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Liskov substitution principle

Liskov substitution principle (LSP): Derived classes must be substitutable for their base classes. -- proposed by Barbara Liskov

LSP sounds the same as substitutability but it goes beyond substitutability; LSP implies that a subclass should not be more restrictive than the behavior specified by the superclass. As you know, Java has language support for substitutability. However, if LSP is not followed, substituting a subclass object for a superclass object can break the functionality of the code.

Suppose the Payroll class depends on the adjustMySalary(int percent) method of the Staff class. Furthermore, the Staff class states that the adjustMySalary method will work for all positive percent values. Both the Admin and Academic classes override the adjustMySalary method.

Now consider the following:

  • The Admin#adjustMySalary method works for both negative and positive percent values.
  • The Academic#adjustMySalary method works for percent values 1..100 only.

In the above scenario,

  • The Admin class follows LSP because it fulfills Payroll’s expectation of Staff objects (i.e., it works for all positive values). Substituting Admin objects for Staff objects will not break the Payroll class functionality.
  • The Academic class violates LSP because it will not work for percent values over 100 as expected by the Payroll class. Substituting Academic objects for Staff objects can potentially break the Payroll class functionality.

Another example


Exercises:

Is this LSP?