如何使用 Stream API 查找出现次数最多的字母和出现次数？

Question

我有里面有单个字母的列表。我需要计算所有重复项并找到最频繁的重复项。该列表是随机生成的，因此它可能包含几个最常见的字母。

是否可以在一个 Stream 中只创建一个 map 或将第二个 map 放在 Stream 中？我只需要一个 Stream 链，使用方法 groupingBy().

public static void mostFrequentlyDuplicateLetters(List<String> letters) {   
     Map<String, Long> collect = letterList
            .stream()
            .collect(Collectors.groupingBy(String::valueOf, Collectors.counting()))
            .entrySet()
            .stream()
            .filter(// How to find the most frequent letters and put them on a map?))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        }

Answer 1

首先，您需要的 return 类型实际上是 Map.Entry，而不是整个地图，因为您只需要出现次数最多的条目。

你可以这样试试：

   public Map.Entry<String,Long> mostFrequentlyDuplicateLetters(List<String> letterList){
        return letterList
                .stream()
                .collect(Collectors.groupingBy(String::valueOf, Collectors.counting()))
                .entrySet()
                .stream().max(Map.Entry.comparingByValue())
                .get();
    }

Answer 2

it may contain several most frequent letters

下面的解决方案允许从给定列表中确定 all the letters 具有 maximum frequency.

    public static void main(String[] args) {
        Map<String, Long> frequencies = getFrequencyMap(List.of("A", "B", "B", "C", "C", "C", "B", "D"));
        long max = getMaxFrequency(frequencies);
        System.out.println("max = " + max);
        System.out.println(mostFrequentlyDuplicateLetters(frequencies, max));
    }

    public static Map<String, Long> getFrequencyMap(List<String> letters) {
        return letters.stream()
                .collect(Collectors.groupingBy(UnaryOperator.identity(), Collectors.counting()));
    }

    public static long getMaxFrequency(Map<String, Long> frequencies) {
        return frequencies.values().stream()
                .mapToLong(Long::longValue)
                .max()
                .orElse(0);
    }

    public static List<String> mostFrequentlyDuplicateLetters(Map<String, Long> frequencies, 
                                                              long frequency) {
        return frequencies.entrySet().stream()
                .filter(entry -> entry.getValue() == frequency)
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());
    }

输出（字母B和C的频率都为3）

max = 3
[B, C]

Is it possible to create only one map inside one Stream

如果您希望您的代码既高效又干净，答案是 NO。通过尝试在一种方法中融合多个关注点，您将违反 SOLID 原则中的第一个 - The single responsibility principle.

如何使用 Stream API 查找出现次数最多的字母和出现次数？

How to find the most frequent letters and the number of occurrences, using Stream API?

java

java-stream

collectors