java - Why will happen strange thins，when I reuse the List Collector and put it to HashSet. - 


    public class leetcode3sum {      public static void main(string[] args) {         int[] = {-1, 0, 1, 2, -1, -4};         list<list<integer>> lists = foursum(a, 0);     }      public static list<list<integer>> foursum(int[] nums, int target) {         if (nums == null || nums.length < 4 )             return null;          int len = nums.length;         list<integer> ns = new arraylist<>();         hashset<list<integer>> set = new hashset<>();          (int = 0; < len; ++i)         {             (int j = + 1; j < len; ++j)             {                 (int k = j + 1; k < len; ++k)                 {                     //filter sum not equal target                     if (nums[i] + nums[j] + nums[k] != target)                     {                         continue;                     }                      ns.add(nums[i]);                     ns.add(nums[j]);                     ns.add(nums[k]);                     collections.sort(ns);                     system.out.println(ns);                      set.add(ns);                     system.out.println(set);                     ns.clear();                 }             }         }          list<list<integer>> lists = new arraylist<>();         (list<integer> s : set)             lists.add(s);          return lists;     } }

the strange result is:

[-1, 0, 1] [[-1, 0, 1]] [-1, -1, 2] [[-1, -1, 2], [-1, -1, 2]] [-1, 0, 1] [[-1, 0, 1], [-1, 0, 1]]

what except is:

[-1, 0, 1] [[-1, 0, 1]] [-1, -1, 2] [[-1, -1, 2], [-1, 0, 1]] [-1, 0, 1] [[-1, -1, 2], [-1, 0, 1]]

i don't want new arraylist object every time. if don't it, work will!

i read jdk1.8's resource code, think should not happen strange things. want know why ?


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