在Applet中生成随机三角形（动画）

Question

I try to solve this question, but I have an infinite loop and don't know how to solve this problem. Guide me. I'm new in Java.

In terminal:

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at ThreadApplet.RandGen(ThreadApplet.java:10)
        at ThreadApplet.paint(ThreadApplet.java:30)
        at java.awt.Container.update(Container.java:1988)
        at sun.awt.RepaintArea.updateComponent(RepaintArea.java:255)

Thanks for HELP!

Question: Write an applet that displays randomly generated triangles in different colors.

    /////////////Applet

import java.awt.*;

import java.applet.*;

public class ThreadApplet extends Applet{
    MyThread thread;
    int x[],y[];
    public boolean ctrl=true;

    public void RandGen(){
        for (int i=0; i<3;i++){
            x[i]=(int)Math.random()*100;
            y[i]=(int)Math.random()*100;
        }       
    }

    public void start(){
        if (thread==null){
            thread = new MyThread(this);
            thread.start();
        }
    }

    public void stop(){
        thread = null;
    }

    public void paint(Graphics g){
        if (ctrl==true){
            g.setColor(Color.blue);
        } else {
            g.setColor(Color.red);
        }
        RandGen();
        g.fillPolygon(x,y,3);   
    }
}

public class MyThread extends Thread{
    ThreadApplet applet;

    public MyThread (ThreadApplet applet){
        this.applet=applet;
    }
    public void run(){
        Thread thisThread = Thread.currentThread();
        while (this==thisThread){           
            applet.repaint();
            try{Thread.sleep(50);}
            catch(InterruptedException e){} 
        }       
    }
}

Answer 1

You need to initialize your arrays before usage:

 int x[] = new int[17];

for example.