Muhammad Hewedy Muhammad Hewedy - 21 days ago 10
Java Question

Group by counting in Java8 stream API

I try to find a simple way in Java8 stream API to do the grouping, I come out with this complex way!

List<String> list = new ArrayList<>();

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

Map<String, List<String>> collect = list.stream().collect(
Collectors.groupingBy(o -> o));
System.out.println(collect);

List<String[]> collect2 = collect
.entrySet()
.stream()
.map(e -> new String[] { e.getKey(),
String.valueOf(e.getValue().size()) })
.collect(Collectors.toList());

collect2.forEach(o -> System.out.println(o[0] + " >> " + o[1]));


I appreciate your input.

Answer

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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