StringBuilder modified by multiple threads

The question I am asking is related to Difference between StringBuilder and StringBuffer but not the same. I want to see what really happens if a StringBuilder is modified by two threads at the same time.

I wrote the following classes:

public class ThreadTester
{
    public static void main(String[] args) throws InterruptedException
    {
        Runnable threadJob = new MyRunnable();
        Thread myThread = new Thread(threadJob);
        myThread.start();

        for (int i = 0; i < 100; i++)
        {
            Thread.sleep(10);
            StringContainer.addToSb("a");
        }

        System.out.println("1: " + StringContainer.getSb());
        System.out.println("1 length: " + StringContainer.getSb().length());
    }
}

public class MyRunnable implements Runnable
{
    @Override
    public void run()
    {
        for (int i = 0; i < 100; i++)
        {
            try
            {
                Thread.sleep(10);
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
            StringContainer.addToSb("b");
        }

        System.out.println("2: " + StringContainer.getSb());
        System.out.println("2 length: " + StringContainer.getSb().length());
    }
}

public class StringContainer
{
    private static final StringBuffer sb = new StringBuffer();

    public static StringBuffer getSb()
    {
        return sb;
    }

    public static void addToSb(String s)
    {
        sb.append(s);
    }
}

Initially I kept a StringBuffer in the StringContainer. Since StringBuffer is thread-safe, at a time, only one thread can append to it, so the output is consistent - either both threads reported the length of the buffer as 200, like:

1: abababababababababbaabababababababbaababababababababababababbabaabbababaabbaababababbababaab