Java8利用Stream分組統計List（或數組）中元素的個數，結果存放在Map

int[] arr=new int[]{5,1,3,4,1};
//若統計的是int數組，先轉化為List
List<Integer> list= Arrays.stream(arr).boxed().collect(Collectors.toList());
//groupingBy分組
Map<Integer, Long> map = list.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
//控制台輸出map
map.forEach((k,v)->{
      System.out.println("k="+k+",v="+v);
});

k=1,v=2
k=3,v=1
k=4,v=1
k=5,v=1