public static void swap(List<?> list, int i, int j) {
        // instead of using a raw type here, it's possible to capture
        // the wildcard but it will require a call to a supplementary
        // private method
        final List l = list;
        l.set(i, l.set(j, l.get(i)));
    }
    //NB l.set(i, l.set(j,l.get(i)));  l.set(j,)返回j位置的之前的值。
    
    

    @SuppressWarnings("unchecked")
    public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
        if (c==null)
            return binarySearch((List<? extends Comparable<? super T>>) list, key);

        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)//根据有没有整数索引或者list小于5000分成两种查询
            return Collections.indexedBinarySearch(list, key, c);  
        else
            return Collections.iteratorBinarySearch(list, key, c); 
    }
       
    private static <T> int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
        int low = 0;
        int high = list.size()-1;

        while (low <= high) {
            int mid = (low + high) >>> 1;// 写的mid 迭代比较好。 无符号左移一位，速度快
            Comparable<? super T> midVal = list.get(mid);
            int cmp = midVal.compareTo(key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

    private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
        int low = 0;
        int high = l.size()-1;
        ListIterator<? extends T> i = l.listIterator(); //AbstractList 实现了这个listIterator方法，初始cursor＝0

        while (low <= high) {
            int mid = (low + high) >>> 1;
            T midVal = get(i, mid);
            int cmp = c.compare(midVal, key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }


    public static void reverse(List<?> list) {
        int size = list.size();
        if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {//根据是不是小于18或者是不是有整数索引分成两种reverse方式。
            for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
                swap(list, i, j);
        } else {
            // instead of using a raw type here, it's possible to capture
            // the wildcard but it will require a call to a supplementary
            // private method
            ListIterator fwd = list.listIterator();
            ListIterator rev = list.listIterator(size);
            for (int i=0, mid=list.size()>>1; i<mid; i++) {
                Object tmp = fwd.next();
                fwd.set(rev.previous());
                rev.set(tmp);
            }
        }
    }