Liskov-Substitutionsprinzip

Objects should be replaceable with their subtype without affecting the correctness of the code

Let's understand this with inheritance(Is-a relationship)
example: Ostrich is a bird, Hunchback is a car, etc.

Example: Racing-car is a car

public class Car{
    public double getCabinWidth(){
        //return cabin width
    }
}

public class RacingCar extends Car{
    @Override
    public double getCabinWidth(){
        //UNIMPLEMENTED
    }
    public double getCockpitWidth(){
        //return the cockpit width of the racing car
    }
}

RacingCar overrides the getCabinWidth() of the car class but leaves it unimplemented because Racing cars don't have cabin width( if you see a Formula 1 racing car, it does not have any interior space, all it has is just a cockpit where the driver sits)
Hence the interior space in a racing car is called a cockpit.
Note: Racing cars have some specifications that might not match the generic car

public class CarUtil{
    Car car1 = new Car();
    Car car2 = new Car();
    Car car3 = new RacingCar();

    List<Car> myCars = new ArrayList<>();
    myCars.add(car1);
    myCars.add(car2);
    myCars.add(car3);
    // this will not work in 3rd iteration, because the getCabinWidth() in RacingCar is not implemented 
    for(Car car  : myCars){
        System.out.println(car.getCabinWidth());
    }
}

This is a design that has been exposed, as the for loop will fail for the third iteration.
To fix this we have to strike at the root which is Inheritance itself.

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Solution 1 : (Breaking the Hierarchy)

We have to break the inheritance, instead, we will come up with a common parent for both Car and RacingCar

We will create a very generic parent class called Vehicle

public class Vehicle{
    public double getInteriorWidth(){
        //return the interior width
    }
}

public class Car extends Vehicle{
    @Override
    public double getInteriorWidth(){
        return this.getCabinWidth();
    }
    public double getCabinWidth(){
        //return cabin width
    }
}

public class RacingCar extends Vehicle{
    @Override
    public double getInteriorWidth(){
        return this.getCockpitWidth();
    }

    public double getCockpitWidth(){
        //return the cockpit width of the racing car
    }
}

public class VehicleUtils{
    Vehicle vehicle1 = new Car();
    Vehicle vehicle2 = new Car();
    Vehicle vehicle2 = new RacingCar();

    List<Vehicle> vehicles = new ArrayList<>();
    vehicles.add(vehicle1);
    vehicles.add(vehicle2);
    vehicles.add(vehicle3);
    for(Vehicle vehicle : vehicles){
        System.out.println(vehicle.getInteriorWidth());
    } 
}

**Breaking the hierarchy: Break the hierarchy if the substitution fails

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Solution 2: Tell don't ask

Let's take another example of Amazon
Amazon offers x amount of discount on all the third-party products.
And offers 1.5 times x on all its in-house products (Amazon Basics products are Amazon in-house products)

public class Product{
    public double discount = 20;//x amount of discount on all the third-party products on Amazon
    public double getDiscount(){
        return discount;
    }
}

public class InHouseProduct extends Product{
    public void applyDiscount(){
        discount  = discount*1.5;// 1.5 times more discount on InHouseProducts
    }
}

public class PricingUtils{
    Product p1 = new Product();
    Product p2 = new Product();
    Product p2 = new InHouseProduct();
    List<Product> products = new ArrayList<>();
    products.add(p1);
    products.add(p2);
    products.add(p2);
    for(Product product : products){
        if(product instanceOf InHouseProduct){
            ((InHouseProduct)product).applyDiscount();
        }
        System.out.println(product.getDiscount());
    }
}

Note that the if statement is involved in updating the discount amount of InHouseProduct which is against the Liskov substitution principle ( as we should have been able to replace the object Product with its subtype InHouseProduct), but here in if statement we are manually updating the discount amount which should not be done.

A slight modification in the InHouseProduct class will fix this issue

public class InHouseProduct extends Product{

    @Override
    public double getDiscount(){
        applyDiscount();
        return discount;
    }
    public void applyDiscount(){
        discount  = discount*1.5;
    }
}

And finally removing the if statement from the PricingUtils class

public class PricingUtils{
    Product p1 = new Product();
    Product p2 = new Product();
    Product p2 = new InHouseProduct();
    List<Product> products = new ArrayList<>();
    products.add(p1);
    products.add(p2);
    products.add(p2);
    for(Product product : products){
        System.out.println(product.getDiscount());
    }
}

Tell don't ask: Here we are telling the utils class to print all the discounts and the utils class does not have to ask anything ( as it was asking by if statement earlier)

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