Java의 불변 클래스

class Address{
    private String houseNumber;
    private String street;
    private String city;
}

final class User {
    private final int id;
    private final String name;
    private final String email;
    private final Address address;

    /*
     * To clone the address.
     */
    private static Address cloneAddress(Address address) {
        return new ObjectMapper().convertValue(address, Address.class);
    }

    User(int id, String name, String email, Address address) {
        this.id = id;
        this.name = name;
        this.email = email;
        /*
         * Any field contains reference of any mutable 
                 * object must be
         * initialized with the cloned object
         */
        this.address = cloneAddress(address);
    }
    public int getId() {
        return id;
    }
    public String getName() {
        return name;
    }
    public String getEmail() {
        return email;
    }
    public Address getAddress() {
        /*
         * Getter method must return the reference of cloned object.
         */
        return cloneAddress(address);
    }
}