java如何向synchronizedList添加线程安全方法

Question

public class ListHelper {
    private final List<String> list= Collections.synchronizedList(new ArrayList<>());

    public boolean putIfAbsent(String x) {
        System.out.println(Thread.currentThread().getName()+" start putIfAbsent");
        synchronized (list){
            boolean absent=!list.contains(x);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            if (absent){
                System.out.println("add前hashcode: "+list.hashCode());
                list.add(x);
            }
            System.out.println(Thread.currentThread().getName()+" end putIfAbsent");
            return absent;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ListHelper helper = new ListHelper();
        CountDownLatch countDownLatch = new CountDownLatch(2);
        String a = "a";
        new Thread(()->{
            System.out.println(Thread.currentThread().getName()+"-"+helper.putIfAbsent(a));
            countDownLatch.countDown();
        },"A").start();
        new Thread(()->{
            System.out.println(Thread.currentThread().getName()+"-list.add() start");
            System.out.println(Thread.currentThread().getName()+"-"+helper.list.add(a));
            countDownLatch.countDown();
        },"B").start();
        countDownLatch.await();
        for (Object o : helper.list) {
            System.out.println(o);
        }
    }
}

我认为使用list作为锁是正确的，但程序的输出让我惊讶。 程序输出如下所示：

A start putIfAbsent
B-list.add() start
A end putIfAbsent
A-true
B-true
a
a

list似乎并不相互排斥