SOLID Principles

⚡ 1-Line Idea: Five guiding design principles that make software modular, testable, and maintainable under change.

1. Quick Revision Card

• S - Single Responsibility Principle (SRP): One class, one responsibility, one reason to change.
• O - Open/Closed Principle (OCP): Open for extension by adding new subclasses/strategies; closed for modification of existing tested code.
• L - Liskov Substitution Principle (LSP): Subtypes must be substitutable for their base types without breaking application invariants.
• I - Interface Segregation Principle (ISP): Clients should not be forced to depend on interfaces they do not use (lean, role-specific interfaces).
• D - Dependency Inversion Principle (DIP): High-level modules should depend on abstractions (interfaces), never concrete low-level implementations.

2. Real-Life Analogy

Think of a modular smartphone. The battery, camera module, and charging port communicate through standard interfaces (USB-C). You can swap in a higher-capacity battery (extension) without rewiring the motherboard (modification).

3. Bad vs Good Code (Open/Closed Principle Example)

Bad: Violating OCP with Type Checking

public class NotificationService {
    public void send(String type, String message) {
        if (type.equals("EMAIL")) {
            System.out.println("Sending Email: " + message);
        } else if (type.equals("SMS")) {
            System.out.println("Sending SMS: " + message);
        }
        // Adding PUSH requires editing this tested method!
    }
}

Good: OCP with Polymorphic Strategy

public interface NotificationSender {
    void send(String message);
}

public class EmailSender implements NotificationSender {
    @Override
    public void send(String message) {
        System.out.println("Sending Email: " + message);
    }
}

public class SmsSender implements NotificationSender {
    @Override
    public void send(String message) {
        System.out.println("Sending SMS: " + message);
    }
}

// Open for extension: new sender types plug in without changing NotificationService
public class NotificationService {
    public void notifyUser(NotificationSender sender, String message) {
        sender.send(message);
    }
}

4. How to Say it in an Interview

"I am applying the Open/Closed Principle here by introducing an interface so we can plug in new payment gateways without altering existing payment processing logic."

5. Memory Trick

S-O-L-I-D: Single job, Open plug, Legit swap, Interface lean, Depend on abstract.