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Developers often wonder why SOLID principles are so important and why senior developers and solution architects put so much emphasis on them. Usually, developers just focus on making code work. And when it does, it feels like a win. But over time, they face challenges like maintaining, scaling and fixing the code without breaking anything else.
This is where many projects start to struggle. Code becomes hard to understand, small changes take hours, and adding new features feels risky.
That’s why understanding SOLID principles is so important. They help developers move from writing code that simply works to building systems that are clean, flexible and scalable.
SOLID is a set of five object-oriented design principles that guide developers in writing better code. Each letter in SOLID represents a principle that focuses on one aspect of good software design.
These principles teach you to think and structure instead of teaching you what to code, which is more important. They help you design code that can survive real-world use.
In simple terms, SOLID principles help you:
SOLID is not about writing more code or complex abstractions. It’s about thinking before you code and validating whether your design makes sense in the long run.
In the next sections, we’ll break down each of the five SOLID principles, understand the problem they solve, and see why they are essential for building well-designed software systems.
SOLID is an acronym formed from the first letters of five core design principles in object-oriented programming:
Each of these principles focuses on solving a specific design problem that developers commonly face as applications grow in size and complexity. Let’s start with these principles one by one for your better understanding. We are going to explain each principle with examples, so make sure you read it to the end.
The Single Responsibility Principle says:
“A class should have only one responsibility, and only one reason to change.”
In simple words:
SRP helps you avoid writing confusing, tightly coupled code that becomes hard to fix later. At many times, developers may feel natural and logical to put everything (logic, validation, data, output, etc) in one class. At first, this works. But as the project grows, even small changes become risky.
Let’s understand this with a simple example.
Let’s say we are building an application that works with shapes like circles and squares. The goal of the application is to take a collection of shapes and calculate the total area of all shapes.
To do this, we first need to define our shape objects and store the values required to calculate their area.
Each shape needs specific information to calculate its area.
So we create separate classes for each shape and pass the required values using constructors.
Square Class
class Square {
double-sided;
Square(double side) {
this.side = side;
}
}
Circle Class
class Circle {
double radius;
Circle(double radius) {
this.radius = radius;
}
}So far, everything looks fine. Each class is only responsible for holding shape data.
Now that we have our shape classes, the next step is to make sure each shape knows how to calculate its own area. This is important because:
We create a commonShape interface that defines one rule. Every shape must know how to calculate its area.
interface Shape {
double area();
}This keeps our design clean and consistent.
Square Class
class Square implements Shape {
double side;
Square(double side) {
this.side = side;
}
public double area() {
return side * side;
}
}The Square class now:
Nothing more. Nothing less.
Circle Class
class Circle implements Shape {
double radius;
Circle(double radius) {
this.radius = radius;
}
public double area() {
return Math.PI * radius * radius;
}
}The Circle class is responsible only for:
Now we create a class whose only job is to add up areas.
class AreaCalculator {
double calculateTotalArea(List<Shape> shapes) {
double totalArea = 0;
for (Shape shape : shapes) {
totalArea += shape.area();
}
return totalArea;
}
}This class:
The Open-Closed Principle says:
“Software entities should be open for extension but closed for modification.” In simple words, you should be able to add new behavior without changing existing code. This is important because changing existing code can introduce bugs.
Because we already followed SRP correctly:
Now, if we want to add a new shape, we don’t touch existing code.
class Rectangle implements Shape {
double length;
double width;
Rectangle(double length, double width) {
this.length = length;
this.width = width;
}
public double area() {
return length * width;
}
}That’s it. We don’t need any change in AreaCalculator and no change in Square or Circle This means our design is open for extension and closed for modification.
The Liskov Substitution Principle says:
“Objects of a child class should be able to replace objects of the parent class without breaking the program.”
In simpler terms, if a class implements an interface, it must behave correctly. The program should not care which concrete class it is using.
All shapes implement the Shape interface:
Shape shape = new Circle(5);
or
Shape shape = new Square(4);Both work perfectly because:
The AreaCalculator can safely use any shape without worrying.
If one shape behaves differently or breaks expectations, the whole system becomes unreliable. Because all shapes follow the same contract, LSP is satisfied.
The Interface Segregation Principle says:
“Clients should not be forced to depend on methods they do not use.”
In simple words, interfaces should be small and focused. It should not force classes to implement unnecessary methods.
Now, let’s understand this with our example:
Our Shape interface looks like this:
interface Shape {
double area();
}This is good because every shape needs to calculate its area. And no extra methods are forced here.
The Dependency Inversion Principle says: “High-level modules should not depend on low-level modules. Both should depend on abstractions.”
To follow DIP, we introduce an interface between the reminder logic and the sending logic.
interface MessageService {
void sendMessage(String message);
}This interface represents any way of sending a message.
class EmailService implements MessageService {
public void sendMessage(String message) {
// send email logic
}
}class PasswordReminder {
MessageService messageService;
PasswordReminder(MessageService messageService) {
this.messageService = messageService;
}
void sendReminder() {
messageService.sendMessage("Reset your password");
}
}Now:
If tomorrow you want to add SMS:
class SMSService implements MessageService {
public void sendMessage(String message) {
// send SMS logic
}
}No change needed in PasswordReminder
Because of DIP:
SOLID principles are not rules meant to be memorized or applied blindly. They exist to help you pause and think about your design before writing code. As an engineering student or developer, your goal should not be to write the most code, but to write code that makes sense when the project grows.
A simple way to validate your design is to ask yourself a few honest questions:
If your design answers these questions well, you’re already thinking in terms of SOLID. Good software design is not about being clever. It’s about being clear, flexible, and intentional.
When you learn to structure your code this way, it separates a coder from a software engineer.
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