1，Arrays 中 实现的二分查找

public static int binarySearch(int[] a, int fromIndex, int toIndex,                                   int key) {        rangeCheck(a.length, fromIndex, toIndex);           // 边界测试        return binarySearch0(a, fromIndex, toIndex, key);   // private method    }    // Like public version, but without range checks.    private static int binarySearch0(int[] a, int fromIndex, int toIndex,                                     int key) {        int low = fromIndex;                                // 包含头，不包含尾        int high = toIndex - 1;        while (low <= high) {                               // 有等号            int mid = (low + high) >>> 1;                   // 绝对右移            int midVal = a[mid];            if (midVal < key)                low = mid + 1;            else if (midVal > key)                high = mid - 1;            else                return mid; // key found        }        return -(low + 1);  // key not found.               // 没有找到的话，返回的就是插入位置（负数）    }

2，Collections 中的二分查找实现

public static 
    
         int binarySearch(List
     > list, T key) {        if (list instanceof RandomAccess || list.size()
     
          int indexedBinarySearch(List
      > list, T key) {        int low = 0;        int high = list.size()-1;        while (low <= high) {            int mid = (low + high) >>> 1;            Comparable
       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 
      
           int iteratorBinarySearch(List
       > list, T key)    {        int low = 0;        int high = list.size()-1;        ListIterator
       > i = list.listIterator();        while (low <= high) {            int mid = (low + high) >>> 1;            Comparable
        midVal = get(i, mid);    // 注意： i 这个 iterator 在查找过程中没有再重新初始化            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    }    /**     * Gets the ith element from the given list by repositioning the specified     * list listIterator.     */    private static 
       
         T get(ListIterator
         i, int index) {        T obj = null;        int pos = i.nextIndex();        if (pos <= index) {            do {                obj = i.next();             // 往后找            } while (pos++ < index);        } else {            do {                obj = i.previous();         // 往前找            } while (--pos > index);        }        return obj;    }