How can I make this small pseudocode comply with Liskov principle?

Question

Keeping things tidy and simple, here is my initial Java pseudocode as an example:

public abstract class Vehicle {        
     private String owner;
     private id plate;

     public removeVehicle() {
         if (typeof(this) == Car) {this.removeCar();}
         if (typeof(this) == Truck) {this.removeTruck();}
     }
}

public class Car extends Vehicle {
     public removeCar() {
        this.removeVehicle(this);
     }
}

public class Truck extends Vehicle {
     public removeTruck() {
        this.removeVehicle(this);
     }         
}

From the book Agile principles patterns and practices in C# by Robert, Micah Martin says that it does not comply with Liskov principle since Vehicle, in order to remove other vehicles types (e.g. Motorcicle), needs to extend code and add another if statement for this new type of vehicle.

I tried to follow solutions provided from here, but most are just taken from the book I just referrenced and I'm actively trying to properly understand.

How can I modify this pseudocode so it does comply with Liskov?

score 1 · Accepted Answer · answered Mar 30 '22 at 19:03

According to

"Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it."

see https://en.wikipedia.org/wiki/SOLID

The target of your exercise could be to define removeVehicle in class Vehicle as abstract and implement it in the classes which extend Vehicle.

How can I make this small pseudocode comply with Liskov principle?

1 Answers1