国产av日韩一区二区三区精品,成人性爱视频在线观看,国产,欧美,日韩,一区,www.成色av久久成人,2222eeee成人天堂

The function we hope to achieve is to provide an atomic operation for List: if not, add it. Because ArrayList itself is not thread-safe, it is converted into a thread-safe class through the collection Collections.synchronizedList, and then through an auxiliary method to ListAchieve such a function.

class BadListHelper <E> {  
    public List<E> list = Collections.synchronizedList(new ArrayList<E>());  

    public synchronized boolean putIfAbsent(E x) {  
        boolean absent = !list.contains(x);  
        if (absent)  
            list.add(x);  
        return absent;  
    }  
}  
 

Is this code thread-unsafe? If so, can you prove it? Thanks