How does Thread.yield prevent a print statement from executing in a while loop given that the main thread changes the 'while' condition

Question

Below is the example code of the problem:We a have main thread and another thread which calls the enters the run function.

public class WhatDoesThisDo {

private static boolean ready;

private static int number;



private static class ReaderThread extends Thread {

    public void run() {

        while (!ready)

            Thread.yield();



        System.out.println(number);

    }       

}

public static void main(String[] args) {

    new ReaderThread().start();

    number = 42;

    ready = true;

}

}

The correct answer according to some is that it either prints 42, 0 or nothing. Please explain why it can print nothing.