Visual Guide to SOLID Principles: Understand Each Rule with Illustrated Examples

If you are familiar with object‑oriented programming, you have probably heard of the SOLID principles, a set of five design rules advocated by Robert C. Martin to make software easier to extend and maintain.

The purpose of this article is to illustrate each principle with pictures, helping visual learners grasp their intent.

Single Responsibility

A class should have only one responsibility. When a class handles many duties, the chance of errors rises because changes to one responsibility may unintentionally affect others.

Example (wrong): a single class acts as chef, gardener, painter, and driver. Example (correct): four separate classes each handle one role.

Goal: isolate variables so that changes in one responsibility do not cause errors elsewhere.

Open‑Closed

A class should be open for extension but closed for modification. Extend functionality without altering existing code.

Example (wrong): change a "Cut" action to "Paint" by modifying the class. Example (correct): extend the class to support both "Cut" and "Paint" without changing existing behavior.

Goal: add new features without altering the current implementation of a class.

Liskov Substitution

If S is a subtype of T, any instance of T can be replaced by an instance of S without affecting program correctness.

Example (wrong): a parent class Sam provides coffee; the child class Eden fails to deliver coffee when Sam is absent. Example (correct): Eden can deliver any coffee (e.g., Cappuccino) just like Sam.

Goal: ensure subclasses can be used wherever their base class is expected without introducing errors.

Interface Segregation

Clients should not be forced to depend on methods they do not use.

Example (wrong): a class is required to implement a messy, unrelated interface. Example (correct): the class implements a clean, focused interface.

Goal: keep interfaces small and specific so that classes only implement the behavior they need.

Dependency Inversion

High‑level modules should not depend on low‑level modules; both should depend on abstractions. Abstractions should not depend on details, but details should depend on abstractions.

High‑level module (or class): uses tools to perform actions.

Low‑level module (or class): provides the tools.

Abstraction: the interface that connects the two.

Details: how the tool works.

The principle states that classes and interfaces should not know how the tools work; they only need to satisfy the interface contract.

Goal: reduce the dependency of high‑level modules on low‑level modules by introducing abstractions.

Conclusion

We have discussed all five SOLID principles and highlighted their goals. Applying them will help you write code that is easier to adjust, extend, and test, moving you toward a more perfect design.

Thank you for reading; I hope you enjoy the topic as much as I enjoy writing code.