标签：归并排序 mergesort collections.sort
本文是对JDK6中Collections.sort方法的源码解析，也可以看作是对Comparison method violates its general contract!的后续分析。在JDK6中，该方法底层使用的是经过优化后的归并排序，废话不多说，直接看源码。public static <T> void sort(List<T> list, Comparator<? super T> c) {
Object[] a = list.toArray();
Arrays.sort(a, (Comparator)c);
ListIterator i = list.listIterator();
for (int j=0; j<a.length; j++) {
i.next();
i.set(a[j]);
}
} 将list转成数组，然后调用Arrays.sort方法排序，最后将排好顺序的值覆盖到原list上。public static <T> void sort(T[] a, Comparator<? super T> c) {
T[] aux = (T[])a.clone();
if (c==null)
mergeSort(aux, a, 0, a.length, 0);
else
mergeSort(aux, a, 0, a.length, 0, c);
} 克隆一个数组，如果比较器为空，mergeSort(aux, a, 0, a.length, 0);如果比较器不为空，mergeSort(aux, a, 0, a.length, 0, c);二者内部算法实现一致，只是比较元素的方法不一样。下面来看归并排序的实现，看其是如何优化的。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++];
}
} 我们分段来看。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;
} 这里有一个常量INSERTIONSORT_THRESHOLD。/** * Tuning parameter: list size at or below which insertion sort will be * used in preference to mergesort or quicksort. */ private static final int INSERTIONSORT_THRESHOLD = 7;当数组长度<7时，这里使用了直接插入排序。直接插入排序的过程可以看这个视频，插入排序适用于小数列的排序。这里是JDK6中归并排序的第一个优化。
// Recursively sort halves of dest into src int destLow = low; int destHigh = high; low += off; high += off; int mid = (low + high) >>> 1;// 中间索引，相当于(low + high) / 2 mergeSort(dest, src, low, mid, -off, c);// 排序左边 mergeSort(dest, src, mid, high, -off, c);// 排序右边这里开始递归排序，我们不需要关注off变量，这个变量是排序数组中部分区域的时候使用的，而我们要排序的是整个数组。
// 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;
} 左边和右边排好序之后，开始合并。这时src[low ~ mid - 1]和src[mid ~ high - 1]都是有序的，这时比较src[mid - 1]和src[mid]，如果前者比后者小，那么皆大欢喜，真个src数组就是有序的了，只需将其复制到目标数组后，就完成了排序，不过这种碰运气的几率会比较小。这里是JDK6中归并排序的第二个优化。// 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++];
} 程序执行到这里，进行传统的合并操作。其过程如下图：
最终：
Collections.sort in JDK6：MergeSort
标签：归并排序 mergesort collections.sort
原文地址：http://blog.csdn.net/ghsau/article/details/42060651
