class Class
  
  alias_method :old_new, :new
  
  def abstract_class
    @is_abstract = true
    @abstract_methods = []
  end
  
  def is_abstract?
    return !@is_abstract.nil?
  end
  
  def new(*params)
    raise Exception, "#{self} is an abstract class" if is_abstract?
    
    if self.superclass.is_abstract?
      sc = self.superclass
      
      sc.abstract_methods.each do |method|
        if !method_defined?(method)
          methods = sc.abstract_methods.join(", ")
          raise Exception, "#{self} must implement the inherited abstract " + 
              "methods from #{sc}: #{methods}"
        end
      end
    end
    
    return old_new(*params)
  end
  
  def abstract_method(method)
    
    raise Exception, "#{self} is not an abstract class" if !is_abstract?
    
    if @abstract_methods.include?(method)
        raise Exception, "#{method} is already defined as an abstract method" 
    end
    
    if method_defined?(method)
      raise Exception, "Abstract methods do not specify a body" 
    end
    
    @abstract_methods << method
  end
  
  def abstract_methods
    return @abstract_methods
  end
 
  def method_added(method)
    if is_abstract? && @abstract_methods.include?(method)
      raise Exception, "Abstract methods do not specify a body" 
    end
  end
  
end