Java like interface in ruby.

engine.rb

require './engine_interface'

class Engine
  def self.init_with_name name
    new
    @name = name
  end

  def start
    puts "Starting"
  end

  def stop
    puts "Stopping"
  end

  # must be included at the bottom
  include EngineInterface
end

e = Engine.new
e.start

engine_interface.rb

require './interface'

module EngineInterface
  include Interface

  def self.included klass
    ensure_instance_methods klass, start: 0, stop: 0
    ensure_class_methods klass, init_with_name: 1
  end

end

interface.rb

module Interface
  def self.included klass
    klass.extend ClassMethods
  end

  module ClassMethods
    def ensure_instance_methods(klass, **methods_hash)
      ensure_methods(klass.new, methods_hash)
    end

    def ensure_class_methods(klass, **methods_hash)
      ensure_methods(klass, methods_hash)
    end

    private
    # methods_hash sample
    # {:meth1_name => args_count, :meth2_name => args_count}
    def ensure_methods(klass, methods_hash)
      methods_hash.each do |method_name, args_count|
        if klass.is_a? Class
          class_name  = klass.name
          method_type = ".#{method_name}"
        else
          class_name  = klass.class.name
          method_type = "##{method_name}"
        end
        if klass.respond_to? method_name
          if klass.method(method_name).arity != args_count
            raise NotImplementedError, "#{class_name}#{method_type} should receive #{args_count} parameter(s)"
          end
        else
          raise NotImplementedError, "Class '#{class_name}' must implement the method '#{method_type}'"
        end
     end
    end
  end

end