@Entity
public class User extends Model {

    @Id
    @Constraints.Email
    public String email;

    @Constraints.MinLength(3)
    @Constraints.MaxLength(255)
    public String firstName;

    @Constraints.MinLength(3)
    @Constraints.MaxLength(255)
    public String lastName;

    @Constraints.MinLength(3)
    @Constraints.MaxLength(255)
    public String username;

    @Constraints.MinLength(16)
    @Constraints.MaxLength(255)
    public String password;

    public static Finder<String, User> finder = new Finder<>(User.class);

    public static User create(User user){
        user.password = BCrypt.hashpw(user.password, BCrypt.gensalt(12));
        user.save();
        return user;
        }
     ...
}

@Test
    public void createEmptyUser(){
        User user = new User();
        user.email="";
        user.save();

        assertTrue(user.email.isEmpty());
        assertNotNull(User.finder.byId(user.email));
        assertEquals(true, User.findByEmail(user.email).isPresent());
    }