java - 如何使用 Collectors.collectionAndThen 和 Collectors.groupingBy

Question

以下面的列表为例:

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

我需要获取每个元素的相对频率并将其打印为字符串格式。为此，我使用了 2 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);

并得到结果

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

我被告知要重构上述内容并使用 Collectors.collectionAndThen 一次性完成，但我似乎找不到正确的语法(编译错误 Operator '*' cannot be applied to '<lambda parameter>', 'double')。有人可以帮助更正以下内容以获得与上述相同的结果吗？

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

你很接近。

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

• collectingAndThen 的第一个参数是 counting() 以获得频率。
• 然后您通过将频率转换为格式化的百分比来处理该频率。

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);

打印每个条目集产生

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