How to implement an abstract class in Ruby

I know there is no concept of an abstract class in Ruby. But if it needs to be implemented, how do I go about it? I tried something like this:

class A
  def self.new
    raise 'Doh! You are trying to write Java in Ruby!'
  end
end

class B < A
  ...
  ...
end

But, when I try to instantiate B, it is internally going to call A.new which is going to raise the exception.

Also, modules cannot be instantiated, but they cannot be inherited too. Making the new method private will also not work.

Does anyone have any pointers?

I don't like using abstract classes in Ruby (there's almost always a better way). If you really think it's the best technique for the situation though, you can use the following snippet to be more declarative about which methods are abstract:

module Abstract
  def abstract_methods(*args)
    args.each do |name|
      class_eval(<<-END, __FILE__, __LINE__)
        def #{name}(*args)
          raise NotImplementedError.new("You must implement #{name}.")
        end
      END
      # important that this END is capitalized, since it marks the end of <<-END
    end
  end
end

require 'rubygems'
require 'rspec'

describe "abstract methods" do
  before(:each) do
    @klass = Class.new do
      extend Abstract

      abstract_methods :foo, :bar
    end
  end

  it "raises NoMethodError" do
    proc {
      @klass.new.foo
    }.should raise_error(NoMethodError)
  end

  it "can be overridden" do
    subclass = Class.new(@klass) do
      def foo
        :overridden
      end
    end

    subclass.new.foo.should == :overridden
  end
end

Basically, you just call abstract_methods with the list of methods that are abstract, and when they get called by an instance of the abstract class, a NotImplementedError exception will be raised.