为什么不能使用类的实例创建新的线程？

Question

I wanted to know, why following code does not work:

public class ClockRunnable extends Applet implements Runnable {

Thread m_zeit;
Thread m_background;

 public void init() {
    m_zeit = new Thread(new ClockRunnable());
    m_background = new Thread(new Background());

    m_zeit.start();
    m_background.start();

  }
}

This is of course only an example code. I was wondering, because if I create a new Background Thread it works like written above. But if I call a new Thread for my ClockRunnable class, it throws me a NullPointerExecption. But if I change the new Thread command to

        m_zeit = new Thread(this);

It works perfectly.

If the example above is not enough, here is the whole code + ErrorOutput:

import java.util.*;
import java.text.*;
import java.awt.*;
import java.applet.*;

public class UhrzeitRunnable extends Applet implements Runnable {
String m_aktZeit;
DateFormat m_formatierer;
Font m_anzeigeFont;
Color m_farbe;
Thread m_zeit;
Thread m_background;

public void init() {
    m_anzeigeFont = new Font("Serif",Font.BOLD,22);
    m_formatierer = DateFormat.getTimeInstance();
    m_aktZeit = m_formatierer.format(new Date());
    m_zeit = new Thread(new UhrzeitRunnable());   // if I change it to "this" it works
    m_background = new Thread(new Background());
    m_zeit.start();
    m_background.start();

}

public void run() {
    while(true) {
        m_aktZeit = m_formatierer.format(new Date());  //NullPointerExeption on this line
        repaint();
        try {
            Thread.sleep(1000); 
        } catch (InterruptedException e) {
            return;
        }
    }
}

public void start() {
    if(m_zeit == null) {
        m_zeit = new Thread(new UhrzeitRunnable());
        m_zeit.start();
    }
    if (m_background==null) {
        m_background = new Thread(new Background());
        m_background.start();
    }
}

public void stop() {
    if(m_zeit!=null){
        m_zeit.interrupt();
        m_zeit = null;
    }
}

public void destroy() {
    if(m_zeit !=null) {
        m_zeit.interrupt();
        m_zeit = null;
    }
}

public void paint(Graphics g) {
    g.setFont(m_anzeigeFont);
    g.setColor(Color.blue);
    this.setBackground(m_farbe);
    g.drawString(m_aktZeit, 20, 45);
}


public class Background implements Runnable {
public void run() {
    while (true) {
        m_farbe = new Color((int) (255*Math.random()),(int) (255*Math.random()), (int)(255*Math.random()));
            repaint();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {

        }
    }
}
}
}

And the Error output. The background thread works perfectly by the way.

Exception in thread "Thread-3" java.lang.NullPointerException
at kapitel15.UhrzeitRunnable.run(UhrzeitRunnable.java:29)
at java.lang.Thread.run(Unknown Source)

Answer 1

Applets are strange beasts, and it is very unusual for you to call a constructor on one. Usually this is done by the web page's Java applet driver which then calls the the Applet's init method. And when you do call the constructor, the init() is not called, leaving some of your key fields null. But why would you want to do this? The Applet that you create is a completely distinct object from the one that is being displayed, and changes in its state will not be reflected in the displayed object.