How can I count occurrences with groupBy?

java functional-programming java-8
Muhammad Hewedy picture Muhammad Hewedy · Aug 22, 2014 · Viewed 127k times · Source

I want to collect the items in a stream into a map which groups equal objects together, and maps to the number of occurrences.

List<String> list = Arrays.asList("Hello", "Hello", "World");
Map<String, Long> wordToFrequency = // what goes here?

So in this case, I would like the map to consist of these entries:

Hello -> 2
World -> 1

How can I do that?

Answer

Jon Skeet picture Jon Skeet · Aug 22, 2014

I think you're just looking for the overload which takes another Collector to specify what to do with each group... and then Collectors.counting() to do the counting:

import java.util.*;
import java.util.stream.*;

class Test {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();

        list.add("Hello");
        list.add("Hello");
        list.add("World");

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

        System.out.println(counted);
    }
}

Result:

{Hello=2, World=1}

(There's also the possibility of using groupingByConcurrent for more efficiency. Something to bear in mind for your real code, if it would be safe in your context.)

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