1.1 Liskov Substitution Principle (LSP) with Solution in Java - SOLID Principles of Low Level Design

Concept && Coding - by Shrayansh
4 May 202308:56

Summary

TLDRIn this insightful video, Shreyansh explains the Liskov Substitution Principle (LSP), a fundamental concept in object-oriented programming that ensures subclasses can replace parent classes without disrupting code functionality. Using a practical example of a `Vehicle` class and its child classes, including `Motorcycle`, `Car`, and `Bicycle`, he highlights potential issues when a subclass undermines the parent class's capabilities. Shreyansh offers a solution by introducing a separate `EngineVehicle` class to maintain functionality while avoiding null references, emphasizing the importance of proper inheritance design to ensure code safety and stability.

Takeaways

  • 😀 The Liskov Substitution Principle (LSP) states that subclasses should be interchangeable with their parent classes without breaking the functionality of the program.
  • 🚗 A basic `Vehicle` class can have different subclasses like `Motorcycle`, `Car`, and `Bicycle`, each representing different types of vehicles.
  • 🔄 When substituting objects, the program should continue to function correctly regardless of which subclass is used.
  • ⚠️ Violating the LSP can lead to runtime errors, such as null pointer exceptions, when a subclass does not adhere to the expected behavior of the parent class.
  • 🔧 In the example, adding `Bicycle`, which has overridden `hasEngine()` to return null, breaks the code when checking if it has an engine.
  • 📏 To avoid LSP violations, separate vehicle classes into those that have engines and those that do not, using an `EngineVehicle` subclass.
  • 👨‍💻 By creating specific subclasses for vehicles with engines, the base `Vehicle` class remains generic and applicable to all types of vehicles.
  • 🛠️ Client code should only access methods defined in the parent class to ensure safety and maintainability of the code.
  • 📚 Proper inheritance and method overriding are essential to uphold the Liskov Substitution Principle.
  • ✅ Following LSP allows for a more robust and error-free code structure, enabling easier maintenance and scalability.

Q & A

  • What is the Liskov Substitution Principle (LSP)?

    -The Liskov Substitution Principle states that objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program.

  • How does the Liskov Substitution Principle relate to object-oriented programming?

    -LSP is a fundamental principle of object-oriented programming that ensures derived classes can stand in for their base classes without causing issues in the code's functionality.

  • What example is used to illustrate the LSP in the transcript?

    -The example involves a parent class `Vehicle` and child classes `Motorcycle`, `Car`, and `Bicycle` to demonstrate how substituting these objects can lead to errors if the principles of LSP are violated.

  • What problem arises when adding the `Bicycle` subclass to the list of vehicles?

    -The `Bicycle` subclass violates the LSP because it overrides the `hasEngine()` method to return null, leading to a NullPointerException when the client code tries to call methods on it.

  • How can one avoid violating the Liskov Substitution Principle?

    -To avoid violating LSP, define generic methods in the parent class and use composition for specific features in separate classes, ensuring that all subclasses adhere to the same interface.

  • What changes were suggested to the class structure to adhere to LSP?

    -The suggested changes include creating an `EngineVehicle` class that inherits from `Vehicle` for vehicles with engines, while `Bicycle` should only inherit from `Vehicle` to prevent breaking the code.

  • What is the role of the `Vehicle` class in this example?

    -The `Vehicle` class serves as the parent class containing common methods applicable to all vehicle types, such as `getNumberOfWheels()`.

  • What happens when you attempt to add a `Bicycle` to an `EngineVehicle` list?

    -Attempting to add a `Bicycle` to an `EngineVehicle` list results in a compile-time error because `Bicycle` does not inherit from `EngineVehicle`.

  • Why is it important for the client code to be able to substitute different vehicle objects?

    -It's important because it allows for flexibility and code reusability, enabling developers to work with various vehicle types without modifying the core logic of the program.

  • What is the consequence of not following the Liskov Substitution Principle in a codebase?

    -Not following LSP can lead to fragile code that breaks when subclasses are used interchangeably, resulting in errors that can be difficult to trace and fix.

Outlines

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Mindmap

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Keywords

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Highlights

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Transcripts

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Liskov PrincipleOOP ConceptsSoftware DesignInheritanceCode QualityProgramming TipsDeveloper AudienceBest PracticesClient CodeNull Pointer
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