01.mock_example.rb

class Jason
  def say(x)
    puts "Original definition of say... #{x}"
  end
end

# expect(Jason).to receive(:say).with(...).and_return(...)
class Jason
  alias_method :orig_say, :say
  
  def say(x)
    expected_input = 'hello'
    if x != expected_input
      raise "I expected to receive #{expected_input.inspect}, but I received #{x.inspect} instead!"
    else
      puts "hello, how are you?"
    end
  end
end

Jason.new.say "hello"
Jason.new.say "goodbye"

02.mock_example.rb

class Jason
  def say(x)
    puts "Original definition of say... #{x}"
  end
end

class Jason
  alias_method :orig_say, :say
  
  def say(x)
    expected_input = 'hello'
    if x != expected_input
      orig_say(x)
    else
      puts "hello, how are you?"
    end
  end
end

Jason.new.say "hello"
Jason.new.say "goodbye"

03.mock_example.rb

class Jason
  def say(x)
    puts "Original definition of say... #{x}"
  end
end

class Jason
  alias_method :orig_say, :say
  
  def say(x)
    expected_inputs = %w[hello goodbye]
    if !expected_inputs.include?(x)
      orig_say(x)
    elsif x == 'hello'
      puts "hello, how are you?"
    elsif x == 'goodbye'
      puts "goodbye to you, sir!"
    end
  end
end

Jason.new.say "hello"
Jason.new.say "goodbye"