What Is the Substitutability Principle in Object-Oriented Programming?


Angela Bailey

The Substitutability Principle in Object-Oriented Programming

In the world of object-oriented programming (OOP), there are several important principles that guide developers in writing clean, maintainable, and extensible code. One such principle is the Substitutability Principle. This principle, also known as the Liskov Substitution Principle (LSP), is a fundamental concept that ensures the correct behavior of polymorphism in object-oriented systems.

Understanding Polymorphism

Before we delve into the Substitutability Principle, let’s first understand polymorphism. In OOP, polymorphism allows objects of different classes to be treated as objects of a common superclass. This means that you can have multiple classes that share a common set of behaviors defined by a superclass or interface.

The Liskov Substitution Principle (LSP)

The Liskov Substitution Principle states that if S is a subtype of T, then objects of type T can be replaced with objects of type S without affecting the correctness of the program. In other words, if you have a reference to an object of type T, you should be able to substitute it with an object of any subtype S without breaking the functionality.


Let’s consider an example to illustrate this principle. Suppose we have a superclass called Shape and two subclasses called Circle and Square.

Both Circle and Square inherit from Shape and override its abstract methods like `getArea()` and `draw()`. According to LSP, we should be able to use instances of Circle or Square wherever we expect an instance of Shape.

  • Violating LSP:

Shape shape = new Square();
shape.setDimensions(5, 5);

In this example, we create a reference `shape` of type Shape but assign it an instance of Square. The `setDimensions()` method is not defined in the Shape class but is specific to the Square class. This violates LSP since we are using a subclass in a way that doesn’t adhere to the behavior of its superclass.

  • Adhering to LSP:

Square square = new Square();
square.setDimensions(5, 5);

In this corrected example, we directly create an instance of Square and use the `setDimensions()` method provided by the Square class. This ensures that we are using the subclass-specific behavior appropriately.

Benefits of Substitutability Principle

By adhering to the Substitutability Principle, you can ensure that your code remains flexible and maintainable. It allows for easy extensibility by enabling new subclasses to be added without affecting existing code that relies on the superclass.

Using LSP also helps in writing robust and reliable code. It ensures that objects can be substituted without causing unexpected behaviors or breaking existing functionality. This makes it easier to reason about your code’s correctness and simplifies debugging and testing processes.


The Substitutability Principle, also known as the Liskov Substitution Principle, is a vital concept in object-oriented programming. By adhering to this principle, you can write more maintainable and extensible code while ensuring correct behavior during polymorphism. Remember, using proper inheritance hierarchies and following LSP guidelines will lead to cleaner and more reliable software systems.

Now that you understand the importance of the Substitutability Principle, try applying it in your own projects. Happy coding!

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