OOP in Ruby - UK Income Tax Calculator

Today I wanted to learn more about OOP and do some actual coding. I started with a procedural paradigm and then recoded to OOP and learn from it. I wrote a very simple program in Ruby which calculates income tax in UK (very original!! hehe). I decided to use three objects....but can you come up with something different?

#!/usr/bin/env ruby

class Employee

 attr_reader :salary, :type, :taxtotal

 def initialize( salary, type )
  @salary = salary
  @type = type
 end

 def computetaxbands
  case type  # current tax rates are the same for single and married persons
  when 1
   bandA = 2790
   bandB = 32010
   bandC = 150000
   rateA = 0.1
   rateB = 0.2
   rateC = 0.4
   rateD = 0.5
  end
  
  case salary
  when 0..bandA
   calctax( IncomeTaxBand.new( salary, rateA ) )
  when bandA+1..bandB
   calctax( IncomeTaxBand.new( salary, rateB ) )
  when bandB+1..bandC
   calctax( IncomeTaxBand.new( bandB, rateB ), IncomeTaxBand.new( salary - bandB, rateC ) )
  else
   calctax( IncomeTaxBand.new( bandB, rateB ), IncomeTaxBand.new( bandC, rateC ), IncomeTaxBand.new( salary - bandC, rateD )  )
  end
  
 end

 def calctax( *taxbands ) #taxbands is an array of IncomeTaxBand objects
  @taxtotal = 0
  taxband_inst = taxbands
  taxbands.each { | taxband_inst | @taxtotal += taxband_inst.tax }
 end
 
end

class IncomeTaxBand
 
 attr_reader :tax
 
 def initialize( amount, pct )
  @tax = amount * pct
 end

end

class TaxViewer

 def inputvalid?(salary)
  if (salary.respond_to?(:match) && salary.match(/\d/))
   true
  else
   false
  end
 end 
 
 def annual(salary, tax)
  netannual = (salary - tax).round  
 end
 
 def monthly(salary, tax)
  netmonthly = ((salary - tax) / 12).round
 end
 
 def weekly(salary, tax)
  netweekly = ((salary - tax) / 52).round
 end

end

puts "UK Income tax calculator (2013-2014):\n\n"
loop do

 viewer = TaxViewer.new
 print "Please enter your gross annual Salary: £"
 salary = gets.chomp

 if viewer.inputvalid? salary

  emp = Employee.new(salary.to_i, 1)
  emp.computetaxbands

  printf "Total Tax: £%d\n", emp.taxtotal
  printf "Net Salary: £%d\n", viewer.annual(salary.to_i, emp.taxtotal)
  printf "Net Month: £%d\n", viewer.monthly(salary.to_i, emp.taxtotal)
  printf "Net Week: £%d\n", viewer.weekly(salary.to_i, emp.taxtotal) 
  break
 else
  print "\nPlease enter a valid number!\n\n"
 end

end

You can pull this code from Github for easier readability - please note that I am using 3 Classes -
1. Employee - reflecting a real world object
2. IncomeTaxBand - another real world object
3. TaxViewer - since Ruby encourages MVC, any viewer methods can be placed as an object too

This post is by no means any guide or tutorial - but if you need to ask me any queries please do. What I wanted to illustrate is an example of how a simple program which I personally used to undertake in a procedural approach in Bash or Perl in the past, can be made as OOP. The advantages of this design are too many to mention although some people might think it looks overbloated or complex.