A https://github.com/joyclark/cookie-jar in Ruby

cookies.rb

class CookieJar
  def initialize(initial_count)
    @count = initial_count
  end

  def count_cookies
    @count
  end

  def remove_cookies(amount)
    raise CookieOutOfBoundsException.new if amount > @count

    @count -= amount
  end

  def add_cookies(amount)
    @count += amount
  end
end

class Kitten
  def initialize(cookie_jar)
    @cookie_jar = cookie_jar
  end

  def eat_cookie
    @cookie_jar.remove_cookies(1)
  end
end

class Grandma
  def initialize(cookie_jar)
    @cookie_jar = cookie_jar
  end

  def bake_cookies(amount)
    @cookie_jar.add_cookies(amount)
  end
end

class CookieMonster
  def initialize(cookie_jar)
    @cookie_jar = cookie_jar
  end

  def eat_cookies(amount)
    @cookie_jar.remove_cookies(amount)
  end
end

class CookieOutOfBoundsException < RangeError
end

cookies_spec.rb

require_relative "cookies"

describe "the workflow" do
  it "should end in a CookieOutOfBounds Exception" do
    cookie_jar = CookieJar.new(5)
    kitten = Kitten.new(cookie_jar)
    grandma = Grandma.new(cookie_jar)
    cookie_monster = CookieMonster.new(cookie_jar)

    kitten.eat_cookie
    kitten.eat_cookie
    grandma.bake_cookies(5)
    cookie_monster.eat_cookies(5)
    expect { cookie_monster.eat_cookies(10) }.to raise_error(CookieOutOfBoundsException)
  end
end

describe CookieJar do
  subject { CookieJar.new(3) }

  it "should be able to count the cookies within" do
    expect(subject.count_cookies).to be 3
  end

  it "should be able to remove cookies" do
    expect { subject.remove_cookies(2) }.to change { subject.count_cookies }.from(3).to(1)
  end

  it "should be able to add cookies" do
    expect { subject.add_cookies(5) }.to change { subject.count_cookies }.from(3).to(8)
  end

  it "should raise a CookieOutOfBoundsException when containg less than 0 cookies" do
    expect { subject.remove_cookies(4) }.to raise_error(CookieOutOfBoundsException)
  end
end

describe Kitten do
  subject { Kitten.new(cookie_jar) }
  let(:cookie_jar) { instance_double(CookieJar) }

  it "should eat a single cookie" do
    expect(cookie_jar).to receive(:remove_cookies).with(1)
    subject.eat_cookie
  end
end

describe Grandma do
  subject { Grandma.new(cookie_jar) }
  let(:cookie_jar) { instance_double(CookieJar) }

  it "should add cookies to the jar after baking them" do
    expect(cookie_jar).to receive(:add_cookies).with(5)
    subject.bake_cookies(5)
  end
end

describe CookieMonster do
  subject { CookieMonster.new(cookie_jar) }
  let(:cookie_jar) { instance_double(CookieJar) }

  it "should eat multiple cookies from the jar" do
    expect(cookie_jar).to receive(:remove_cookies).with(7)
    subject.eat_cookies(7)
  end
end

Gemfile

# frozen_string_literal: true
source "https://rubygems.org"

gem "rspec"