Here are the top 50 Java interview questions and answers with examples, categorized into different sections like Core Java, OOP Concepts, Exception Handling, etc.

Core Java

1. What is Java?

Java is a high-level, object-oriented programming language developed by Sun Microsystems. It is platform-independent due to its JVM (Java Virtual Machine).

2. What are the main features of Java?

Simple, Object-Oriented, Platform-Independent, Secure, Robust, Multithreaded, and High Performance.

3. What is the difference between JDK, JRE, and JVM?

JDK (Java Development Kit) is used to develop Java programs. JRE (Java Runtime Environment) is used to run Java programs. JVM (Java Virtual Machine) converts bytecode into machine-specific code.

4. Explain OOP Concepts in Java.

The four main OOP principles are:

• Encapsulation – Wrapping data and methods into a single unit.
• Inheritance – Mechanism where one class inherits the properties of another.
• Polymorphism – Ability to perform a single action in different ways.
• Abstraction – Hiding complex implementation details.

5. What is the difference between == and .equals() in Java?

== checks for reference equality (whether two objects point to the same memory location), while .equals() checks for content equality.

6. What is the difference between a constructor and a method?

A constructor is used to initialize objects, while a method is used to perform actions or return values.

7. What are static variables and methods?

Static variables are shared among all instances of a class, and static methods can be called without creating an instance of the class.

8. What is the final keyword in Java?

The final keyword can be used with variables (constant values), methods (cannot be overridden), and classes (cannot be inherited).

9. What is the difference between ArrayList and LinkedList in Java?

ArrayList is backed by a dynamic array, while LinkedList is backed by a doubly linked list. ArrayList is faster for accessing elements, while LinkedList is faster for inserting and deleting elements.

10. What is the String pool?

Java’s String pool is a special memory region where string literals are stored. If a string is already in the pool, a new string with the same value will reference the existing one, saving memory.

Exception Handling

11. What is Exception Handling in Java?

Exception Handling is a mechanism to handle runtime errors, allowing the program to continue execution.

12. Explain the difference between checked and unchecked exceptions.

Checked exceptions are checked at compile-time (e.g., IOException), whereas unchecked exceptions occur at runtime (e.g., ArithmeticException, NullPointerException).

13. What are the key components of exception handling in Java?

try, catch, finally, throw, and throws.

14. What is the purpose of the finally block?

The finally block always executes, regardless of whether an exception is thrown or caught. It is typically used for cleanup code.

15. Can we have a try block without a catch block?

Yes, but it must be followed by a finally block or the method should declare exceptions using the throws keyword.

Multithreading and Concurrency

16. What is multithreading in Java?

Multithreading is the concurrent execution of two or more threads to maximize the utilization of CPU.

17. How do you create a thread in Java?

By extending the Thread class or by implementing the Runnable interface.

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running");
    }
}

18. What is the difference between sleep() and wait()?

sleep() pauses the thread for a specified time, while wait() causes a thread to wait until another thread invokes notify() or notifyAll() on the same object.

19. What is a daemon thread?

A daemon thread runs in the background to support non-daemon threads (like the Garbage Collector). It terminates when all non-daemon threads finish execution.

20. What is the synchronized keyword?

It ensures that only one thread can access a synchronized method or block at a time to prevent thread interference.

Collections Framework

21. What is the Java Collections Framework?

The Java Collections Framework provides a set of classes and interfaces to handle data structures like lists, sets, queues, and maps.

22. What is the difference between HashMap and HashTable?

HashMap is not synchronized and allows one null key, while HashTable is synchronized and does not allow null keys or values.

23. What is a Set in Java?

A Set is a collection that does not allow duplicate elements. Examples include HashSet and TreeSet.

24. What is the difference between HashSet and TreeSet?

HashSet is unordered, while TreeSet stores elements in sorted order.

25. How does the Iterator work in Java?

An Iterator allows you to traverse a collection, removing elements safely during iteration.

26. What is the ConcurrentHashMap?

A thread-safe version of HashMap where read operations are non-blocking, and write operations are performed with locks.

OOP Concepts

27. What is an interface in Java?

An interface defines abstract methods that a class must implement. It provides multiple inheritance in Java.

28. Can an interface implement another interface?

Yes, interfaces can extend other interfaces.

29. What is the difference between an abstract class and an interface?

An abstract class can have concrete methods, while an interface can only have abstract methods (until Java 8, when default methods were introduced).

30. What is method overloading and overriding?

Overloading is when methods have the same name but different parameter lists, while overriding is when a subclass provides a specific implementation for a method declared in a parent class.

31. What is the use of the super keyword?

The super keyword is used to refer to the parent class’s methods or constructors.

Java 8 Features

32. What is a Lambda expression?

Lambda expressions provide a way to implement functional interfaces concisely in Java 8.

(parameter) -> expression

33. What are Streams in Java 8?

Streams provide a way to process collections of objects in a functional programming style (e.g., filtering, mapping, and reducing).

34. What is the difference between map() and flatMap() in streams?

map() transforms each element, while flatMap() flattens nested collections into a single collection.

35. What is Optional in Java?

Optional is a container object used to represent null values and avoid NullPointerException.

Advanced Topics

36. What is the Java Memory Model (JMM)?

The JMM defines how threads interact through memory and what behaviors are allowed in a multithreaded environment.

37. What is Garbage Collection in Java?

Garbage Collection is the automatic process of reclaiming memory by removing objects that are no longer in use.

38. What is a ClassLoader?

The ClassLoader is responsible for loading Java classes into memory during runtime.

39. What is Reflection in Java?

Reflection allows inspection and modification of classes, interfaces, fields, and methods during runtime.

40. What is the Singleton design pattern?

A Singleton ensures a class has only one instance and provides a global point of access to it.

41. What is the Factory design pattern?

A Factory pattern provides a way to instantiate objects without exposing the instantiation logic to the client.

J2EE/Java EE

42. What is JDBC?

JDBC (Java Database Connectivity) is an API for connecting and executing queries on a database.

43. What is a servlet in Java?

A servlet is a Java class used to extend the capabilities of servers hosting web applications. It handles HTTP requests and responses.

44. What is JSP?

JSP (JavaServer Pages) is a technology used for developing web pages that support dynamic content.

45. What is Hibernate?

Hibernate is a framework that simplifies database interaction by mapping Java objects to database tables.

46. What is Spring Framework?

Spring is a powerful framework for building Java applications, especially for enterprise-level applications. It supports dependency injection, aspect-oriented programming, and more.

47. What is REST in Java?

REST (Representational State Transfer) is an architectural style for designing networked applications. Java frameworks like JAX-RS are used to implement RESTful web services.

48. What is Maven in Java?

Maven is a build automation tool used for managing dependencies and project builds.

49. What is a JAR file?

A JAR (Java ARchive) file is a compressed file format that bundles multiple Java classes and resources into a single file for distribution.

50. What are annotations in Java?

Annotations provide metadata to Java code, giving instructions to the compiler or runtime. Examples include @Override, @Deprecated, and custom annotations.