How do you test or verify that your interfaces adhere to the Liskov substitution principle?

1 Use contract tests

Contract tests are a type of unit tests that check that a class that implements an interface meets the specifications of that interface. For example, if you have an interface that defines a method to calculate the area of a shape, you can write a contract test that asserts that the method returns a positive value for any valid shape. Then, you can run this test on any class that implements the interface, such as a circle, a square, or a triangle, and verify that they all pass the test. This way, you can ensure that any class that implements the interface behaves consistently and correctly.

2 Use code analysis tools

Code analysis tools are software tools that can detect potential violations of the LSP by analyzing the source code of your program. For example, some tools can warn you if you override a method in a subclass and change its signature, return type, or exception handling. This could indicate that you are breaking the LSP, because you are changing the behavior of the method in a way that could confuse or surprise the clients that use the interface. By using code analysis tools, you can catch these errors early and fix them before they cause problems in your program.

3 Use behavioral subtyping

Behavioral subtyping is a concept that formalizes the LSP by defining a set of rules and properties that a subclass must satisfy to be a valid subtype of its superclass. For example, one of the rules is that a subclass must preserve the invariants of its superclass, meaning that it must not violate any assumptions or conditions that the superclass relies on. Another rule is that a subclass must be substitutable for its superclass in any context, meaning that it must not require any additional or different input or output from the clients that use the interface. By applying behavioral subtyping, you can design your interfaces and classes in a way that respects the LSP and avoids unexpected or undesirable behavior.

4 Use design by contract

Design by contract is a methodology that defines the responsibilities and expectations of each component of a system in terms of preconditions, postconditions, and invariants. Preconditions are the conditions that must be true before a method is called, postconditions are the conditions that must be true after a method is executed, and invariants are the conditions that must be true at all times for a class or an interface. By using design by contract, you can specify the behavior and requirements of your interfaces and classes in a clear and precise way, and ensure that they are consistent and compatible with each other.

5 Use polymorphism wisely

Polymorphism is the ability of an object to take different forms depending on its type or context. It is a powerful feature of object-oriented programming that allows you to write generic and reusable code that can work with different types of objects that implement the same interface. However, polymorphism also comes with some challenges and risks, especially when it comes to the LSP. For example, if you use polymorphism to invoke a method on an object that implements an interface, you must be careful not to assume anything about the internal state or implementation of that object, because it could vary depending on the subclass. Similarly, if you use polymorphism to pass an object as an argument to a method that expects an interface, you must be careful not to violate the preconditions or postconditions of that method, because they could depend on the superclass. By using polymorphism wisely, you can leverage the benefits of the LSP without compromising the quality or correctness of your code.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?