How do you design interfaces that follow the Liskov substitution principle?

1 Use abstraction

One way to design interfaces that follow the LSP is to use abstraction. Abstraction means hiding the implementation details and exposing only the essential features and behaviors of an entity. By using abstract classes or interfaces, you can define a common contract that all subclasses must adhere to, regardless of their specific details. For example, if you have an abstract class Animal with a method makeSound(), you can have subclasses like Dog, Cat, and Bird that implement this method in different ways, but still conform to the Animal contract.

2 Avoid type checking

Another way to design interfaces that follow the LSP is to avoid type checking. Type checking means using conditional statements or operators to determine the type of an object at runtime and perform different actions based on that. This can violate the LSP because it creates a dependency on the concrete implementation of an object, rather than its abstract behavior. For example, if you have a function that takes an Animal as a parameter and checks if it is a Dog or a Cat, then you are limiting the function to work only with those two subclasses, and not with any other Animal subclass that may exist or be added later.

3 Follow the design by contract principle

A third way to design interfaces that follow the LSP is to follow the design by contract principle. This principle states that every class or method should have a clear and precise specification of its preconditions, postconditions, and invariants. Preconditions are the requirements that must be met before a method is executed, postconditions are the guarantees that must be fulfilled after a method is executed, and invariants are the properties that must remain unchanged throughout the execution of a method. By following this principle, you can ensure that every subclass respects the contract of its superclass, and that the clients can rely on the expected behavior of any object that implements the interface.

4 Use composition over inheritance

A fourth way to design interfaces that follow the LSP is to use composition over inheritance. Composition means using objects as components or attributes of other objects, rather than inheriting from them. This can reduce the complexity and coupling of your code, and allow you to reuse functionality without violating the LSP. For example, if you have a class Car that inherits from a class Vehicle, and you want to add a new feature like electric engine, you may have to create a new subclass ElectricCar that overrides some methods of Vehicle. However, this may break the LSP if some methods of Vehicle are not applicable or consistent with ElectricCar. A better solution would be to use composition and have a class Engine as an attribute of Car, and then have different types of engines like GasEngine or ElectricEngine that implement an Engine interface.