Trying to open an Android class in RubyMotion

main_activity.rb

# throws NameError: uninitialized constant `Android'
class Android::App::Activity
  def say_hello
    puts "hello!"
  end
end

class MainActivity < Android::App::Activity
  def onCreate(savedInstanceState)
    super
    say_hello
  end
end

main_activity2.rb

# throws com.rubymotion.NoMethodError: undefined method `say_hello' for #<MainActivity:0xbee810bc>
module Android
  module App
    class Activity
      def say_hello
        puts "hello!"
      end
    end
  end
end

class MainActivity < Android::App::Activity
  def onCreate(savedInstanceState)
    super
    say_hello
  end
end

main_activity3.rb

# This is another variation on the problem: monkey patching a Ruby class works fine, but
# the patch is not picked up by the Java version of the same class (e.g. Hash and
# java.util.HashMap). This will be needed when working with 3rd party Java libraries that
# will be returning objects into RubyLand, e.g. Volley

class Hash
  def say_hello
    puts "hello"
  end
end

class MainActivity < Android::App::Activity
  def onCreate(savedInstanceState)
    super

    # this works
    h = {}
    h.say_hello

    # this raises: undefined method `say_hello' for {}:java.util.HashMap
    hash = Java::Util::HashMap.new
    hash.say_hello

  end
end