What are SOLID design principles?

SOLID is a set of five object-oriented design principles that focus on creating elegant, robust, and maintainable object-oriented code. Think of them as the pillars of good software architecture. Let's break each one down:

S: Single Responsibility Principle (SRP)

Concept: A class should have only one reason to change, meaning it should have only one job or responsibility.

Real-World Analogy: Like a chef in a restaurant, who is responsible only for cooking and not for taking orders or serving food.

In Practice: If you have a class called OrderProcessor, it should only process orders, not handle database storage or error logging.

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SOLID Design Principles

O: Open/Closed Principle

Concept: Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

Real-World Analogy: Think of a graphic design software. It allows you to add new functionalities (like new shapes) without changing the core drawing engine.

In Practice: Implementing new functionalities via interfaces or abstract classes, allowing new functionalities to be added without changing existing code.

L: Liskov Substitution Principle

Concept: Objects of a superclass should be replaceable with objects of subclasses without affecting the correctness of the program.

Real-World Analogy: If you're driving a vehicle, you should be able to replace it with a different type of vehicle (like switching a car for a truck) and still be able to drive without issues.

In Practice: Ensuring that subclasses don't alter expected behavior. For example, if you have a class Bird with a method fly(), a subclass Penguin should not implement fly() since penguins can't fly.

I: Interface Segregation Principle

Concept: Clients should not be forced to depend on interfaces they do not use.

Real-World Analogy: It’s like having a universal remote control with so many buttons, most of which you never use. Instead, you would prefer a simpler remote with only the buttons you need.

In Practice: Creating specific interfaces (like IFlyable, IWalkable) instead of one general-purpose interface, so classes only implement the methods they need.

D: Dependency Inversion Principle

Concept: High-level modules should not depend on low-level modules. Both should depend on abstractions. Also, abstractions should not depend on details; details should depend on abstractions.

Real-World Analogy: Like using a standard power socket in your home – it doesn’t need to know what type of device will be plugged in (the high-level policy). The devices (low-level modules) are designed to fit the socket.

In Practice: Instead of a class directly instantiating dependencies (like new DatabaseConnection()), dependencies are provided to the class (often using interfaces), e.g., through constructor injection.

The Essence of SOLID

Adhering to SOLID principles helps in building software that is easier to understand, maintain, and extend. It's particularly useful in complex systems and large teams where code readability, scalability, and maintainability are critical.

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Object-Oriented Programming
System Design Fundamentals
System Design Interview
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