coupling_example.rb

class Invoice
  def initialize *lines
    @lines = lines
  end
  
  def total
    @lines.inject(0){ |subtotal, line| subtotal += line.total }
  end
end

class InvoiceLine
  def initialize quantity, product
    @quantity = quantity
    @product = product
  end
  
  def total
    @product.price.multiply_by @quantity
  end
end

class Product
  def initialize name, price
    @name = name
    @price = Money.new price
  end
  
  def price
    return @price
  end
end

class Money
  def initialize money
    @money = money
  end
  
  def multiply_by quantity
    @money * quantity
  end
end

ice_tea = Product.new "ice tea", 300
coca_cola = Product.new "Coca Cola", 1500

line1 = InvoiceLine.new 3, ice_tea
line2 = InvoiceLine.new 2, coca_cola

invoice = Invoice.new line1, line2
print invoice.total