final_bob.rb

class Bob
  def reply_to(statement)
    public_send("reply_to_#{Statement.build(statement).class}".downcase.to_sym)
  rescue NoMethodError
    default_reply
  end

  def reply_to_silence
    "Fine. Be that way!"
  end

  def reply_to_yelling
    "Woah, chill out!"
  end

  def reply_to_question
    "Sure."
  end

  def default_reply
    "Whatever."
  end
end

class Question < Statement
  def self.match?(statement)
    statement.end_with?("?")
  end
end

class Yelling < Statement
  def self.match?(statement)
    statement.upcase == statement && statement.downcase != statement
  end
end

class Silence < Statement
  def self.match?(statement)
    statement.strip.empty?
  end
end

class NullStatement < Statement
  def self.match?(statement)
    true
  end
end

class Statement
  def self.build(statement)
    descendants.detect { |klass| klass.match? statement }.new(statement)
  end

  def self.inherited(klass)
    descendants.push klass
  end

  def self.descendants
    @descendants ||= []
  end
end