🌟 SOLID Principles

SOLID is a set of five object-oriented design principles that help you write clean, maintainable, and flexible code.

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S – Single Responsibility Principle (SRP)

One class = One responsibility

✔ A class should do only one job
✔ It should have only one reason to change

Example:
A Book class should only store book data.
Validation should be in another class.

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O – Open/Closed Principle (OCP)

Open for extension, closed for modification

✔ You should be able to add new features
✔ Without changing existing code

Example:
Instead of modifying a class, create a subclass to extend behavior.

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L – Liskov Substitution Principle (LSP)

Child classes should behave like parent classes

✔ A subclass should be usable wherever the parent class is used
✔ It should not break the program

Example:
A Square should behave like a Rectangle if it extends Rectangle.

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I – Interface Segregation Principle (ISP)

No class should be forced to implement methods it doesn’t use

✔ Create smaller, specific interfaces
✔ Avoid “fat” interfaces with many unrelated methods

Example:
Instead of one big Animal interface with walk(), fly(), swim(),
create smaller interfaces like Walkable, Flyable, Swimmable.

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D – Dependency Inversion Principle (DIP)

Depend on abstraction, not concrete classes

✔ High-level classes should depend on interfaces, not concrete classes
✔ Makes code flexible and easy to modify/test

Example:
Use interface Database instead of directly using MySQL class.

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🌈 SUPER EASY Summary (One Line Each)

Principle	Meaning
S	One class, one job
O	Add new things without changing old code
L	Subclasses must work like parents
I	Many small interfaces > one big interface
D	Depend on interfaces, not implementations