A piece of code about Java keyword volatile

Question

I have write a piece of code as below to test the function of the keyword volatile. If I run this, the Thread "BBB" will never jumps out of the loop.

I can add volatile to field number of MyDate to make it visible between different thread, which can make the program finish normally. But without the volatile keyword, if I write some irrelevant statement in the While block of RunnableTwo.run(), it can also make the program run normally. So What exactly happens here?

class MyData {
    int number = 0;

    public void changeNumber() {
        this.number = 60;
    }
}

class RunnableOne implements Runnable {
    private MyData myData;

    public RunnableOne(MyData myData) {
        this.myData = myData;
    }

    @Override
    public void run() {
        {
            System.out.println("Thread" + Thread.currentThread().getName() + "init with the data:" + myData.number);
            try {
                Thread.sleep(200);
            } catch (Exception e) {
                e.printStackTrace();
            }
            myData.changeNumber();
            System.out.println("Thread" + Thread.currentThread().getName() + "finish with the data:" + myData.number);
        }

    }
}


class RunnableTwo implements Runnable {
    private MyData myData;

    public RunnableTwo(MyData myData) {
        this.myData = myData;
    }

    @Override
    public void run() {


        System.out.println("Thread" + Thread.currentThread().getName() + "init with the data:" + myData.number);
        while (myData.number == 0) {
            //System.out.println("wait");

        }

        System.out.println("Thread" + Thread.currentThread().getName() + "finish with the data:" + myData.number);

    }


}


public class VolatileDemo {
    public static void main(String[] args) {
        MyData myData = new MyData();
        new Thread(new RunnableOne(myData), "AAA").start();
        new Thread(new RunnableTwo(myData), "BBB").start();
    }
}