title: How to judge whether a method has defined in a class?
body: <pre><code>class C1
    unless method_defined? :hello  # Certainly, it's not correct. I am asking to find something to do this work.
      def_method(:hello) do
        puts 'Hi Everyone'
      end
    end
  end
  </code></pre>
  
  <p>So, how to judge whether a method has defined or not?</p>