public boolean addAll(Collection<? extends E> c) {
    Object[] cs = (c.getClass() == CopyOnWriteArrayList.class) ?
        ((CopyOnWriteArrayList<?>)c).getArray() : c.toArray();
    if (cs.length == 0)
        return false;
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        if (len == 0 && cs.getClass() == Object[].class)
            setArray(cs);
        else {
            Object[] newElements = Arrays.copyOf(elements, len + cs.length);
            System.arraycopy(cs, 0, newElements, len, cs.length);
            setArray(newElements);
        }
        return true;
    } finally {
        lock.unlock();
    }
}