TDD by example chapter 5/6 equality / privacy

tdd.rb

require 'minitest/autorun'
# WE NEED TO BE ABLE TO ... AND ... GIVEN ...

# WE NEED TO BE ABLE TO add amounts in different currencies AND convert the result GIVEN a set of exchange rates
# $5 + 10 CHF = $10 if CHF:USD rate is 2:1

# WE NEED TO BE ABLE TO multiply a number (of shares) with an amount (price per share) AND receive an amount 
# $5 * 2 = $10
# money rounding, make amount private, equals(), hashCode(), equal null, equal object?
class Dollar
  def initialize(amount)
		@amount = amount
	end
	def times(multiplier)
		Dollar.new(@amount * multiplier)
	end
	def equals?(object)
		@amount == object.amount
	end
	def amount
		@amount
	end
end
describe "test multiplication" do
	it "must multiply an amount (price per share) with a number (of shares) and return an amount (total price)" do
		five = Dollar.new(5)
		# assert_equal 10, five.times(2).amount
		assert Dollar.new(10).equals?(five.times(2))
		# assert_equal 15, five.times(3).amount
		assert Dollar.new(15).equals?(five.times(3)) # Triangulation + prevent dollar side effect of remembering previous calculations
	end
end

describe "test equality" do # equals? implementation needed because it is value object
	it "must be equal every time you create $5" do
		assert Dollar.new(5).equals?(Dollar.new(5))
		refute Dollar.new(5).equals?(Dollar.new(6)) # triangulation
	end
end