Liskov Substitution Principle is the “L” in the SOLID Principles. It states if you have a parent class and a child class, any instance of the child class should be able to replace any instance of the parent class without having unexpected behaviors.
This is a classic example of a violation that Bob Martin uses in his book Agile Software Development
This is a violation because the Square class modifies both height and width when
#set_width are called. Where as in the Rectangle class when
#set_width only one instance variable is being changed by each method. Any instance of
Square cannot be replaced for any instance of
Rectangle because modifying the other instance variable in the methods of the Square class can cause unintended consequences.
This can be rewritten like this so it is not a violation
Below is another example of a violation.
This is a violation because any instance of Dog cannot replace any instance of Animal and be expected to behave the same way. If we create an instance of Animal and an instance of Dog and call
#walk on both of the instances we will receive back two different data structures - a string and an array respectively.
Instead we need consistency. Both implementations of
#walk need to return the same data structure.
Other violations include different method signatures or errors (the child class produces an error in a method that the parent class does not)
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