JDK（三）JDK1.8源码分析【排序】mergeSort

归并排序是JDK对象数组的排序算法之一，接下来开始分析JDK的归并排序算法。

    /**
     * 将指定范围的对象数组按自然顺序升序排序
     * 
     * Src is the source array that starts at index 0
     * Dest is the (possibly larger) array destination with a possible offset
     * low is the index in dest to start sorting
     * high is the end index in dest to end sorting
     * off is the offset to generate corresponding low, high in src
     * To be removed in a future release.
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    private static void mergeSort(Object[] src,
                                  Object[] dest,
                                  int low,
                                  int high,
                                  int off) {
        int length = high - low;

        // 规模很小的数组的排序，直接插入排序的效率反而比归并要高
        // 规模定在INSERTIONSORT_THRESHOLD = 7之内
        // Insertion sort on smallest arrays
        if (length < INSERTIONSORT_THRESHOLD) {
            for (int i=low; i<high; i++)
                for (int j=i; j>low &&
                         ((Comparable) dest[j-1]).compareTo(dest[j])>0; j--)
                    swap(dest, j, j-1);
            return;
        }

        // 递归排序dest的一半元素并赋值给src  
        // Recursively sort halves of dest into src
        int destLow  = low;
        int destHigh = high;
        low  += off;
        high += off;
        int mid = (low + high) >>> 1;
        mergeSort(dest, src, low, mid, -off);
        mergeSort(dest, src, mid, high, -off);

        // 如果低子列表中的最高元素小于高子列表中的最低元素，则忽略合并
        // 如果需要归并的两端low~(mid-1)，mid~high已经有序，即src[mid-1]==src[mid]，那么只需要将src的low~high赋值对应的dest即可，无需再归并
        // If list is already sorted, just copy from src to dest.  This is an
        // optimization that results in faster sorts for nearly ordered lists.
        if (((Comparable)src[mid-1]).compareTo(src[mid]) <= 0) {
            System.arraycopy(src, low, dest, destLow, length);
            return;
        }

        // 将src的两个部分合并，并赋值给dest
        // Merge sorted halves (now in src) into dest
        for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
            if (q >= high || p < mid && ((Comparable)src[p]).compareTo(src[q])<=0)
                dest[i] = src[p++];
            else
                dest[i] = src[q++];
        }
    }