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Comparator升序降序实现
int compare(Object o1, Object o2) 返回一个基本类型的整型
1) 如果要按照升序排序,
则o1 小于o2,返回-1(负数),相等返回0,01大于02返回1(正数)
那这是为什么呢?通过底层源码了解一下:
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); } }
可以看出应该是进入到了else中的if,那么legacyMergeSort是个什么鬼呢,继续往下看
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); }
就是他了,可以看出来这里也应该是进入到了else中,那么继续往下看,mergesort又是个什么鬼呢。
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++]; } }这段代码就比较长了,重点来了(敲黑板!!!)
for (int j=i; j>low && c.compare(dest[j-1], dest[j])>0; j--) swap(dest, j, j-1);这个看到了么, 如果调用compare方法大于0,就把前一个数和后一个数交换,也就是把大的数放后面了,即所谓的 升序了。那么再想想看return arg0-arg1是不是返回的与这里正好匹配呢。差不多说这么多,相信你应该明白了。