多线程不起作用。 什么不对？

Question

It's a little program written with a purpose of studying multithreading. I expected to get in main method different random numbers after run. About 4 numbers per second. But I got many thousands of zeros. Where is an error?

Main Class:

public class Main {

public static void main(String[] args) {
        ExternalWorld externalWorld = new ExternalWorld();
        externalWorld.start();

        int x = 0;
        while (true) {
            while(!externalWorld.signal){
                System.out.println("qqq");}
            System.out.println(++x + ") " + externalWorld.getAnInt());
        }
    }
}

ExternalWorld Class:

import java.util.Random;

public class ExternalWorld extends Thread {

    private int anInt = 0;
    public boolean signal = false;

    @Override
    public void run() {
        Random random = new Random(100);
        while(true) {
            anInt = random.nextInt(100);
            signal = true;
            try {
                Thread.sleep(200);
                signal = false;
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public int getAnInt() {
        if (!signal) {
            int p = 1 / 0;
        }
        int result = anInt;
        anInt = 0;
        return result;
    }
}

Answer 1

problem:

private int anInt = 0;
public boolean signal = false;

You are access those variables from one thread to another thus giving you 0 and false on the main thread

solution:

use volatile keyword to access those variables from multiple threads

sample:

private volatile int anInt = 0;
public volatile boolean signal = false;