Java Interview Questions

1. What is Java and What are its Features?

2. Difference Between JVM, JRE and JDK

3. What are the OOP Concepts in Java?

4. How Does Inheritance Work in Java?

5. What is Polymorphism in Java?

6. What is Encapsulation in Java?

7. What is Abstraction in Java?

8. Interface vs Abstract Class in Java

9. Method Overloading vs Overriding in Java

10. Why are Strings Immutable in Java?

11. String vs StringBuilder vs StringBuffer

12. Checked vs Unchecked Exceptions in Java

13. What is the Java Collections Framework?

14. ArrayList vs LinkedList in Java

15. How Does HashMap Work Internally in Java?

16. HashMap vs Hashtable in Java

17. equals() vs hashCode() in Java

18. Difference Between final, finally and finalize

19. What is the static Keyword in Java?

20. this vs super Keyword in Java

21. How Does Garbage Collection Work in Java?

22. What is Multithreading in Java?

23. What is the Thread Lifecycle in Java?

24. What is the synchronized Keyword in Java?

25. Runnable vs Thread in Java

26. Comparable vs Comparator in Java

27. What are Generics in Java?

28. What are Streams in Java 8?

29. What are Functional Interfaces in Java?

30. What is Autoboxing and Unboxing in Java?

31. JDK vs JRE vs JVM: What's the Difference?

32. What is Object-Oriented Programming (OOP) in Java?

33. What are Java Data Types?

34. What is the Java Collections Framework?

35. What is a Constructor in Java?

36. What is the final Keyword in Java?

37. What is a Java Thread?

38. String vs StringBuilder: What's the Difference?

39. What is Autoboxing in Java?

40. What are Generics in Java?

41. What is a lambda expression in Java?

42. What is the Stream API in Java?

43. What is the Java Memory Model and what does happens-before actually guarantee?

44. How do you choose between Java's garbage collectors, and what are the trade-offs?

45. What is the difference between ClassNotFoundException and NoClassDefFoundError?

46. How does class loading work in Java, and what is parent delegation?

47. What are virtual threads, and how do they differ from platform threads?

48. How do you compose asynchronous work with CompletableFuture without blocking?

49. What goes wrong in HashMap and HashSet when equals and hashCode are implemented badly?

50. How does laziness work in Java streams, and when does a stream do no work at all?

51. When is parallelStream a bad idea, and what goes wrong in production?

52. What are records in Java, and where should you not use one?

53. What problem do sealed classes and interfaces solve in Java?

54. How does ThreadLocal cause memory leaks, and how do you use it safely?

55. What are the risks of Java's built-in serialization, and what should you use instead?

56. Why are naive Java microbenchmarks wrong, and what does JIT warmup change?

57. ConcurrentHashMap is thread-safe, so why can code using it still be racy?

58. How do you make a Java class genuinely immutable, and where do defensive copies matter?

59. How does try-with-resources work, and what are suppressed exceptions?

60. How do you diagnose the different kinds of OutOfMemoryError in a Java service?

61. When do you use an atomic class rather than volatile or a lock?