证明ArrayList线程不安全以及如何安全

在知乎上看到了一个话题，如何判断 Java 工程师的基础知识是否扎实？
看到一个回答印象很深刻，其中说到证明ArrayList线程不安全以及如何使之安全，写了一份代码：

public class ThreadSafeDemo {
    public static int demo(final List list, final int testCount) throws InterruptedException {
        ThreadGroup group = new ThreadGroup(list.getClass().getName() + "@" + list.hashCode());
        final Random rand = new Random();

        Runnable listAppender = new Runnable() {
            public void run() {
                try {
                    Thread.sleep(rand.nextInt(2));
                } catch (InterruptedException e) {
                    return;
                }
                list.add("0");
            }
        };

        for (int i = 0; i < testCount; i++) {
            new Thread(group, listAppender, "InsertList-" + i).start();
        }

        while (group.activeCount() > 0) {
            Thread.sleep(10);
        }

        return list.size();
    }

    public static void main(String[] args) throws InterruptedException {
        List unsafeList = new ArrayList();
        List safeList = Collections.synchronizedList(new ArrayList());
        final int N = 10000;
        for (int i = 0; i < 10; i++) {
            unsafeList.clear();
            safeList.clear();
            int unsafeSize = demo(unsafeList, N);
            int safeSize = demo(safeList, N);
            System.out.println("unsafe/safe: " + unsafeSize + "/" + safeSize);
        }
    }
}

测试结果：