L = Liskov Substitution Principle
This is all about using Object Oriented Programming to its fullest. So what the Wikipedia article says is that: “If S is a sub-type of T then objects of type T maybe replaced with type S”. So what does that mean ?
Well it means that when you create your class hierarchy, and you create your base methods, you have to think about the broader implications. For example if your root parent class was “Bird” it would have methods like “Fly” , “Eat” and “Walk”. And then you would classes like “Hawk” , “Blue Jay”, “Robin”, “Penguin”, and “Ostrich”. Now we should be able to put any of these child classes in place of the parent class, and use them. Can you see the problem ?
The problem is that Penguins and Ostriches can’t fly, which violates the “Liskov Substitution Principle”. You can get around this by instead having the two different children inherit from the “Bird” class: “FlyingBird” and “NonFlyingBird”.