我们都知道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]。
也就是说，compare的参数a，b是序列相邻的两个元素，compare定义了什么时候交换这2个元素值，当返回值大于0时交换，小于等于0时不做操作。

参考
[1]:Comparator的compare方法如何定义升序降序