Comprehensive Ruby Programming

The class that knew too much

Here we have an Invoice class that seems to be relatively straightforward:

• It prints out details about the invoice
• It calculates sales tax
• It emails the invoice with its details

Following is the implementation of the Invoice class:

class Invoice  def initialize(customer:, state:, total:)    @customer = customer    @state = state    @total = total  end  def details    "Customer: #{@customer}, Total: #{@total}"  end  def sales_tax    case @state    when 'AZ' then 5.5    when 'TX' then 3.2    when 'CA' then 8.7    end  end  def email_invoice    puts "Emailing invoice..."    puts details  endendinvoice = Invoice.new(customer: "Google", state: "AZ", total: 100)puts invoice.sales_taxinvoice.email_invoice 

When we run this code, everything works fine and ...