import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class MapViews {

    public static void main(String[] args) {
        mapChangeModifiesValuesSet();
        mapChangeModifiesKeysSet();
        valuesSetChangeModifiesMap();
    }

    static void mapChangeModifiesValuesSet() {
        Map<Integer, User> usersMap = loadUsersFromWherever();
        UseValues withValues = new UseValues(usersMap.values());
        usersMap.put(4, new User(4, "Donald"));
        withValues.doSomething();
    }

    static UseKeys mapChangeModifiesKeysSet() {
        Map<Integer, User> usersMap = loadUsersFromWherever();
        UseKeys withKeys = new UseKeys(usersMap.keySet());
        usersMap.put(4, new User(4, "Donald"));
        withKeys.doSomething();
        return withKeys;
    }

    static void valuesSetChangeModifiesMap() {
        Map<Integer, User> usersMap = loadUsersFromWherever();
        UseValues withValues = new UseValues(usersMap.values());
        withValues.clear();
        System.out.println("Map content: " + usersMap);
    }

    static class UseValues {

        private final Collection<User> users;

        public UseValues(Collection<User> users) {
            this.users = users;
            System.out.println("UseValues constructor: " + users);
        }

        public void doSomething() {
            System.out.println("UseValues doSomething: " + users);
        }

        public void clear() {
            users.clear();
        }

    }

    static class UseKeys {

        private final Collection<Integer> ids;

        public UseKeys(Collection<Integer> ids) {
            this.ids = ids;
            System.out.println("UseKeys constructor: " + ids);
        }

        public void doSomething() {
            System.out.println("UseKeys doSomething: " + ids);
        }

    }

    static class User {

        private final Integer id;
        private final String name;

        public User(Integer i, String name) {
            this.id = i;
            this.name = name;
        }

        public Integer getId() {
            return id;
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return id + ": " + name;
        }

    }

    static Map<Integer, User> loadUsersFromWherever() {
        Map<Integer, User> users = new HashMap<>();
        users.put(1, new User(1, "John"));
        users.put(2, new User(2, "Peter"));
        users.put(3, new User(3, "Rose"));
        return users;
    }

}