Here’s a clean, interview-ready README for the SOLID Principles, suitable for a Java / Spring Boot developer like you. You can directly use this in GitHub 📘
SOLID is a set of five object-oriented design principles that help developers write clean, maintainable, scalable, and testable code. These principles are widely used in Java, Spring Boot, and microservices architectures.
SOLID stands for:
- S – Single Responsibility Principle
- O – Open/Closed Principle
- L – Liskov Substitution Principle
- I – Interface Segregation Principle
- D – Dependency Inversion Principle
A class should have only one reason to change.
class UserService {
void saveUser(User user) {}
void sendEmail(User user) {}
}class UserService {
void saveUser(User user) {}
}
class EmailService {
void sendEmail(User user) {}
}- Easier maintenance
- Better readability
- Simplified testing
Software entities should be open for extension but closed for modification.
class DiscountService {
double calculate(String type) {
if(type.equals("STUDENT")) return 10;
if(type.equals("EMPLOYEE")) return 20;
return 0;
}
}interface Discount {
double calculate();
}
class StudentDiscount implements Discount {
public double calculate() { return 10; }
}
class EmployeeDiscount implements Discount {
public double calculate() { return 20; }
}- Avoids breaking existing code
- Supports scalability
Subclasses must be substitutable for their base classes.
class Bird {
void fly() {}
}
class Penguin extends Bird {
void fly() {
throw new UnsupportedOperationException();
}
}interface Bird {}
interface FlyingBird extends Bird {
void fly();
}
class Sparrow implements FlyingBird {
public void fly() {}
}
class Penguin implements Bird {}- Prevents runtime errors
- Improves design correctness
Clients should not be forced to depend on interfaces they do not use.
interface Worker {
void work();
void eat();
}interface Workable {
void work();
}
interface Eatable {
void eat();
}- Smaller, cleaner interfaces
- Better flexibility
High-level modules should not depend on low-level modules. Both should depend on abstractions.
class Car {
Engine engine = new Engine();
}interface Engine {
void start();
}
class PetrolEngine implements Engine {
public void start() {}
}
class Car {
private Engine engine;
Car(Engine engine) {
this.engine = engine;
}
}- Loose coupling
- Easier unit testing
- Supports Spring’s Dependency Injection