[NP] Membership

public class Membership {

    static List<User> users =new ArrayList<>();;
    static Map<String,Integer> userByNumberOfRegistrations= new HashMap<>();


    public static void createUser(String username, String password, String email, String country, Integer age) {
           Membership.users.add(new User(username,password,email,country,age));
           userByNumberOfRegistrations.putIfAbsent(email,0);
           userByNumberOfRegistrations.computeIfPresent(email,(k,v)->v=v+1);
    }

    public static Map<String,Integer> getNDifferentUsersByEmail() {
        return userByNumberOfRegistrations.entrySet().stream()
                .sorted(Comparator.comparing((Map.Entry<String,Integer> entry)->entry.getValue(),Comparator.reverseOrder()))
                .collect(Collectors.toMap(
                        entry->entry.getKey(),
                        entry->entry.getValue(),
                        (e1, e2) -> e1,
                        LinkedHashMap::new
                ));

    }

    public static List<User> getUsersOrderedByEmail() {
        return users.stream().sorted().collect(Collectors.toList());
    }

    public static boolean deleteUser(int userId) {
        User u = users.stream().filter(i->i.id == userId).findAny().orElse(null);
        if(u != null){
            users.remove(u);
            userByNumberOfRegistrations.computeIfPresent(u.getEmail(),(k,v)->v=Math.max(0,v-1));
            if(userByNumberOfRegistrations.get(u.getEmail())==0){
                userByNumberOfRegistrations.remove(u.getEmail());
            }
            return true;
        }
        return false;
    }
}

public class User implements Comparable<User>{
    String username;
    String password;
    String email;
    PersonalInformation personalInformation;
    int id;
    public static int ID_GENERATOR=0;

    public User(String username, String password, String email, String country, Integer age) {
        this.username= username;
        this.password = password;
        this.email = email;
        this.personalInformation = new PersonalInformation(country, age);
        ++ID_GENERATOR;
        this.id = ID_GENERATOR;
    }

    public String getUsername() {
        return username;
    }

    public String getPassword() {
        return password;
    }

    public String getEmail() {
        return email;
    }

    public PersonalInformation getPersonalInformation() {
        return personalInformation;
    }

    public int getId() {
        return id;
    }

    @Override
    public String toString() {
        return String.format("%d: %s, %s", id, username, email);
    }

    @Override
    public int compareTo(User o) {
        return Comparator.comparing(User::getEmail).compare(this,o);
    }
}

public class PersonalInformation {
    String country;
    int age;

    public PersonalInformation(String country, int age) {
        this.country = country;
        this.age = age;
    }

    public String getCountry() {
        return country;
    }

    public int getAge() {
        return age;
    }
}

public class MembershipTest {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int length = scanner.nextInt();
        scanner.nextLine();
        for(int i=0;i<length;i++){
            String username = scanner.nextLine();
            String password = scanner.nextLine();
            String email = scanner.nextLine();
            String country = scanner.nextLine();
            Integer age = scanner.nextInt();
            scanner.nextLine();
            Membership.createUser(username,password,email,country,age);
        }

        for(int i=0;i<length;i++){
            System.out.println(Membership.getNDifferentUsersByEmail());
            System.out.println(Membership.getUsersOrderedByEmail());
            System.out.println(Membership.deleteUser(i+1));
        }

        System.out.println(Membership.deleteUser(1));
    }
}