これで Ruby でも Java の匿名クラスみたいに書ける!!!

anonymous_class.rb

module Kernel

  def method_missing(klass_name, *args, &definition)
    klass =  (self.is_a?(Module) ? self : self.class)
    begin
      const = klass.const_get(klass_name)
      if const.instance_of? Class
        define_singleton_method(klass_name) do |*args, &definition|
          obj = const.new *args
          obj.instance_eval(&definition || proc {})
          obj
        end
      elsif const.instance_of? Module
        define_singleton_method(klass_name) do |*args, &definition|
          obj = Object() { extend const }
          obj.instance_eval(&definition || proc {})
          obj
        end
      else
        return
      end
      send(klass_name, *args, &definition)
    rescue
      $@ = caller
      super
    end
  end

end

if __FILE__ == $PROGRAM_NAME
  require "test/unit"
  class Test__call__ < Test::Unit::TestCase

    def test_call
      assert_equal({}, Hash())
    end

    def test_subclass
      assert_equal("hello", Hash() { def hello; "hello"; end  }.hello)
    end

    def test_anoclass
      assert_equal("hello", Kernel() { def hello; "hello"; end  }.hello)
    end

  end
end