java的Collections中二分查詢原始碼分析

java的Collections類中二分查詢（binarySearch）的實現：

/**判斷List集合是否實現標記介面RandomAccess(ArrayList)或者List集合的長度是否小於5000，如果是滿足以上任何一個條件則採用for(int i = 0; i < size; i++) 來遍歷，

否則使用Iterator迭代器來遍歷。判斷原始碼*/
  public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
        if (c==null)
            return binarySearch((List) list, key);

        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
            return Collections.indexedBinarySearch(list, key, c);
        else
            return Collections.iteratorBinarySearch(list, key, c);
    }

//使用for(int i = 0; i < size; i++)來遍歷二分查詢的原始碼
    private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
        int low = 0;
        int high = l.size()-1;

        while (low <= high) {
            int mid = (low + high) >>> 1;
            T midVal = l.get(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
    }

//使用for(Iterator it;it.hasNext()) it.next()的原始碼 
           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();

        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
    }
   private static <T> T get(ListIterator<? extends T> i, int index) {
        T obj = null;
        int pos = i.nextIndex();
        if (pos <= index) {
            do {
                obj = i.next();
            } while (pos++ < index);
        } else {
            do {
                obj = i.previous();
            } while (--pos > index);
        }
        return obj;
    }