Практика: Коллекция Map

public class Main {

    public static void main(String[] args) {
        Map<Double, String> map = new HashMap<>();
        map.put(4.57, "Иванов");
        map.put(5.07, "Сидоров");
        map.put(5.12, "Петров");
        map.put(5.15, "Ёлкин");
        map.put(5.22, "Михалчик");

        map.remove(5.15);

        for (Double key : map.keySet()) {
            String value = map.get(key);
            System.out.println(key + " - " + value);
        }
    }
}

public class Main {

    public static void main(String[] args) {
        Map<Double, String> map = new LinkedHashMap<>();
        map.put(4.57, "Иванов");
        map.put(5.07, "Сидоров");
        map.put(5.12, "Петров");
        map.put(5.15, "Ёлкин");
        map.put(5.22, "Михалчик");

        map.remove(5.15);

        for (Double key : map.keySet()) {
            String value = map.get(key);
            System.out.println(key + " - " + value);
        }
    }
}

public class User {
    private int id;
    private String name;
    private String site;

    public User(int id, String name, String site) {
        this.id = id;
        this.name = name;
        this.site = site;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getSite() {
        return site;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        User user = (User) o;
        return id == user.id &&
                Objects.equals(name, user.name) &&
                Objects.equals(site, user.site);
    }

    @Override
    public int hashCode() {

        return Objects.hash(id, name, site);
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", site='" + site + '\'' +
                '}';
    }
}

public class Main {

    public static void main(String[] args) {
        List<User> users = new ArrayList<>();

        users.add(new User(1, "Виталий", "facebook.com"));
        users.add(new User(2, "Михаил", "facebook.com"));
        users.add(new User(3, "Александр", "facebook.com"));
        users.add(new User(4, "Илья", "twitter.com"));
        users.add(new User(5, "Святослав", "twitter.com"));
        users.add(new User(6, "Дмитрий", "twitter.com"));
        users.add(new User(7, "Пётр", "twitter.com"));


        Map<String, Integer> statistic = new HashMap<>();
        for (User user : users) {
            String site = user.getSite();

            Integer count = statistic.get(site);

            if(count == null) {
                statistic.put(site, 1);
            } else {
                Integer newCount = count + 1;
                statistic.put(site, newCount);
            }
        }

        for (Map.Entry<String, Integer> entry : statistic.entrySet()) {
            System.out.println(entry.getKey() + " - " + entry.getValue());
        }
    }
}