SOLID Principles Interview Questions And Answers

Last Updated on September 9th, 2025

SOLID Principles Interview Questions and Answers: Intermediate to Expert-Level MCQs

The SOLID principles are the cornerstone of object-oriented design. They provide a set of guidelines that help developers write code that is more maintainable, scalable, and reusable. While most developers can name the five principles, truly understanding and applying them in complex scenarios is the mark of an expert. Undoubtedly, theory is essential, putting that knowledge to the test is the best way to prepare.

This article presents advanced-level Multiple-Choice Questions (MCQs) designed for those who want to go beyond the basics. Each question is crafted to test your deep understanding of single responsibility, open-closed, Liskov substitution, interface segregation, and dependency inversion. We’ve included detailed explanations for every answer, so you don’t just know what’s right, but also you understand why. Let’s begin and put your expertise to the test.

Table of Contents

How to Use This Guide for Interview Preparation?

This guide is more than just a list of questions; it’s a tool designed to help you prepare effectively for expert-level interviews. To get the most out of it, we recommend the following approach:

Treat It Like a Mock Interview: Before looking at the answers, read each question carefully and try to solve it on your own. Write down your thought process and your final choice. This simulates the pressure and independent thinking required in a real interview.
Focus on the “Why”: The most valuable part of this guide is the detailed explanations. After answering each question, compare your reasoning with the provided solution. Pay close attention not just to the correct answer, but to the why. A senior-level interview is less about finding the right option and more about justifying your thought process and understanding the trade-offs.
Identify Knowledge Gaps: If you find yourself consistently getting questions wrong on a particular topic, use that as a signal. Go back and review the core concepts in your study materials before moving on. This is a targeted way to strengthen your weaknesses.
Use It as a Review Tool: Once you’ve gone through all the questions, come back to this guide for a quick review before your interview. Re-reading the questions and their explanations can serve as a powerful refresher to keep the core concepts and common pitfalls fresh in your mind.

In this way, you will not only prepare for the questions but also develop the critical thinking skills needed to ace your SOLID principle’s interviews. Good luck!

SOLID Principles Interview Questions and Answers With Explanation (MCQ Format)

Q#1. You are designing a payment system with classes CreditCardPayment, PayPalPayment, and BankTransferPayment. The system should support new payment methods in the future without modifying existing code. Which principles apply? (Multi-select)

A) Open-Closed Principle (OCP)
B) Dependency Inversion Principle (DIP)
C) Interface Segregation Principle (ISP)
D) Liskov Substitution Principle (LSP)

Answer: A, B, D
Explanation:
A is correct (OCP): New payment types can be added without modifying existing classes. B is correct (DIP): High-level PaymentProcessor should depend on a Payment abstraction, not concrete classes. D is correct (LSP): Each payment method must behave consistently when substituted for another. C is incorrect (ISP): There’s no large interface problem here.

Q#2. A ReportGenerator class is responsible for fetching data from a database, processing it, and then formatting it into a PDF. It directly uses a MySQLConnector concrete class and has internal logic for PDF generation. If the database type changes (e.g., to PostgreSQL) or a new report format (e.g., HTML) is needed, the ReportGenerator class needs significant modification. Which three SOLID principles are most directly violated?

A) SRP, OCP, and DIP
B) OCP, LSP, and ISP
C) SRP, LSP, and DIP
D) ISP, OCP, and LSP

Answer: A
Explanation:
A is correct. The ReportGenerator has multiple responsibilities (data fetching, processing, PDF formatting), violating SRP. The need for significant modification when database type or report format changes violates OCP. Directly using MySQLConnector and having internal PDF generation logic means it depends on concretions, violating DIP. B, C, and D are incorrect because they miss one or more of the direct violations or include principles not directly violated.

Q#3. Which of the following are common anti-patterns that lead to SOLID principle violations? (Multi-select)

A) God Objects (classes with too many responsibilities).
B) Tight coupling between modules.
C) Relying heavily on instanceof or switch statements on type.
D) Using interfaces for all public methods.

