Comparator的compare方法如何定义升序降序

最近做算法题用了Comparator接口下的compare方法，思考了一下升序和降序的规则是如何来的，现在做一个补充，方便以后回顾。

 升序代码

    public static void main(String[] args) {
        Integer[] nums = new Integer[]{6, 8, 3, 0, 2};
        Arrays.sort(nums, new Comparator<Integer>() {

            @Override
            public int compare(Integer o1, Integer o2) {
                return o1 - o2;
            }
        });
        for (Integer i : nums) {
            System.out.print(i + "  ");
        }
    }

降序代码

    public static void main(String[] args) {
        Integer[] nums = new Integer[]{6, 8, 3, 0, 2};
        Arrays.sort(nums, new Comparator<Integer>() {

            @Override
            public int compare(Integer o1, Integer o2) {
                return o2 - o1;
            }
        });
        for (Integer i : nums) {
            System.out.print(i + "  ");
        }
    }

所以更多时候我们是直接记住了compare（int o1, int o2）方法 return o1 - o2 是升序，return o2 - o1 是降序。那么原因我们不妨跳进去源码看一下

    public static <T> void sort(T[] a, Comparator<? super T> c) {
        if (c == null) {
            sort(a);
        } else {
            if (LegacyMergeSort.userRequested)
                legacyMergeSort(a, c);
            else
                TimSort.sort(a, 0, a.length, c, null, 0, 0);
        }
    }

可以看出他是进去了else内，不妨先进入legacyMergeSort看一下

    private static <T> void legacyMergeSort(T[] a, Comparator<? super T> c) {
        T[] aux = a.clone();
        if (c==null)
            mergeSort(aux, a, 0, a.length, 0);
        else
            mergeSort(aux, a, 0, a.length, 0, c);
    }

这里很明显也是进去了else内，继续看mergeSort

    private static void mergeSort(Object[] src,
                                  Object[] dest,
                                  int low, int high, int off,
                                  Comparator c) {
        int length = high - low;

        // Insertion sort on smallest arrays
        if (length < INSERTIONSORT_THRESHOLD) {
            for (int i=low; i<high; i++)
                for (int j=i; j>low && c.compare(dest[j-1], dest[j])>0; j--)
                    swap(dest, j, j-1);
            return;
        }

        // 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, c);
        mergeSort(dest, src, mid, high, -off, c);

        // 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 (c.compare(src[mid-1], src[mid]) <= 0) {
           System.arraycopy(src, low, dest, destLow, length);
           return;
        }

        // Merge sorted halves (now in src) into dest
        for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
            if (q >= high || p < mid && c.compare(src[p], src[q]) <= 0)
                dest[i] = src[p++];
            else
                dest[i] = src[q++];
        }
    }

这一段的代码关键就是如下部分

        if (length < INSERTIONSORT_THRESHOLD) {
            for (int i=low; i<high; i++)
                for (int j=i; j>low && c.compare(dest[j-1], dest[j])>0; j--)
                    swap(dest, j, j-1);
            return;
        }

可以看到这里面调用了compare方法，当方法的返回值大于0的时候就将数组的前一个数和后一个数做交换。以升序为例来讲解，升序的话compare方法就 return o1 - o2，那么就是 return dest[j-1] - dest[j]。

当 dest[j-1] > dest[j] 时，就进行交换。当 dest[j-1] <= dest[j] 时位置不变，从而达到数组升序。降序也是一样的道理，就不多讲了。