Java 8 - count unique String using Collectors.groupingBy

Quick solution:

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class CountStrings {

    public static void main(String[] args) {

        List<String> usernames = new ArrayList<>();
        usernames.add("Kate"); // Kate - 1st time
        usernames.add("John");
        usernames.add("Ann");
        usernames.add("Kate"); // Kate - 2nd time
        usernames.add("Kate"); // Kate - 3rd time

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

        usersToCount.forEach(
                (user, count) -> System.out.println(user + ", " + count)
        );
    }
}

Output:

Ann, 1
Kate, 3
John, 1