Clean Architecture: A Craftsman's Guide to Software Structure and Design

9LSP: THE LISKOV SUBSTITUTION PRINCIPLE

In 1988, Barbara Liskov wrote the following as a way of defining subtypes.

What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.¹

To understand this idea, which is known as the Liskov Substitution Principle (LSP), let’s look at some examples.

GUIDING THE USE OF INHERITANCE

Imagine that we have a class named License, as shown in Figure 9.1. This class has a method named calcFee(), which ...