Are static variables shared between threads?

Question

My teacher in an upper level Java class on threading said something that I wasn't sure of.

He stated that the following code would not necessarily update the ready variable. According to him, the two threads don't necessarily share the static variable, specifically in the case when each thread (main thread versus ReaderThread) is running on its own processor and therefore doesn't share the same registers/cache/etc and one CPU won't update the other.

Essentially, he said it is possible that ready is updated in the main thread, but NOT in the ReaderThread, so that ReaderThread will loop infinitely.

He also claimed it was possible for the program to print 0 or 42. I understand how 42 could be printed, but not 0. He mentioned this would be the case when the number variable is set to the default value.

I thought perhaps it is not guaranteed that the static variable is updated between the threads, but this strikes me as very odd for Java. Does making ready volatile correct this problem?

He showed this code:

public class NoVisibility {  
    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;  
    }  
}