A recent problem with the compare algorithm method under the Comparator interface, thought about how the rules are to ascending and descending, and now do a supplement to facilitate later review.
Ascending Code
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 + " "); } }
Descending Code
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 + " "); } }
More so when we remember is directly compare (int o1, int o2) method return o1 - o2 is ascending, return o2 - o1 is descending. So we might as well jump into the reasons to look at the source code
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); } }
It can be seen that he is a go inside else, may wish to look into the legacyMergeSort
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); }
Here it is clear also go inside else, continue to look 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++]; } }
This section of the code that follows is a key part
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; }
It can be seen that the compare method is called inside, when the return value is greater than 0, the method will be an array of a number of front and rear of a number in exchange. In ascending order as an example to explain, in ascending order, then compare the method to return o1 - o2, then that return dest [j-1] - dest [j].
When dest [j-1]> When dest [j], to be exchanged. When dest [j-1] <= dest [j] the same position, so as to achieve an array in ascending order. Descending is the same reason, do not speak.