#vulnerable code 
                                @Override
    public boolean addAll(int index, Collection<? extends T> c) {
        for (T element : c) {
            internalIndexedAdd(index++, element, false);
        }
        refreshStorage(true);
        return true;
    } 
                            #location 4 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Override
    public boolean addAll(int index, Collection<? extends T> c) {
        for (T element : c) {
            internalIndexedAdd(index++, element);
        }
        return true;
    }