } catch (Exception e) {

        }

        pm1 = getImage(base, "data/pac_1.png");
        pm2 = getImage(base, "data/pac_2.png");
        pm3 = getImage(base, "data/pac_3.png");
        pm4 = getImage(base, "data/pac_4.png");
        pm5 = getImage(base, "data/pac_5.png");
        pm6 = getImage(base, "data/pac_6.png");
        pm7 = getImage(base, "data/pac_7.png");
        pm8 = getImage(base, "data/pac_8.png");
        pm9 = getImage(base, "data/pac_9.png");

        bl1 = getImage(base, "data/bl_1.png");
        bl2 = getImage(base, "data/bl_2.png");
        bl3 = getImage(base, "data/bl_3.png");
        bl4 = getImage(base, "data/bl_4.png");
        bl5 = getImage(base, "data/bl_5.png");
        bl6 = getImage(base, "data/bl_6.png");
        bl7 = getImage(base, "data/bl_7.png");
        bl8 = getImage(base, "data/bl_8.png");

        bg = getImage(base, "data/bg1.png");

        current = pm1;
        current_bl = bl1;
    }

    @Override
    public void start() {
        map = new Map();
        pacman = new Hero();
        blinky = new Blinky(map,pacman);

        Thread graphics_thread = new Thread(this);
        Thread blinky_thread = new Thread(blinky);
        Thread pacman_thread = new Thread(pacman);
        graphics_thread.start();
        blinky_thread.start();
        pacman_thread.start();
    }

    public Map getMap() {
        return map;
    }

    public void setMap(Map map) {
        this.map = map;
    }

    @Override
    public void run() {
        while (true) {
            //pacman.update();
            //blinky.update(pacman.getCenterX(), pacman.getCenterY());
            if (pacman.isMvRight()) {
                current = pm2;
                current_bl = bl2;
            } else if (pacman.isMvLeft()) {
                current = pm4;
                current_bl = bl4;
            } else if (pacman.isMvUp()) {
                current = pm6;
                current_bl = bl6;
            } else if (pacman.isMvDown()) {
                current = pm8;
                current_bl = bl8;
            }
            repaint();
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    @Override
    public void update(Graphics g) {
        if (image == null) {
            image = createImage(this.getWidth(), this.getHeight());
            second = image.getGraphics();
        }

        second.setColor(getBackground());
        second.fillRect(0, 0, getWidth(), getHeight());
        second.setColor(getForeground());
        paint(second);

        g.drawImage(image, 0, 0, this);
    }

    @Override
    public void paint(Graphics g) {
        g.drawImage(bg, 0, 0,this);
        g.drawImage(current, pacman.getCenterX(), pacman.getCenterY(), this);
        g.drawImage(current_bl, blinky.getCenterX(), blinky.getCenterY(), this);
    }

    @Override
    public void keyPressed(KeyEvent e) {

        switch (e.getKeyCode()) {
        case KeyEvent.VK_UP:
            pacman.setMvUp(true);
            //System.out.println("hello");
            pacman.moveUp();
            //System.out.println("hello");

            break;

        case KeyEvent.VK_DOWN:
            //System.out.println("hello");
            pacman.setMvDown(true);
            pacman.moveDown();
            break;

        case KeyEvent.VK_LEFT:
            pacman.setMvLeft(true);
            pacman.moveLeft();
            break;

        case KeyEvent.VK_RIGHT:
            pacman.setMvRight(true);
            pacman.moveRight();
            break;

        case KeyEvent.VK_SPACE:
            pacman.stop();
            break;

        }

    }

    @Override
    public void keyReleased(KeyEvent e) {

    }

    @Override
    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub

    }

}

This is the error the Java Console is showing me.这是 Java 控制台向我显示的错误。

java.lang.NullPointerException
    at pacman.InitialClass.init(InitialClass.java:34)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.init(Unknown Source)
    at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

I packed it into a jar through eclipse, yet I still get NullPointerException.我通过 eclipse 将它打包到一个 jar 中，但我仍然收到 NullPointerException。 What might be causing it?可能是什么原因造成的？

Answer 1

    Frame frame = (Frame) this.getParent().getParent();
    frame.setTitle("Pac-Man");  // line 34

The only thing that might be null on line 34 is the frame attribute.第 34 行唯一可能为null是frame属性。 At this point in the applet life cycle, the applet is apparently yet to be added to a root frame.在applet 生命周期的这一点上，applet 显然还没有被添加到根框架中。

Applet 上的 Java NullPointerException

问题描述

1 个解决方案

解决方案1
1 已采纳 2014-04-09 07:55:53

Applet 上的 Java NullPointerException

问题描述

1 个解决方案

解决方案1 1 已采纳 2014-04-09 07:55:53

解决方案1
1 已采纳 2014-04-09 07:55:53