What is an example of the Liskov Substitution Principle?

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The Liskov Substitution Principle (LSP) is one of the five SOLID principles of object-oriented programming and states that objects of a derived class should be able to replace objects of the base class without affecting the correctness of the program. In other words, if a class A is a subtype of class B, you should be able to use an object of class A wherever an object of class B is expected, and the program should still work correctly.

Here's an example in Python to illustrate the Liskov Substitution Principle:

python
class Bird:
    def fly(self):
        pass

class Sparrow(Bird):
    def fly(self):
        print("Sparrow is flying")

class Ostrich(Bird):
    def fly(self):
        print("Ostrich cannot fly")

def make_bird_fly(bird):
    bird.fly()

# Using Liskov Substitution Principle
sparrow = Sparrow()
ostrich = Ostrich()

make_bird_fly(sparrow)  # Output: Sparrow is flying
make_bird_fly(ostrich)  # Output: Ostrich cannot fly

In this example:

  1. We have a base class Bird with a fly method that doesn't do anything.

  2. We have two derived classes, Sparrow and Ostrich, both of which inherit from the Bird class and provide their own implementations of the fly method.

  3. The make_bird_fly function takes an argument of type Bird and calls its fly method.

  4. We can use the Liskov Substitution Principle to substitute instances of Sparrow and Ostrich for a Bird object in the make_bird_fly function. This demonstrates that objects of derived classes can be used interchangeably with objects of the base class without affecting the correctness of the program.

The key takeaway here is that both Sparrow and Ostrich are subclasses of Bird, and they can be used in a way that adheres to the Liskov Substitution Principle, allowing us to write more flexible and extensible code.

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