快速排序算法的JAVA实现

package Utils.Sort;

/**

*快速排序，要求待排序的数组必须实现Comparable接口

*/

public class QuickSort implements SortStrategy

{

       private static final int CUTOFF = 3;             //当元素数大于此值时采用快速排序

       /**

       *利用快速排序算法对数组obj进行排序，要求待排序的数组必须实现了Comparable接口

       */

       public void sort(Comparable[] obj)

       {

              if (obj == null)

              {

                     throw new NullPointerException("The argument can not be null!");

              }

              quickSort(obj, 0, obj.length - 1);

       }

       /**

       *对数组obj快速排序

       *@param obj　待排序的数组

       *@param left 数组的下界

       *@param right 数组的上界

       */

       private void quickSort(Comparable[] obj, int left, int right)

       {

              if (left + CUTOFF > right)

              {

                     SortStrategy ss = new ChooseSort();

                     ss.sort(obj);

              }

              else

              {

                     //找出枢轴点，并将它放在数组最后面的位置

                     pivot(obj, left, right);

                    

                     int i = left, j = right - 1;

                     Comparable tmp = null;

                     while (true)

                     {

                            //将i, j分别移到大于/小于枢纽值的位置

                            //因为数组的第一个和倒数第二个元素分别小于和大于枢纽元，所以不会发生数组越界

                            while (obj[++i].compareTo(obj[right - 1]) < 0)    {}

                            while (obj[--j].compareTo(obj[right - 1]) > 0)      {}

                           

                            //交换

                            if (i < j)

                            {

                                   tmp = obj[i];

                                   obj[i] = obj[j];

                                   obj[j] = tmp;

                            }

                            else

                                   break;

                     }

                     //将枢纽值与i指向的值交换

                     tmp = obj[i];

                     obj[i] = obj[right - 1];

                     obj[right - 1] = tmp;

                     //对枢纽值左侧和右侧数组继续进行快速排序

                     quickSort(obj, left, i - 1);

                     quickSort(obj, i + 1, right);

              }

       }

       /**

       *在数组obj中选取枢纽元，选取方法为取数组第一个、中间一个、最后一个元素中中间的一个。将枢纽元置于倒数第二个位置，三个中最大的放在数组最后一个位置，最小的放在第一个位置

       *@param obj 要选择枢纽元的数组

       *@param left 数组的下界

       *@param right 数组的上界

       */

       private void pivot(Comparable[] obj, int left, int right)

       {

              int center = (left + right) / 2;

              Comparable tmp = null;

             

              if (obj[left].compareTo(obj[center]) > 0)

              {

                     tmp = obj[left];

                     obj[left] = obj[center];

                     obj[center] = tmp;

              }

              if (obj[left].compareTo(obj[right]) > 0)

              {

                     tmp = obj[left];

                     obj[left] = obj[right];

                     obj[right] = tmp;

              }

              if (obj[center].compareTo(obj[right]) > 0)

              {

                     tmp = obj[center];

                     obj[center] = obj[right];

                     obj[center] = tmp;

              }

              //将枢纽元置于数组的倒数第二个

             

              tmp = obj[center];

              obj[center] = obj[right - 1];

              obj[right - 1] = tmp;

       }

}