Why does this Java program terminate despite that apparently it shouldn't (and didn't)?

Question

A sensitive operation in my lab today went completely wrong. An actuator on an electron microscope went over its boundary, and after a chain of events I lost $12 million of

Answer 1

You could use an object to synchronize the writes and reads. Otherwise, as others said before, a write to currentPos will occur in the middle of the two reads p.x+1 and p.y.

new Thread() {
    void f(Point p) {
        if (p.x+1 != p.y) {
            System.out.println(p.x+" "+p.y);
            System.exit(1);
        }
    }
    @Override
    public void run() {
        while (currentPos == null);
        while (true)
            f(currentPos);
    }
}.start();
Object sem = new Object();
while (true) {
    synchronized(sem) {
        currentPos = new Point(currentPos.x+1, currentPos.y+1);
    }
}