What's wrong with the Square and Rectangle inheritance?

Question

I\'ve read some of article about the practice that making Square an inheritance class of Rectangle class is a bad practice, saying it violate the LSP (Liskov substitution p

Answer 1

What's wrong with it from an Liskov substitution principle (LSP) perspective is that your Rectangles and Squares are mutable. That means that you have to explicitly reimplement the setters in the subclass, and lose the benefits of inheritance. If you make Rectangles immutable, i.e., if you want a different Rectangle you create a new one rather than altering the measurements of an existing one, then there's no problem with violating LSP.

class Rectangle
  attr_reader :width, :height

  def initialize(width, height)
    @width = width
    @height = height
  end

  def area
    @width * @height
  end
end

class Square < Rectangle
  def initialize(length)
    super(length, length)
  end
end

Using attr_reader gives getters but not setters, hence the immutability. With this implementation both Rectangles and Squares provide visibility to height and width, for a square those will always be the same, and the concept of area is consistent.