class Shape2D
def initialize
raise NotImplementedError.
new("#{self.class.name} is an abstract class.")
end
def area
raise NotImplementedError.
new("#{self.class.name}#area is an abstract method.")
end
end
class Square < Shape2D
def initialize(length)
@length = length
end
def area
@length ** 2
end
end
Square.new(10).area # => 100
# NotImplementedError: Shape2D is an abstract class.
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