java - 词频计数 Java 8

Question

Java 8中如何统计List的词频？

List <String> wordsList = Lists.newArrayList("hello", "bye", "ciao", "bye", "ciao");

结果必须是:

{ciao=2, hello=1, bye=2}

Answer 1

我想分享我找到的解决方案，因为一开始我希望使用 map-and-reduce 方法，但它有点不同。

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

或者对于整数值:

Map<String,Integer> collect = wordsList.stream()
     .collect( Collectors.groupingBy( Function.identity(), Collectors.summingInt(e -> 1) ));

编辑

我添加了如何按值对 map 进行排序:

LinkedHashMap<String, Long> countByWordSorted = collect.entrySet()
            .stream()
            .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
            .collect(Collectors.toMap(
                    Map.Entry::getKey,
                    Map.Entry::getValue,
                    (v1, v2) -> {
                        throw new IllegalStateException();
                    },
                    LinkedHashMap::new
            ));