What are SOLID Principles?
Learn the five SOLID principles — Single Responsibility, Open/Closed, Liskov, Interface Segregation and Dependency Inversion — with a Java example and Q&A.
Expected Interview Answer
SOLID is five object-oriented design principles — Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion — that together make code more maintainable, flexible, and testable.
Single Responsibility: a class should have one reason to change. Open/Closed: open for extension, closed for modification. Liskov Substitution: subtypes must be usable in place of their base type without breaking behavior. Interface Segregation: prefer many small, specific interfaces over one large one. Dependency Inversion: depend on abstractions, not concrete implementations. Following them reduces coupling and makes systems easier to extend and change.
- Lower coupling and higher cohesion
- Easier to extend without breaking existing code
- More testable, maintainable designs
AI Mentor Explanation
SOLID is like running a well-organized team. Single Responsibility: each player has one clear role (a wicketkeeper doesn’t open the bowling). Open/Closed: add a new specialist without retraining the whole side. Liskov: any keeper you sub in must actually keep wickets. Interface Segregation: don’t force batters to attend bowling drills. Dependency Inversion: the captain plans around "a fast bowler", not one specific person. Five habits that keep the team adaptable.
Step-by-Step Explanation
Step 1
S — Single Responsibility
A class should have one reason to change.
Step 2
O — Open/Closed
Open for extension, closed for modification.
Step 3
L — Liskov Substitution
Subtypes must be usable wherever the base type is expected.
Step 4
I — Interface Segregation
Many small, specific interfaces beat one large one.
Step 5
D — Dependency Inversion
Depend on abstractions, not concrete implementations.
What Interviewer Expects
- All five principles named and briefly defined
- A concrete example of at least one (often SRP or DIP)
- Why they reduce coupling and aid maintainability
- Awareness that they are guidelines, not absolute rules
Common Mistakes
- Forgetting one of the five principles
- Confusing Open/Closed with Liskov Substitution
- Defining Dependency Inversion as just dependency injection
- Reciting definitions with no example
Best Answer (HR Friendly)
“SOLID is five design principles — Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. Together they keep classes focused, loosely coupled, and easy to extend, which makes software more maintainable and testable.”
Code Example
// Depend on an abstraction, not a concrete class
interface MessageSender { void send(String msg); }
class EmailSender implements MessageSender {
public void send(String msg) { /* ... */ }
}
class Notifier {
private final MessageSender sender; // abstraction
Notifier(MessageSender sender) { this.sender = sender; }
void notifyUser(String msg) { sender.send(msg); }
}Follow-up Questions
- Can you give an example that violates the Single Responsibility Principle?
- What is the difference between Dependency Inversion and Dependency Injection?
- How does Open/Closed relate to polymorphism?
- What is a Liskov Substitution violation?
MCQ Practice
1. The "S" in SOLID stands for?
Single Responsibility: a class should have one reason to change.
2. "Open for extension, closed for modification" is which principle?
The Open/Closed Principle lets you extend behavior without modifying existing code.
3. Depending on abstractions rather than concrete classes is?
Dependency Inversion says high-level modules should depend on abstractions.
Flash Cards
S? — Single Responsibility — one reason to change per class.
O and L? — Open/Closed — extend without modifying; Liskov — subtypes substitutable for base types.
I? — Interface Segregation — many small interfaces over one large one.
D? — Dependency Inversion — depend on abstractions, not concretions.