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]);
}
}
將list轉成數組,然後調用Arrays.sort方法排序,最後將排好順序的值覆蓋到原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);
}
克隆一個數組,如果比較器爲空,mergeSort(aux, a, 0, a.length, 0);如果比較器不爲空,mergeSort(aux, a, 0, a.length, 0, c);二者內部算法實現一致,只是比較元素的方法不一樣。下面來看歸併排序的實現,看其是如何優化的。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++];
}
}
我們分段來看。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;
}
這裏有一個常量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;
當數組長度<7時,這裏使用了直接插入排序。直接插入排序的過程可以看這個視頻,插入排序適用於小數列的排序。這裏是JDK6中歸併排序的第一個優化。// Recursively sort halves of dest into src
int destLow = low;
int destHigh = high;
low += off;
high += off;
int mid = (low + high) >>> 1;// 中間索引,相當於(low + high) / 2
mergeSort(dest, src, low, mid, -off, c);// 排序左邊
mergeSort(dest, src, mid, high, -off, c);// 排序右邊
這裏開始遞歸排序,我們不需要關注off變量,這個變量是排序數組中部分區域的時候使用的,而我們要排序的是整個數組。// 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;
}
左邊和右邊排好序之後,開始合併。這時src[low ~ mid - 1]和src[mid ~ high - 1]都是有序的,這時比較src[mid - 1]和src[mid],如果前者比後者小,那麼皆大歡喜,真個src數組就是有序的了,只需將其複製到目標數組後,就完成了排序,不過這種碰運氣的機率會比較小。這裏是JDK6中歸併排序的第二個優化。// 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++];
}
程序執行到這裏,進行傳統的合併操作。其過程如下圖:初始狀態:
循環一次後:
每次都比較src[p]和src[q],將較小的元素存儲到dest[i],不斷的循環比較,直至整個數組都有序。
最終: