A Ruby module that can make your classes final. A final class cannot be subclassed, and its instance methods cannot be redefined.

gem install final

gem cert --add <(curl -Ls https://raw.githubusercontent.com/djberg96/final/main/certs/djberg96_pub.pem)

require 'final'

class Alpha
  include Final

  def foo
    puts "Hello"
  end
end

# Error! You cannot subclass Alpha.
class Beta < Alpha; end

# Error! You cannot redefine an existing method.
class Alpha
  def foo
    puts "Hi"
  end
end

# Not an error. You can add new methods to an existing class.
class Alpha
  def bar
    puts "World"
  end
end

This library is merely a proof of concept that was published as the result of a tweet made by James Edward Gray II, where he commented that Ruby core classes should never be subclassed. Knowing that static languages like Java implement the "final" keyword as a way to prevent subclassing, I wanted to see if something similar could be done for Ruby. Turns out it can.

I do not, however, necessarily advocate actually using this library in production code. Freezing a class this way is contrary to the very nature of Ruby and dynamic languages in general. If you're going to use it, you had better have a very good reason for doing so!

Only instance methods are prevented from redefinition. Singleton methods are not final. This change may happen in a future release.

Apache-2.0

(C) 2003-2021 Daniel J. Berger All Rights Reserved.

This package is provided "as is" and without any express or implied warranties, including, without limitation, the implied warranties of merchantability and fitness for a particular purpose.

Daniel J. Berger