计数流的元素 [英] Counting elements of a Stream
问题描述
我想计算一个流的不同元素，我想知道为什么
I want to count the different elements of a stream and am wondering why
Stream<String> stream = Stream.of("a", "b", "a", "c", "c", "a", "a", "d");
Map<String, Integer> counter1 = stream.collect(Collectors.toMap(s -> s, 1, Integer::sum));
无法使用。 Eclipse告诉我
doesn't work. Eclipse tells me
类型Collectors中的toMap（Function，Function，BinaryOperator）方法不适用于参数（（s） > {}，int，Integer :: sum）
The method toMap(Function, Function, BinaryOperator) in the type Collectors is not applicable for the arguments (( s) -> {}, int, Integer::sum)
顺便说一下，我知道这个解决方案：
By the way, I know about that solution:
Map<String, Long> counter2 = stream.collect(Collectors.groupingBy(s -> s, Collectors.counting()));
所以我有两个问题：
- 我的第一种方法有什么错误？
- 如何实现这样的计数器？
EDIT：我自己解决了第一个问题：
I solved the first question by myself:
Map<String, Integer> counter1 = stream.collect(Collectors.toMap(s -> s, s -> 1, Integer::sum));
Java正在等待第二个参数。
Java is expecting a function as second argument.
推荐答案
确实有几种方法。您没有提到的是 .collect（groupingBy（x - > x，summingInt（x - > 1）））;
There are indeed several ways to do it. The one you haven't mentioned is .collect(groupingBy(x -> x, summingInt(x -> 1)));
性能有一些差异。
如果每个桶中的对象非常少，则方法1将处于最佳状态。在每个桶只有1个对象的理想情况下，您将立即结束最终映射，无需修改条目。
Approach #1 is going to be at its best if there are very few objects per bucket. In the ideal case of only 1 object per bucket, you end up with the final map right away with no need to modify the entries. In the worst case of having a very large number of repeated objects, it will have to do a lot of boxing/unboxing.
方法＃2依赖于 counting（） collector，它没有指定它应该如何做计数。当前实现转发到减少，但可能会改变。
Approach #2 relies on counting() collector, which doesn't specify exactly how it should do the counting. The current implementation forwards to reducing but that might change.
summingInt 方法将累积 int 中的计数，而不是整数，因此不需要任何装箱/取消装箱。
The summingInt approach will accumulate the count in int rather than Integer and thus will not require any boxing/unboxing. It will be at its best if objects repeat a very large number of times.
对于选择哪一个，最好是编写清晰度并优化，当它变成必要。对我来说， groupingBy（x-> x，counting（））表达的意图最清楚，所以这是我赞成的。
As for which one to choose, it is best to code for clarity and optimize when it becomes necessary. To me, groupingBy(x->x, counting()) expresses the intent most clearly, so that's the one I would favor.
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