✅ 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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📊 Excel Interview Question

❓ What is the difference between VLOOKUP and XLOOKUP?

🧠 Key Differences Explained Simply:

🔹 Lookup Direction

• VLOOKUP → Can only search left to right
• XLOOKUP → Can search both left → right and right → left

🔹 Column Dependency

• VLOOKUP → Depends on column number (breaks if columns move)
• XLOOKUP → No column number required (more reliable)

🔹 Error Handling

• VLOOKUP → Returns #N/A if value not found
• XLOOKUP → Built-in option to handle missing values gracefully

🔹 Flexibility & Performance

• VLOOKUP → Limited and outdated
• XLOOKUP → Modern, flexible, and recommended by Microsoft

🚀 Final Verdict:

If Excel version allows, always prefer XLOOKUP for cleaner, safer, and future-proof formulas.

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✅ Top 50 Coding Interview Questions You Must Prepare 💻🧠

1. What is the difference between compiled and interpreted languages?
2. What is time complexity? Why does it matter in interviews?
3. What is space complexity?
4. Explain Big O notation with examples.
5. Difference between array and linked list.
6. What is a stack? Real use cases.
7. What is a queue? Types of queues.
8. Difference between stack and queue.
9. What is recursion? When should you avoid it?
10. Difference between recursion and iteration.
11. What is a hash table? How does hashing work?
12. What are collisions in hashing? How do you handle them?
13. Difference between HashMap and HashSet.
14. What is a binary tree?
15. What is a binary search tree?
16. Difference between BFS and DFS.
17. What is a balanced tree?
18. What is heap data structure?
19. Difference between min heap and max heap.
20. What is a graph? Directed vs undirected.
21. What is adjacency matrix vs adjacency list?
22. What is sorting? Name common sorting algorithms.
23. Difference between quick sort and merge sort.
24. Which sorting algorithm is fastest and why?
25. What is searching? Linear vs binary search.
26. Why binary search needs sorted data?
27. What is dynamic programming?
28. Difference between greedy and dynamic programming.
29. What is memoization?
30. What is backtracking?
31. What is a pointer?
32. Difference between pointer and reference.
33. What is memory leak?
34. What is segmentation fault?
35. Difference between process and thread.
36. What is multithreading?
37. What is synchronization?
38. What is deadlock?
39. Conditions for deadlock.
40. Difference between shallow copy and deep copy.
41. What is exception handling?
42. Checked vs unchecked exceptions.
43. What is mutable vs immutable object?
44. What is garbage collection?
45. What is REST API?
46. What is JSON?
47. Difference between HTTP and HTTPS.
48. What is version control? Why Git matters?
49. Explain a coding problem you optimized recently.
50. How do you approach a new coding problem in interviews?

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