Here the code

class Aux implements Runnable {

    private Boolean isOn = false;
    private String statusMessage;
    private final Object lock;

    public Aux(String message, Object lock) {
        this.lock = lock;
        this.statusMessage = message;
    }

    @Override
    public void run() {
        for (;;) {
            synchronized (lock) {

                if (isOn && "left".equals(this.statusMessage)) {
                    isOn = false;
                    System.out.println(statusMessage);
                } else if (!isOn && "right".equals(this.statusMessage)) {
                    isOn = true;
                    System.out.println(statusMessage);
                }

                if ("left".equals(this.statusMessage)) {
                    System.out.println("left " + isOn);
                }

            }
        }
    }
}

public class Question {
    public static void main(String [] args) {
        Object lock = new Object();

        new Thread(new Aux("left", lock)).start();
        new Thread(new Aux("right", lock)).start();
    }
}

In this code I expect to see:

left, right, left right and so on,

but when Thread with "left" message changes isOn to false, Thread with "right" message don't see it and I get ("right true" and "left false" console messages), left thread don't get isOn in true, but right Thread can't change it cause it always see old isOn value (true).

When i add volatile modifier to isOn nothing changes, but if I change isOn to some class with boolean field and change this field then threads are see changes and it works fine

Thanks in advance.