SOLID Coding. Why Squares Aren’t Rectangles: Liskov Substitution Principle

What every developer needs to know about substitution

Programming is all about creating systems that work seamlessly and predictably, even as they grow in complexity. One principle that plays a crucial role in achieving this is the Liskov Substitution Principle (LSP). As one of the foundational pillars of the SOLID principles, LSP ensures that your code is flexible, maintainable, and free from unexpected behavior.

So, today we’re diving into the “L” in SOLID, we’ll keep it light, with some Ruby examples and the occasional tractor for good measure. (Yes, tractors 🚜 You’ll see.)

Non-members can read this article for free using this link.

Before we start, if you’re new to SOLID, you might want to check out the Single Responsibility Principle (SRP) first. It’s all about teaching your code to focus on one job at a time — trust me, it’s life-changing.

The Single Responsibility Saga: One Job, One Focus

A Quick History Lesson

Let’s rewind to 1987, when Barbara Liskov, a computer scientist, presented a talk at a conference. Fun fact: Barbara was the first woman in the U.S. to earn a Ph.D. in computer…