public class ImmutableUser {
    private final String username;
    private final String password;
    private final String firstname;
    private final String lastname;
    private final String email;
    private final Date creationDate;
    private ImmutableUser(UserBuilder builder) {
        this.username = builder.username;
        this.password = builder.password;
        this.creationDate = builder.creationDate;
        this.firstname = builder.firstname;
        this.lastname = builder.lastname;
        this.email = builder.email;
    }

    public static class UserBuilder {
        private final String username;
        private final String password;
        private final Date creationDate;
        private String firstname;
        private String lastname;
        private String email;
        public UserBuilder(String username, String password) {
            this.username = username;
            this.password = password;
            this.creationDate = new Date();
        }

        public UserBuilder firstName(String firsname) {
            this.firstname = firsname;
            return this;
        }

        public UserBuilder lastName(String lastname) {
            this.lastname = lastname;
            return this;
        }

        public ImmutableUser build() {
            return new ImmutableUser(this);
        }
    }

    public Date getCreationDate() {
        return new Date(creationDate.getTime());
    }
}