public class CopyOnWriteArrayList {
    private transient volatile Object[] array;

    public boolean add(E e) {
        // 貌似定义一个本地变量指向属性，对性能会有优化？
        // https://stackoverflow.com/questions/3866537/why-j-u-c-copyonwritearraylist-creates-local-lock-variable-inside-methods
        final ReentrantLock lock = this.lock;
        // 只有写写操作才要加锁互斥
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            // 修改时先复制一份数组快照，带上预计的容量
            // 复制快照的成本昂贵
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            // 修改完快照后，再让内部指针指向新数组
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

    public boolean addIfAbsent(E e) 
        Object[] snapshot = getArray();
        // 调用了indexOf检查元素的存在，实质上是遍历，性能不好
        return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :
            addIfAbsent(e, snapshot);
    }
    
    // indexOf使用了遍历，性能不好
    private static int indexOf(Object o, Object[] elements,
                               int index, int fence) {
        if (o == null) {
            for (int i = index; i < fence; i++)
                if (elements[i] == null)
                    return i;
        } else {
            for (int i = index; i < fence; i++)
                if (o.equals(elements[i]))
                    return i;
        }
        return -1;
    }
}