Top 50 Java Interview Questions & Answers

1. What is Java?

Java is a high-level, object-oriented programming language designed for portability using JVM.

2. Why is Java platform independent?

Because Java code compiles into bytecode which runs on JVM across platforms.

3. What is JVM?

JVM executes Java bytecode and provides platform independence.

4. What is JDK?

JDK is a development kit including compiler, tools, and JRE.

5. What is JRE?

JRE provides runtime environment to execute Java programs.

6. What is OOP?

Object-oriented programming organizes code using objects and classes.

7. What are OOP pillars?

Encapsulation, Inheritance, Polymorphism, Abstraction.

8. What is encapsulation?

Binding data and methods together with restricted access.

9. What is inheritance?

Mechanism where one class inherits properties of another.

10. What is polymorphism?

Ability of an object to take many forms.

11. Method overloading?

Same method name with different parameters.

12. Method overriding?

Subclass redefining superclass method.

13. Abstract class?

Class with abstract and concrete methods.

14. Interface?

Defines contract with abstract methods.

15. Constructor?

Special method to initialize objects.

16. this keyword?

Refers to current object instance.

17. super keyword?

Refers to parent class.

18. final keyword?

Used to restrict modification.

19. finally block?

Executes after try-catch.

20. finalize()?

Called before garbage collection.

21. Collection framework?

Set of classes/interfaces for data structures.

22. List vs Set?

List allows duplicates, Set does not.

23. Map?

Stores key-value pairs.

24. HashMap vs HashTable?

HashMap is not synchronized, Hashtable is.

25. ArrayList vs LinkedList?

ArrayList faster for access, LinkedList for insertion.

26. Generics?

Allow type safety at compile time.

27. Lambda expressions?

Short way to write anonymous functions.

28. Streams?

Used to process collections functionally.

29. Optional?

Avoids null pointer exceptions.

30. Functional interface?

Interface with one abstract method.

31. Multithreading?

Running multiple threads simultaneously.

32. Thread lifecycle?

New → Runnable → Running → Waiting → Terminated.

33. Synchronization?

Controls access to shared resources.

34. Deadlock?

Threads waiting indefinitely for each other.

35. Executor framework?

Manages thread pools efficiently.

36. Exception handling?

Managing runtime errors using try-catch.

37. Checked vs unchecked?

Checked compile-time, unchecked runtime.

38. Serialization?

Converting object to byte stream.

39. Deserialization?

Converting byte stream back to object.

40. Garbage collection?

Automatic memory management.

41. String vs StringBuilder?

String immutable, StringBuilder mutable.

42. Immutable class?

Object whose state cannot change.

43. Comparable vs Comparator?

Comparable defines natural order, Comparator custom order.

44. Reflection?

Inspect and modify runtime behavior.

45. Dependency Injection?

Providing dependencies externally.

46. Spring Framework?

Framework for building enterprise Java apps.

47. Hibernate?

ORM tool for database interaction.

48. Microservices?

Architecture with small independent services.

49. REST API?

Web services using HTTP methods.

50. JVM tuning?

Optimizing memory and performance.

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