immutability

class Money
  include Comparable

  attr_reader :amount, :currency

  def initialize(amount, currency = 'USD')

import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.Currency;
import java.util.Objects;

public final class Money {

class PriceBreakdown < Struct.new(:subtotal_cents, :discount_cents, :tax_cents, keyword_init: true)
  def initialize(*)
    super
    freeze
  end