如何将Collectors.collectingAndThen与Collectors.groupingBy结合使用

Question

例如，给出了以下列表：

List<String> input = List.of("FOO", "FOO", "FOO", "FOO", "FOO", "BAR", "BAR", "BAZ", "BAZ", "BAZ", "DOO", "DOO"); 

我需要得到每个元素的相对频率，并打印它格式化为一个字符串。为此，我使用了两个步骤，首先创建一个频率图，然后计算相对百分比和格式：

Map<String, Long> relativeFrequency =
        input.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));

Map<String, String> relativeFrequency2 = relativeFrequency.entrySet().stream()
        .collect(Collectors.toMap(Map.Entry::getKey,
                entry -> String.format ( "%.02f%%", entry.getValue() * 100.  / input.size() )));

System.out.println(relativeFrequency2);

得到结果

{BAR=16.67%, DOO=16.67%, FOO=41.67%, BAZ=25.00%}

有人告诉我要重构上面的内容，并使用Collectors.collectingAndThen，一次完成，但是我似乎找不到正确的语法(编译错误Operator '*' cannot be applied to '<lambda parameter>', 'double')。有人能帮助纠正以下情况以获得与上面相同的结果吗？

Map<String, String> relativeFrequency3 = 
input.stream().collect(Collectors.collectingAndThen(Collectors.groupingBy(Function.identity(), Collectors.counting(),
                total -> String.format ( "%.02f%%", total * 100.  / input.size() ))
));

Answer 1

你们关系很好。

List<String> input =
        List.of("FOO", "FOO", "FOO", "FOO", "FOO", "BAR",
                "BAR", "BAZ", "BAZ", "BAZ", "DOO", "DOO");

frequency.

• then
• collectingAndThen的第一个参数是counting()，通过将其转换为格式化百分比来获得处理频率的counting()。

Map<String, String> map = input.stream().collect(Collectors
        .groupingBy(a -> a, Collectors.collectingAndThen(
                Collectors.counting(),
                count -> "%.2f%%".formatted((double) count
                        / input.size() * 100))));
        
map.entrySet().forEach(System.out::println);

打印每一项集的产量

BAR=16.67%
DOO=16.67%
FOO=41.67%
BAZ=25.00%

Answer 2

import java.util.*;
import java.util.stream.*;
import java.util.function.Function;

class Main {
    public static void main(String args[]) {
        var input = List.of("FOO", "FOO", "FOO", "FOO", "FOO", "BAR", "BAR", "BAZ", "BAZ", "BAZ", "DOO", "DOO");
        var out = input.stream()
            .collect(Collectors.groupingBy(
                w -> w,
                Collectors.collectingAndThen(
                    Collectors.summingInt(w -> 1),
                    s -> String.format("%.02f%%", s * 100. / input.size()))
            ));
        System.out.println(out);
    }
}