Liskov Substitution Principle: The Real Meaning of Inheritance

This article delves into the Liskov Substitution Principle (LSP), a frequently misunderstood SOLID principle. Using the classic rectangle-square problem, it illustrates common LSP violations: subtypes failing to fully substitute base types. The author advocates using composition and interfaces over inheritance to avoid LSP violations, providing a practical example with payment processing. Common LSP violations, such as throwing unexpected exceptions and returning inconsistent results with the base type, are outlined, along with how to ensure LSP compliance through contract testing and clear pre/postconditions. Ultimately, the article emphasizes that LSP is about more than just inheritance—it's about behavioral compatibility and meeting expectations. Following LSP leads to more reliable and maintainable code.