Introduction

Prepare for your Java interviews with these top questions and clear, concise answers. This guide covers Java basics, OOP concepts, collections, multithreading, and real-world debugging, ideal for freshers and experienced candidates.

Java Interview Questions and Answers

1. What is Java?
A high-level, object-oriented programming language developed by Sun Microsystems.

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

3. What is JVM?
Java Virtual Machine, responsible for running Java bytecode on any platform.

4. What is JDK?
Java Development Kit, a software development environment for writing Java programs.

5. What is JRE?
Java Runtime Environment, providing libraries and JVM to run Java applications.

6. Explain OOP concepts in Java.
Encapsulation, Inheritance, Polymorphism, and Abstraction.

7. What is encapsulation?
Wrapping data and code together into a single unit.

8. What is inheritance in Java?
Allows one class to acquire properties and methods of another class.

9. What is polymorphism?
Ability of an object to take many forms, method overloading and overriding.

10. What is abstraction?
Hiding implementation details and showing only functionality.

11. Difference between abstract class and interface?
Abstract class can have method implementations; interface cannot (prior to Java 8).

12. What is method overloading?
Defining multiple methods with the same name but different parameters.

13. What is method overriding?
Subclass providing a specific implementation of a superclass method.

14. Can you override a static method?
No, static methods cannot be overridden.

15. What is the final keyword in Java?
Used to restrict class inheritance, method overriding, and variable reassignment.

16. What is a constructor in Java?
A block of code used to initialize objects.

17. Types of constructors in Java?
Default constructor and parameterized constructor.

18. What is the difference between == and .equals()?
== checks reference equality; .equals() checks content equality.

19. What are access modifiers in Java?
public, private, protected, and default.

20. What is a package in Java?
A namespace to group related classes and interfaces.

21. What is the main method in Java?
The entry point: public static void main(String[] args).

22. What is a static variable?
A variable shared among all instances of a class.

23. What is a static block?
Block of code that runs once when the class is loaded.

24. What is the super keyword?
Used to refer to the immediate parent class object.

25. What is this keyword in Java?
Refers to the current object within a method or constructor.

26. What is the difference between ArrayList and LinkedList?
ArrayList is better for retrieval; LinkedList is better for manipulation.

27. What is a HashMap in Java?
A collection that stores data in key-value pairs with no duplicate keys.

28. What is the difference between HashMap and Hashtable?
HashMap is not synchronized; Hashtable is synchronized.

29. What is multithreading in Java?
Executing multiple threads simultaneously for multitasking.

30. How to create a thread in Java?
By extending Thread class or implementing Runnable interface.

31. What is synchronization in Java?
Controlling access to shared resources in multithreading.

32. What is the difference between process and thread?
Process has its own memory; thread shares memory within a process.

33. What is exception handling in Java?
Handling runtime errors using try, catch, finally, and throw.

34. What is the difference between checked and unchecked exceptions?
Checked exceptions are checked at compile-time; unchecked at runtime.

35. What is a try-catch block?
Used to handle exceptions and execute alternative code.

36. What is finally block?
Block that executes regardless of exception occurrence.

37. What is throw and throws?
throw is used to explicitly throw an exception; throws declares exceptions in a method.

38. What is garbage collection in Java?
Automatic memory management to free unused objects.

39. What is the finalize() method?
Method called before garbage collection on an object.

40. What are wrapper classes in Java?
Classes that convert primitive data types into objects.

41. What is String in Java?
An immutable sequence of characters.

42. Difference between String, StringBuffer, and StringBuilder?
String is immutable; StringBuffer is mutable and thread-safe; StringBuilder is mutable but not thread-safe.

43. What is JDBC?
Java Database Connectivity, used to connect and execute queries with databases.

44. What are the steps in JDBC connectivity?
Load driver, establish connection, create statement, execute query, close connection.

45. What is serialization in Java?
Converting an object into a byte stream for storage or transfer.

46. What is deserialization?
Converting a byte stream back into an object.

47. What is the transient keyword?
Prevents serialization of a variable.

48. What are design patterns in Java?
Reusable solutions to common software design problems.

49. What is the singleton pattern?
Design pattern ensuring only one instance of a class is created.

50. Why is Java platform independent?
Because Java code is compiled into bytecode, which can run on any system with a JVM.

Frequently Asked Questions