Liskov Substitution Principle-javaTutorial-php.cn

Liskov Substitution Principle

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

Let's understand this with inheritance(Is-arelationship)
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 } }

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

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RacingCar overrides the getCabinWidth() of the car class but leaves itunimplementedbecause 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 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()); } }

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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.

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 } }

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

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public class RacingCar extends Vehicle{ @Override public double getInteriorWidth(){ return this.getCockpitWidth(); } public double getCockpitWidth(){ //return the cockpit width of the racing car } }

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

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

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 Basicsproducts 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; } }

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

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public class PricingUtils{ Product p1 = new Product(); Product p2 = new Product(); Product p2 = new InHouseProduct(); List 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()); } }

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Note that the if statement is involved in updating the discount amount of InHouseProduct which isagainst 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; } }

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

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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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