class User {
    private int id;
    private String email;

    User(int id, String email) {
        this.id = id
        this.email = email
    }

    // equals, hashcode
}

@Test
public void assertUsers() {
    Set<User> expected = new HashSet<>() {
        {
            add(new User(1, "1@qwerty.com"));
            add(new User(3, "2@qwerty.com"));
            add(new User(5, "3@qwerty.com"));
            add(new User(7, "4@qwerty.com"));
        }
    }

    Set<User> actual = new HashSet<>() {
        {
            add(new User(2, "1@qwerty.com"));
            add(new User(4, "2@qwerty.com"));
            add(new User(6, "3@qwerty.com"));
            add(new User(8, "4@qwerty.com"));
        }
    }

    assertEquals(expected, actual); // should return true

}

class IdConverter {
    // internal id - external id
    public static Map<Integer, Integer> getIdRelation() {
        Map<Integer, Integer> id_relation = new HashMap<>();
        id_relation.put(1, 2);
        id_relation.put(3, 4);
        id_relation.put(5, 6);
        id_relation.put(7, 8);
        return id_relation;
    }
}