doubles.rb

class Dog
  def initialize(logger)
    @logger = logger
  end

  def bark
    puts "Bark!"
    logger.log("Barked")
  end

  private
  attr_reader :logger
end

describe Dog, '#bark' do
  it 'logs the barking' do
    logger = double("fake logger")
    logger.should_receive(:log).with("Barked")

    Dog.new(logger).bark
  end
end