✅ OOP Interview Questions with Answers Part-4 🧠💻

31. What is SOLID in OOP?
SOLID is a set of 5 design principles for writing maintainable, scalable OOP code. It stands for:
S – Single Responsibility
O – Open/Closed
L – Liskov Substitution
I – Interface Segregation
D – Dependency Inversion

32. Explain each SOLID principle briefly:
• Single Responsibility – A class should do one thing only.
• Open/Closed – Classes should be open for extension, but closed for modification.
• Liskov Substitution – Subclasses should replace their parent classes without breaking functionality.
• Interface Segregation – Prefer small, specific interfaces over large ones.
• Dependency Inversion – Depend on abstractions, not concrete classes.

33. What is Liskov Substitution Principle?
If a class S is a subclass of class T, objects of type T should be replaceable with objects of type S without affecting the program.
Example: A Bird base class with a fly() method may break if Penguin inherits it (Penguins can't fly). So, design must respect capabilities.

34. What is Dependency Inversion Principle?
High-level modules should not depend on low-level modules. Both should depend on abstractions.
Example: A service class should depend on an interface, not a specific implementation.

35. What is object slicing?
Occurs when an object of a derived class is assigned to a base class variable — the extra properties of the derived class are "sliced off."
Example: C++ object slicing when passing by value.

36. What are getters and setters?
Special methods used to get and set values of private variables in a class.
They support encapsulation and validation.

def get_name(self): return self._name  
def set_name(self, name): self._name = name

37. What is a virtual function?
A function declared in the base class and overridden in the derived class, using the virtual keyword (in C++). Enables run-time polymorphism.

38. What is early binding vs late binding?
• Early Binding (Static): Method call is resolved at compile time (e.g., method overloading).
• Late Binding (Dynamic): Method call is resolved at run-time (e.g., method overriding).

39. What is dynamic dispatch?
It’s the process where the method to be invoked is determined at runtime based on the object’s actual type — used in method overriding (late binding).

40. What is a pure virtual function?
A virtual function with no implementation in the base class — makes the class abstract.
Syntax (C++):

virtual void draw() = 0;

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✅ OOP Interview Questions with Answers Part-5 🧠💻

41. What is multiple inheritance?
It means a class can inherit from more than one parent class.
✔ Supported in C++
✖ Not directly supported in Java (handled via interfaces)

42. What are mixins?
Mixins are a way to add reusable behavior to classes without using inheritance.
✔ Used in Python and JavaScript
✔ Promotes code reuse

43. What is the diamond problem in inheritance?
Occurs when two parent classes inherit from a common grandparent, and a child class inherits both.
❌ Creates ambiguity about which method to inherit.

44. How is the diamond problem solved in C++ or Java?
• C++: Uses virtual inheritance
• Java: Avoids it entirely using interfaces (no multiple class inheritance)

45. What are abstract data types in OOP?
ADTs define what operations can be done, not how.
Examples: Stack, Queue, List
✔ Implementation is hidden
✔ Promotes abstraction

46. What is a design pattern in OOP?
Reusable solution to a common software design problem.
✔ Templates for writing clean, maintainable code

47. What are some common OOP design patterns?
• Singleton – one instance
• Factory – object creation logic
• Observer – event-based updates
• Strategy – interchangeable behavior
• Adapter – interface compatibility

48. Interface vs Abstract Class (Real-world use)
• Interface – Contract; use when you want to define capability (e.g., Drivable)
• Abstract Class – Shared structure + behavior; base class for similar types (e.g., Vehicle)

49. What is garbage collection?
Automatic memory management – reclaims memory from unused objects.
✔ Java has a built-in GC
✔ Prevents memory leaks

50. Real-world use of OOP?
• Games – Objects for players, enemies
• Banking – Classes for accounts, transactions
• UI – Buttons, forms as objects
• E-commerce – Products, carts, users as objects

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📊 SQL Interview Queries – Intermediate Level

━━━━━━━━━━━━━━

❓ Query 01: Find employees earning more than the average salary

SELECT *
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

❓ Query 02: Find department-wise employee count

SELECT department, COUNT(*) AS emp_count
FROM employees
GROUP BY department;

❓ Query 03: Find departments with average salary greater than 60,000

SELECT department
FROM employees
GROUP BY department
HAVING AVG(salary) > 60000;

❓ Query 04: Fetch employees who do not belong to any department

SELECT e.*
FROM employees e
LEFT JOIN departments d
ON e.department_id = d.department_id
WHERE d.department_id IS NULL;

❓ Query 05: Find second highest salary

SELECT MAX(salary)
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);

❓ Query 06: Get highest salary in each department

SELECT department, MAX(salary)
FROM employees
GROUP BY department;

❓ Query 07: Fetch employees hired in the last 6 months

SELECT *
FROM employees
WHERE hire_date >= DATE_SUB(CURDATE(), INTERVAL 6 MONTH);

❓ Query 08: Find duplicate email IDs

SELECT email, COUNT(*)
FROM employees
GROUP BY email
HAVING COUNT(*) > 1;

❓ Query 09: Rank employees by salary within each department

SELECT *,
RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS rank
FROM employees;

❓ Query 10: Fetch top 2 highest paid employees from each department

SELECT *
FROM (
SELECT *,
DENSE_RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS rnk
FROM employees
) t
WHERE rnk <= 2;

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