Answer: A, B, C
Explanation:
A is correct. God Objects violate SRP by having too many responsibilities. B is correct. Tight coupling is a result of violating OCP, DIP, and sometimes ISP. C is correct. Relying on instanceof or switch on type often indicates violations of OCP and LSP. D is incorrect. Using interfaces for all public methods is generally a good practice that supports DIP and ISP.

Q#4. You have a UserPreferences class that stores user settings. It has methods loadFromFile(), saveToFile(), and displayOnUI(). The displayOnUI() method directly manipulates UI elements. If the UI framework changes, UserPreferences needs to be updated. Which two SOLID principles are most directly violated?

A) Single Responsibility Principle (SRP) and Open-Closed Principle (OCP)
B) Liskov Substitution Principle (LSP) and Interface Segregation Principle (ISP)
C) Dependency Inversion Principle (DIP) and Single Responsibility Principle (SRP)
D) Open-Closed Principle (OCP) and Dependency Inversion Principle (DIP)

Answer: A
Explanation:
A is correct. The UserPreferences class has multiple responsibilities (persistence and UI display), violating SRP. The need to update it when the UI framework changes indicates it”s not closed for modification, violating OCP. A better design would separate the UI display logic into a dedicated class. B is incorrect because LSP and ISP are not the primary violations. C is incorrect because DIP and SRP are not the primary violations. D is incorrect because while DIP might be violated if it directly depends on a concrete UI renderer, SRP and OCP are more directly evident from the description.

Q#5. Which of the following best describe disadvantages of ignoring SOLID principles? (Multi-select)

A) Tightly coupled code
B) Reduced testability
C) Fragile code with ripple-effect changes
D) Faster delivery of stable software

Answer: A, B, C
Explanation:
A is correct: Ignoring SOLID often leads to tight coupling. B is correct: Lack of abstractions reduces testability. C is correct: Ripple-effect changes are common when SOLID is not followed. D is incorrect: Software may be delivered quickly initially, but stability is compromised.

Q#6. You introduce a caching mechanism into a microservice, but later discover the caching logic is tightly coupled with database queries. When scaling, this becomes problematic. Which principle could have prevented this?

A) OCP
B) DIP
C) ISP
D) SRP

Answer: B
Explanation:
B is correct: DIP would encourage depending on abstractions (e.g., CacheRepository interface), not specific implementations, allowing flexibility. A is incorrect: OCP relates to adding new features, not decoupling. C is incorrect: ISP is not relevant here. D is incorrect: SRP may hint at splitting caching and persistence, but DIP is more directly violated.

Q#7. Which of the following are indicators that your system might be violating the Dependency Inversion Principle? (Multi-select)

A) High-level modules directly import and use concrete low-level classes.
B) Extensive use of new keyword within high-level business logic.
C) Interfaces with too many methods.
D) Classes having multiple, unrelated responsibilities.

Answer: A, B
Explanation:
A is correct. This is a direct violation of DIP, as high-level modules should depend on abstractions. B is correct. Extensive use of new keyword within high-level modules indicates direct instantiation of concrete dependencies, violating DIP. C is incorrect. This is an indicator of ISP violation. D is incorrect. This is an indicator of SRP violation.

Q#8. A ReportFormatter class has a method format(data, formatType). It uses a switch statement to handle different formatType values (e.g., “XML”, “JSON”). It also has a method saveToFile(formattedReport, filename). Which two SOLID principles are most directly violated?

Answer: A
Explanation:
A is correct. The ReportFormatter has multiple responsibilities (formatting and saving), violating SRP. The switch statement for formatType means it”s not closed for modification when new formats are added, violating OCP. A better design would separate formatting and saving into distinct classes. B is incorrect because LSP and ISP are not the primary violations. C is incorrect because DIP and SRP are not the primary violations. D is incorrect because while DIP might be violated if it directly depends on a concrete file writer, SRP and OCP are more directly evident from the description.

Q#9. In a notification system, developers created a single Notifier interface with methods: sendEmail(), sendSMS(), sendPush(). Clients using only SMS are forced to implement unused methods. Which principle should fix this?

