Quick 50 java related interview question answers for revision

1. What is JVM?
   JVM (Java Virtual Machine) is an engine that provides a runtime environment to execute Java bytecode.

2. What are the main features of Java?
   Platform independence, object-oriented, secure, robust, multithreaded, and high performance.

3. What is the difference between JDK, JRE, and JVM?
   JDK includes tools for development; JRE is for running Java applications; JVM executes bytecode.

4. Explain object-oriented programming concepts.
   Encapsulation, inheritance, polymorphism, and abstraction.

5. What is the difference between an interface and an abstract class?
   Interfaces define contracts with abstract methods; abstract classes can have both abstract and concrete methods.

6. What are access modifiers?
   Keywords that set access level: public, private, protected, default.

7. What is a constructor in Java?
   A special method to initialize objects.

8. What is method overloading vs. method overriding?
   Overloading: Same method name, different parameters; overriding: Subclass provides a specific implementation of a superclass method.

9. What is the significance of 'static' keyword?
   Marks members belonging to the class rather than instances.

10. What is the difference between == and .equals()?
    == compares references; .equals() compares object content.

11. What is a final variable/method/class?
    Cannot be changed/overridden/inherited.

12. Explain garbage collection in Java.
    Automatic memory management that removes unused objects.

13. What are checked and unchecked exceptions?
    Checked are checked at compile time; unchecked are runtime exceptions.

14. Difference between throw and throws?
    throw is used to actually throw an exception; throws declares exceptions a method can throw.

15. What is multi-threading?
    Concurrent execution of two or more threads.

16. How to create a thread in Java?
    Implement Runnable or extend Thread class.

17. What is synchronization?
    Controlling access to shared resources to prevent thread interference.

18. What is a deadlock?
    When two or more threads wait indefinitely for resources held by each other.

19. What are Java Collections?
    Framework for data structures like List, Set, Map.

20. Difference between ArrayList and LinkedList?
    ArrayList uses a dynamic array; LinkedList uses doubly linked list.

21. What is a HashMap?
    A hash table-based implementation of the Map interface.

22. Difference between HashMap and TreeMap?
    HashMap is unordered; TreeMap is sorted by keys.

23. What is encapsulation?
    Wrapping data and code into a single unit and restricting access.

24. What is polymorphism?
    Ability to take many forms via method overriding and overloading.

25. Explain inheritance.
    Deriving new classes from existing classes.

26. What is an inner class?
    A class defined inside another class.

27. What is lambda expression?
    Short block of code which takes in parameters and returns a value, introduced in Java 8 for functional programming.

28. Explain Stream API in Java 8.
    Supports functional-style operations on collections, such as map, filter, and reduce.

29. What is the difference between final, finally, and finalize?
    final: keyword; finally: block for cleanup; finalize: method called by GC before destroying an object.

30. What are annotations?
    Metadata that provides data about a program but is not part of the program logic.

31. What is the purpose of the transient keyword?
    To skip serialization of a field.

32. What is the difference between StringBuilder and StringBuffer?
    StringBuilder is faster but not synchronized; StringBuffer is synchronized.

33. What is serialization?
    Converting an object into a byte stream.

34. What is deserialization?
    Reconstructing an object from a byte stream.

35. What is the use of the volatile keyword?
    Ensures visibility of changes to variables across threads.

36. What is reflection in Java?
    API to inspect and manipulate classes at runtime.

37. What is a deadlock and how to avoid it?
    A situation where threads wait forever; avoid by lock ordering or using timed locks.

38. What is the difference between wait() and sleep()?
    wait() releases the lock, sleep() doesn’t.

39. What is Java Memory Model?
    Defines how Java manages memory and threads interaction.

40. What is the role of the ClassLoader?
    Loads class files into JVM.

41. Explain the principles of design patterns used in Java.
    Reusable solutions to common problems (e.g., Singleton, Factory, Observer).

42. What is the difference between shallow copy and deep copy?
    Shallow copy copies object reference; deep copy duplicates objects.

43. What is JIT compiler?
    Just-In-Time compiler compiles bytecode to native machine code at runtime.

44. How do you handle exceptions effectively?
    Use try-catch-finally, create custom exceptions, and follow best practices.

45. What are functional interfaces?
    Interfaces with a single abstract method used in lambda expressions.

46. What is the Optional class?
    A container object to avoid null checks.

47. What is the difference between Comparable and Comparator?
    Comparable defines natural ordering; Comparator is custom ordering.

48. What is method reference?
    Shorter syntax for lambda expressions referring to a method.

49. Explain the Fork/Join framework.
    Framework to take advantage of multiple processors by recursively breaking tasks.

50. What are some key performance considerations in Java?
    Efficient memory usage, avoiding unnecessary object creation, choosing the right data structures, and tuning Garbage Collection.

This list covers foundational and advanced topics Java developers with experience should be familiar with. Detailed explanations and real project examples during interviews will help solidify understanding and impress interviewers.