Low Level Design 107 | Liskov Substitution Principle | 2022 | System Design | Summary and Q&A

TL;DR

Learn about the Liskov Substitution Principle, which ensures that subclass instances can be seamlessly substituted for superclass instances, and how to apply it in class design.

Key Insights

• 📚 The Liskov Substitution Principle (LSP) is named after a famous computer scientist and is a key component of proper inheritance in object-oriented programming. It ensures conceptual correctness and avoids errors in design.
• 🧩 LSP states that a subclass should be able to replace its superclass without breaking the functionality of the code.
• ⚡️ LSP is crucial for abstracting the real world into classes and objects, and it helps in identifying and correcting design errors in inheritance.
• 🍽️ A real-world example of LSP violation is when a function that takes an instance of a parent class is unable to process an instance of a derived class without changing the way it is called.
• 💡 To adhere to LSP, the derived class should have the same functionality as the parent class without requiring changes in the calling code.
• 🥤 In the case of a menu item class hierarchy, a violation of LSP occurs when a function for calculating discounts in the child class requires a different method call compared to the parent class.
• 🔑 To correctly follow LSP, the parent class should define a private function that the child class overrides when necessary, allowing seamless substitution of instances without breaking the code.
• 📐 Another example of LSP violation is when a square class is derived from a rectangle class, leading to complications and the need for excessive checks. LSP suggests inheriting both square and rectangle classes from a common shape class to avoid such violations.

Transcript

hello and welcome to pseudocode in today's video we will learn about the third principle of solid which is called lisk of substitution principle it is named after a famous computer scientist and a tutoring award winner uh the details of the scientist is linked in the description you can check it out before watching this video i would recommend you ... Read More

Questions & Answers

Q: Why is it important to follow the Liskov Substitution Principle in object-oriented programming?

Following the Liskov Substitution Principle ensures that your code is conceptually correct and allows for seamless substitution of subclass instances in place of superclass instances without breaking functionality. This results in a more maintainable and extensible codebase.

Q: How can violations of the Liskov Substitution Principle occur in class design?

Violations can occur when the inheritance relationship is incorrect, such as inheriting a class that is not truly a type of the superclass. Violations can also occur when subclass instances require different behavior or when they cannot substitute superclass instances without impacting functionality.

Q: How can the Liskov Substitution Principle be applied to the example of menu items with discounts?

In this example, ensuring that the function getPrice can be called on both the superclass (MenuItem) and subclass (BeverageItem) instances without changing the client code is crucial. By properly designing the classes and using private functions and method overriding, the Liskov Substitution Principle can be followed.

Q: Can the Liskov Substitution Principle be violated in the case of inheritance between shapes?

Yes, the Liskov Substitution Principle can be violated if a square class extends the rectangle class instead of the shape class. This is because a square is not truly a type of rectangle, and operations on a square may require different behavior than operations on a rectangle.

Summary & Key Takeaways

• The Liskov Substitution Principle ensures that subclass instances can be used in place of superclass instances without breaking the functionality of the code.

• Violating this principle indicates incorrect class design and can lead to conceptual errors.

• Real-world examples, such as menu items and shapes, are used to illustrate the application of the Liskov Substitution Principle.