Java 8 - count unique objects eg Users and return Map<User, Integer> using Collectors.groupingBy

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

import java.util.function.Function;

import java.util.stream.Collectors;

​

public class CountUsers {

​

    public static void main(String[] args) {

​

        List<User> users = new ArrayList<>();

        users.add(new User("Kate", 24)); // Kate - 1st time

        users.add(new User("John", 26));

        users.add(new User("Ann", 25));

        users.add(new User("Kate", 24)); // Kate - 2nd time

        users.add(new User("Kate", 24)); // Kate - 3rd time

​

        Map<User, Long> usersToCount = users.stream().collect(

                Collectors.groupingBy(

                        Function.identity(), Collectors.counting()

                )

        );

​

        usersToCount.forEach(

                (user, count) -> System.out.println(user + ", " + count)

        );

    }

​

    private static class User {

        private final String name;

        private final int age;

​

        public User(String name, int age) {

            this.name = name;

            this.age = age;

        }

​

        public String getName() {

            return name;

        }

​

        public int getAge() {

            return age;

        }

​

        @Override

        public boolean equals(Object o) {

            if (this == o) return true;

            if (o == null || getClass() != o.getClass()) return false;

​

            User user = (User) o;

​

            if (age != user.age) return false;

            return name != null ? name.equals(user.name) : user.name == null;

        }

​

        @Override

        public int hashCode() {

            int result = name != null ? name.hashCode() : 0;

            result = 31 * result + age;

            return result;

        }

​

        @Override

        public String toString() {

            return "User{" +

                    "name='" + name + '\'' +

                    ", age=" + age +

                    '}';

        }

    }

}

User{name='Ann', age=25}, 1

User{name='John', age=26}, 1

User{name='Kate', age=24}, 3