SOLID Principles

What problem is this solving?

Object-oriented code often becomes hard to change when classes take too many responsibilities, depend on concrete tools, or expose contracts that do not match real behavior.

SOLID gives names to these design problems and helps you fix them before every change becomes risky.

Simple definition

SOLID is a group of five object-oriented design principles.

They help you write code that is easier to:

understand
test
extend
change safely

SOLID is not a rule that says every feature needs many classes.

It is a way to notice design problems when code starts becoming hard to change.

Better explanation

S → Single Responsibility Principle
O → Open/Closed Principle
L → Liskov Substitution Principle
I → Interface Segregation Principle
D → Dependency Inversion Principle

Each word matters.

The main idea

Bad object-oriented code usually has these problems:

One class does too many things.
Adding a new feature requires editing old stable code again and again.
Child classes break behavior expected from parent classes.
Interfaces force classes to implement methods they do not need.
Business logic is tightly coupled to concrete tools like database clients, email clients, or payment SDKs.

SOLID gives names to these problems and suggests better structure.

Real example

Imagine an application that handles invoice payment.

Bad design often puts everything in one class:

class InvoiceService {
    void payInvoice(Invoice invoice) {
        // validate invoice
        // calculate tax
        // save payment in database
        // send receipt email
        // write audit log
    }
}

This looks convenient at first.

But later, different changes hit the same class:

Tax rules change.
Database storage changes.
Email provider changes.
Audit logging changes.
Payment validation changes.

That is where SOLID becomes useful.

It helps separate responsibilities, hide change behind contracts, and make extension safer.

SOLID as a tree

SOLID Principles
├── S: Single Responsibility Principle
├── O: Open/Closed Principle
├── L: Liskov Substitution Principle
├── I: Interface Segregation Principle
└── D: Dependency Inversion Principle

Read them one by one.

Do not try to memorize definitions first.

Understand the problem each principle solves.

Quick Summary

Letter	Principle	Simple Question
S	Single Responsibility	Does this class have one clear reason to change?
O	Open/Closed	Can I add new behavior without rewriting stable code?
L	Liskov Substitution	Can a child type safely replace its parent type?
I	Interface Segregation	Is this interface small enough for every implementer?
D	Dependency Inversion	Does business logic depend on an abstraction instead of a concrete tool?

Common mistake

Do not apply SOLID by creating unnecessary abstractions everywhere.

SOLID should reduce complexity.

If it adds ceremony without a real change boundary, keep the code simple.

Interview Answer

If an interviewer asks:

What is SOLID?

You can answer:

SOLID is a set of five object-oriented design principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. These principles help reduce coupling and make code easier to understand, test, and extend. They are guidelines, not strict rules, and should be applied when they reduce complexity.