1 public class MergeSort {
 2
 3     public static <T extends Comparable<? super T>> void mergeSort(T[] arr) {
 4         T[] tmpArray = (T[]) new Comparable[arr.length];
 5         mergeSort(arr, tmpArray, 0, arr.length - 1);
 6     }
 7
 8     /**
 9      *
10      * @param arr an array of Comparable items
11      * @param tmpArray an array to place the merge result
12      * @param left the left-most index of the array
13      * @param right right-most index of the array
14      */
15     private static <T extends Comparable<? super T>> void mergeSort(T[] arr,
16             T[] tmpArray, int left, int right) {
17         if (left < right) {
18             int center = (left + right) / 2;
19             mergeSort(arr, tmpArray, left, center);
20             mergeSort(arr, tmpArray, center + 1, right);
21             merge(arr, tmpArray, left, center + 1, right);
22         }
23     }
24
25     /**
26      *
27      * @param arr an array of Comparable items
28      * @param tmpArray an array to place the merge result
29      * @param leftPos the left-most index of the subarray
30      * @param rightPos the index of the start of the second half
31      * @param rightEnd the right-most index of the subarray
32      */
33     private static <T extends Comparable<? super T>> void merge(T[] arr,
34             T[] tmpArray, int leftPos, int rightPos, int rightEnd) {
35         int leftEnd = rightPos - 1;
36         int numElements = rightEnd - leftPos + 1;
37         int tmpPos = leftPos;// 只使用tmpArray中某一部分区域
38         while (leftPos <= leftEnd && rightPos <= rightEnd) {
39             if (arr[leftPos].compareTo(arr[rightPos]) <= 0)
40                 tmpArray[tmpPos++] = arr[leftPos++];
41             else
42                 tmpArray[tmpPos++] = arr[rightPos++];
43         }
44
45         while (leftPos <= leftEnd)
46             tmpArray[tmpPos++] = arr[leftPos++];// copy rest of left half
47         while (rightPos <= rightEnd)
48             tmpArray[tmpPos++] = arr[rightPos++];// copy rest of right half
49
50         // copy tmpArray back
51         for (int i = 0; i < numElements; i++, rightEnd--)
52             arr[rightEnd] = tmpArray[rightEnd];//只拷贝当前 merge 的部分数组
53
54         /**
55          * 复制了整个数组中的所有元素
56           for(int i = 0; i < tmpArray.length; i++)
57                  arr[i] = tmpArray[i];
58          */
59     }
60
61     //for test purpose
62     public static void main(String[] args) {
63         Integer[] arr = {24,13,26,1,2,27,38,15};
64         mergeSort(arr);
65         for (Integer i : arr)
66             System.out.print(i + " ");
67     }
68 }