The Liskov Substitution Principle (LSP)

SUBTYPES MUST BE SUBSTITUTABLE FOR THEIR BASE TYPES.

하위타입의 객체가 상위 객체를 대체할 수 있어야 한다.

Have you ever seen code that has lots of instanceof expressions in the clauses of if

statements. Though there are some legitimate uses for expressions like this, they are few

and far between. Usually they are a result of violating the LSP, and are themselves a violation

of the OCP.

The LSP says that the users of base classes should not have to do anything special in

order to use derivatives. Specifically, they should not have to use instanceof, or downcasts.

Indeed, they should know nothing about the derivatives at all. Not even that they

exist.

Consider the payroll application shown in Figure 6-8. The Employee class is abstract

and has an abstract method named calcPay. It’s pretty clear that SalariedEmployee

will implement this to return the employee’s salary. It’s also pretty clear that Hourly-

Employee will implement it to return the hourly rate times the sum of the hours on this

week’s time cards.