A) SRP
B) OCP
C) ISP
D) DIP

Answer: C
Explanation:
C is correct (ISP): Split into smaller interfaces (EmailNotifier, SMSNotifier, etc.). A is incorrect: Responsibility distribution isn’t the issue. B is incorrect: Extensibility isn’t the focus here. D is incorrect: Abstraction level is fine; the issue is oversized interfaces.

Q#10. Which of the following are common benefits of using Dependency Injection (DI) in conjunction with DIP? (Multi-select)

A) Increased testability.
B) Reduced coupling.
C) Easier management of dependencies.
D) Eliminates the need for interfaces.

Answer: A, B, C
Explanation:
A is correct. DI makes it easy to swap out real dependencies for mocks or stubs during testing, greatly improving testability. B is correct. By injecting dependencies (often abstractions), modules become less coupled to concrete implementations. C is correct. DI frameworks help manage the lifecycle and creation of dependencies, simplifying the overall architecture. D is incorrect. DI often relies heavily on interfaces to achieve DIP.

Q#11. Which of the following statements are true about SOLID?

SOLID principles are primarily intended for object-oriented systems.
SOLID can help reduce coupling and increase cohesion.
Applying SOLID always reduces the total number of classes in a system.

A) 1 & 2
B) 1 & 3
C) 2 & 3
D) All 3

Answer: A
Explanation:
1 is correct: SOLID emerged in the OOP context. 2 is correct: Cohesion and reduced coupling are major goals. 3 is incorrect: SOLID often increases the number of classes via abstraction and separation.

Q#12. You have a User class that holds user data. It also has a method displayUserProfile() that renders the user’s profile on a web page. If the web framework changes, the User class needs to be updated. Which two SOLID principles are most directly violated?

Answer: A
Explanation:
A is correct. The User class has multiple responsibilities (data and UI display), violating SRP. The need to update it when the web framework changes indicates it”s not closed for modification, violating OCP. A better design would separate the UI display logic into a dedicated UserProfileRenderer class. B is incorrect because LSP and ISP are not the primary violations. C is incorrect because DIP and SRP are not the primary violations. D is incorrect because while DIP might be violated if it directly depends on a concrete UI renderer, SRP and OCP are more directly evident from the description.

Q#13. A Shape interface has a method draw(). You have Circle and Square classes implementing this. A new Line class is introduced, and it also implements Shape, but its draw() method only draws a line, not a closed shape. If a function expects a Shape and calls draw(), and it gets a Line, it might get an unexpected result (a line instead of a closed shape). Which two SOLID principles are violated?

Answer: B
Explanation:
B is correct. Line drawing only a line for draw() violates LSP because it cannot be substituted for Shape without breaking the expected behavior (drawing a closed shape). It also violates ISP because the Shape interface is forcing Line to implement a method (draw()) that is semantically different for an open shape, indicating a fat interface. A better design would be to have separate interfaces like ClosedShape and OpenShape. A is incorrect because SRP and OCP are not the primary violations. C is incorrect because DIP and SRP are not the primary violations. D is incorrect because OCP and DIP are not the primary violations.

Q#14. A ProductService class has methods createProduct(), updateProduct(), and deleteProduct(). It also has a method sendProductUpdateNotification() that sends an email notification. If the email sending mechanism changes, the ProductService needs to be updated. Which two SOLID principles are most directly violated?

Answer: A
Explanation:
A is correct. The ProductService has multiple responsibilities (product management and notification), violating SRP. The need to update it when the email sending mechanism changes indicates it”s not closed for modification, violating OCP. A better design would separate the notification logic into a dedicated NotificationService. B is incorrect because LSP and ISP are not the primary violations. C is incorrect because DIP and SRP are not the primary violations. D is incorrect because while DIP might be violated if it directly depends on a concrete email sender, SRP and OCP are more directly evident from the description.

Q#15. Which of the following are common benefits of achieving loose coupling and high cohesion in software design? (Multi-select)

A) Easier to understand and maintain code.
B) Increased reusability of components.
C) Improved testability.
D) Faster initial development time.

