How can I efficiently count word frequencies in Java 8 using streams?

Word Frequency Counting in Java 8: A Streamlined Approach

In Java 8, counting the frequency of words in a list can be achieved elegantly using streams.

Consider the following example list: List wordsList = Lists.newArrayList("hello", "bye", "ciao", "bye", "ciao");. To count word frequencies, we utilize the Collectors.groupingBy and Collectors.counting collectors.

<code class="java">Map<String, Long> wordFrequencies = wordsList.stream()
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));</code>

This code first groups the list elements by their string identity, effectively creating a map where keys are unique words and values are their counts. The Collectors.counting collector automatically increments the count for each word.

The resulting map, wordFrequencies, will resemble {ciao=2, hello=1, bye=2} when printed.

Alternatively, for integer-valued counts:

<code class="java">Map<String, Integer> wordFrequencies = wordsList.stream()
     .collect(Collectors.groupingBy(Function.identity(), Collectors.summingInt(e -> 1)));</code>

This variant uses Collectors.summingInt to accumulate integer counts.

To sort the resulting map by value, we can chain additional streams and collectors:

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

This sorting ensures that the most frequently occurring words appear first in the map.

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