public class User {
    private final String name;
    private final String email;
    private final int age;
    private final String phone;

    private User(Builder builder) {
        this.name = builder.name;
        this.email = builder.email;
        this.age = builder.age;
        this.phone = builder.phone;
    }

    public static class Builder {
        private String name;
        private String email;
        private int age;
        private String phone;

        public Builder name(String name) {
            this.name = name; return this;
        }
        public Builder email(String email) {
            this.email = email; return this;
        }
        public Builder age(int age) {
            this.age = age; return this;
        }
        public Builder phone(String phone) {
            this.phone = phone; return this;
        }
        public User build() {
            return new User(this);
        }
    }

    public static void main(String[] args) {
        User user = new User.Builder()
            .name("Alice")
            .email("alice@example.com")
            .age(30)
            .build();
    }
}