mutliple_apps.rb

class TestClass
  def initialize
    if defined?(@@app_created)
      Window.new
      return
    end

    @@app_created = true
    puts "app hasn't been created, so I'll make one"
  end
end

class Window
  def initialize
    puts "the app has been created, so I'll make a window instead"
  end
end

TestClass.new # first class
TestClass.new # second class

## output:
# app hasn't been created, so I'll make one
# the app has been created, so I'll make a window instead