LSP

Summary

Let be a property provable of objects of type . Then should be true for objects of type where .

Polymorphism needs to be done carefully as overriding a method can cause the code to fail without implementer changing anything in their code. This cannot be checked by the compiler.

Effectively, a subclass should not break the expectations set by the superclass. If a class is substitutable for a parent class , it should be able to pass all test cases of the parent class .

Example

Given a Restaurant class and a LunchRestaurant subclass, and a canMakeReservation method.

If the canMakeReservation method returns true at all times, but the overridden method in LunchRestaurant does not at certain times: we get the case where a test case fails.

Restaurant r = new Restaurant();
Restaurant lr = new LunchRestaurant();

r.canMakeReservation(1200) == true // expected behaviour
lr.canMakeReservation(1200) == true // fails

Prevent inheritance

To prevent inheritance and/or overriding, consider using the final modifier.

• Adding a final modifier to the class to prevent it from being inherited.
• Adding a final modifier to the methods to prevent it from being overridden.