javafunctional-programmingjava-8

How can I count occurrences with groupBy?


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?


Solution

  • 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.)