Java计数整数数组中每个元素的出现次数 [英] Java count occurrence of each element in an integer array

问题描述

我编写了以下代码段来计算每个元素的出现次数。是否有可能以更短的方式实现这一目标？

I've written the following snippet to count the number of occurrences of each element. Is it possible to achieve this in a much shorter way?

int[] arr = {1, 6, 2, 8, 5, 4, 7, 7, 5, 7};
Arrays.stream(arr)
        .collect(ArrayList::new, ArrayList::add, ArrayList::addAll)
        .stream()
        .collect(Collectors.groupingBy(s -> s))
        .forEach((k, v) -> System.out.println(k+" "+v.size()));

此外，我想只显示超过1次的元素。所以我尝试修改如下导致错误。

Also I would like to display only the elements which occur more than 1 time. So I tried modifying as below which resulted in an error.

.forEach((k, v) -> if(v.size() > 1) System.out.println(k+" "+v.size()));

这样做的正确方法是什么？

What is the correct way to do this?

推荐答案

对于后一个问题，你必须改变

For the latter question, you have to change

.forEach((k, v) -> if(v.size() > 1) System.out.println(k+" "+v.size()));

到

.forEach((k, v) -> {if(v.size() > 1) System.out.println(k+" "+v.size());});

对于第一部分，目前尚不清楚为什么需要第一个 collect 后跟第二个 Stream 管道。
如果目的是将 IntStream 转换为 Stream< Integer> ，请使用 boxed（）：

For the first part, it's not clear why you need the first collect followed by a second Stream pipeline. If the purpose was to convert an IntStream to a Stream<Integer>, use boxed():

Arrays.stream(arr)
      .boxed()
      .collect(Collectors.groupingBy(s -> s))
      .forEach((k, v) -> System.out.println(k+" "+v.size()));

正如Dici建议的那样，你也可以链接收藏家将每个数字与其出现次数分组：

As Dici suggested, you can also chain Collectors to group each number with its number of occurrences :

Map<Integer,Integer> occurrences = 
    Arrays.stream(arr)
          .boxed()
          .collect(Collectors.groupingBy(s -> s, Collectors.counting()));

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