Answer: A, B, C
Explanation:
A is correct. Loosely coupled and highly cohesive modules are easier to reason about and understand, leading to better maintainability. B is correct. Independent and focused components can be easily reused in different parts of the system or in other projects. C is correct. Isolated components with clear responsibilities are much easier to test in isolation. D is incorrect. Achieving loose coupling and high cohesion often requires more upfront design effort, which might slow down initial development but pays off in the long run.

Q#16. You are designing a system for processing financial transactions. You have a TransactionProcessor class with a method process(transaction). Inside process(), it uses a switch statement to handle different transaction types (e.g., Deposit, Withdrawal, Transfer). It also directly calls a FraudDetector concrete class. Which three SOLID principles are most directly violated?

A) SRP, OCP, and DIP
B) OCP, LSP, and ISP
C) SRP, LSP, and DIP
D) ISP, OCP, and LSP

Answer: A
Explanation:
A is correct. The TransactionProcessor has multiple responsibilities (processing different transaction types and fraud detection), violating SRP. The switch statement for types violates OCP (not closed for modification when new types are added). Directly calling FraudDetector (a concrete class) violates DIP (depending on a concretion, not an abstraction). B, C, and D are incorrect because they miss one or more of the direct violations or include principles not directly violated.

Q#17. Observe the below code snippet:

abstract class Shape {
    abstract double area();
}
class Triangle extends Shape {
    private double base, height;
    double area() { return 0.5 * base * height; }
}

What principle is respected by this design?

A) LSP
B) OCP
C) SRP
D) DIP

Answer: A
Explanation:
A is correct (LSP): Triangle correctly substitutes Shape. B is incorrect: OCP is possible here but not demonstrated directly. C is incorrect: No clear multiple responsibility scenario. D is incorrect: No dependency abstraction issue here.

Q#18. You have a Vehicle interface with methods drive(), fly(), and sail(). A Car class implements this interface, but its fly() and sail() methods throw UnsupportedOperationException. Which two SOLID principles are violated?

Answer: B
Explanation:
B is correct. Car throwing UnsupportedOperationException for fly() and sail() violates LSP because it cannot be substituted for Vehicle without breaking the expected behavior of those methods. It also violates ISP because the Vehicle interface is forcing Car to implement methods that are irrelevant to it, indicating a fat interface. A better design would be to have separate interfaces like Drivable, Flyable, and Sailable. A is incorrect because SRP and OCP are not the primary violations. C is incorrect because DIP and SRP are not the primary violations. D is incorrect because OCP and DIP are not the primary violations.

Q#19. A ProductCatalog class has methods addProduct(), removeProduct(), and searchProduct(). It also has a method sendProductAvailabilityAlert() that sends an SMS alert. If the SMS sending service changes, the ProductCatalog needs to be updated. Which two SOLID principles are most directly violated?

Answer: A
Explanation:
A is correct. The ProductCatalog has multiple responsibilities (product management and notification), violating SRP. The need to update it when the SMS sending service changes indicates it’s not closed for modification, violating OCP. A better design would separate the notification logic into a dedicated NotificationService. B is incorrect because LSP and ISP are not the primary violations. C is incorrect because DIP and SRP are not the primary violations. D is incorrect because while DIP might be violated if it directly depends on a concrete SMS sender, SRP and OCP are more directly evident from the description.

Q#20. Which of the following are common indicators of a well-designed, SOLID-compliant system? (Multi-select)

A) Easy to add new features without modifying existing code.
B) Components are highly reusable.
C) Changes in one part of the system have minimal impact on other parts.
D) Complex inheritance hierarchies with deep class trees.

Answer: A, B, C
Explanation:
A is correct. This is a direct benefit of OCP. B is correct. This is a benefit of low coupling and high cohesion, promoted by SOLID principles. C is correct. This is a benefit of low coupling, promoted by SOLID principles. D is incorrect. Deep and complex inheritance hierarchies can often lead to violations of LSP and make the system rigid and fragile. While inheritance is used, SOLID promotes shallow and well-defined hierarchies.

How to Use This Guide for Interview Preparation?

SOLID Principles Interview Questions and Answers With Explanation (MCQ Format)

Further Reading on SOLID Principles

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