Top Java Interview Questions and Answers TO GET YOU HIRED in 2026

1. What is Java?

Answer:
Java is a high-level, object-oriented, platform-independent programming language developed by Sun Microsystems (now Oracle). Java follows the principle "Write Once, Run Anywhere (WORA)" because Java programs run on the JVM.

2. What are the main features of Java?

Answer:

Platform Independent
Object-Oriented
Robust
Secure
Multithreaded
High Performance (with JIT Compiler)
Distributed
Portable
Automatic Garbage Collection

3. What is JVM?

Answer:
JVM (Java Virtual Machine) is responsible for executing Java bytecode. It converts bytecode into machine code and provides platform independence.

4. Difference between JDK, JRE, and JVM?

JDK	JRE	JVM
Development Kit	Runtime Environment	Virtual Machine
Includes JRE + Tools	Includes JVM + Libraries	Executes Bytecode
Used by Developers	Used by Users	Runs Java Programs

5. What is Object-Oriented Programming (OOP)?

Answer:
OOP is a programming paradigm based on objects and classes.

The four pillars are:

Encapsulation
Inheritance
Polymorphism
Abstraction

6. What is Encapsulation?

Answer:
Encapsulation means wrapping data and methods together into a single unit (class) while restricting direct access using private variables.

Example:

class Student{
    private int age;

    public void setAge(int age){
        this.age = age;
    }

    public int getAge(){
        return age;
    }
}

7. What is Inheritance?

Answer:
Inheritance allows one class to acquire properties and methods of another class.

Example:

class Animal{
    void eat(){}
}

class Dog extends Animal{
    void bark(){}
}

8. What is Polymorphism?

Types

Compile-time (Method Overloading)
Runtime (Method Overriding)

9. Method Overloading vs Overriding

Overloading	Overriding
Same method name	Same method name
Different parameters	Same parameters
Compile-time	Runtime
Same class	Parent & Child

10. What is Abstraction?

Answer:
Abstraction hides implementation details while exposing only essential functionality.

Achieved using:

Abstract Classes
Interfaces

11. Abstract Class vs Interface

Abstract Class	Interface
Can have constructors	Cannot have constructors
Supports partial abstraction	Supports full abstraction (plus default/static methods)
Uses `extends`	Uses `implements`

12. What is a Constructor?

Answer:
A constructor initializes objects.

Types:

Default Constructor
Parameterized Constructor

13. What is `this` keyword?

Answer:
this refers to the current object.

Uses:

Refer current instance variable
Call another constructor
Return current object

14. What is `super` keyword?

Answer:
super refers to the parent class.

Uses:

Access parent variables
Call parent methods
Invoke parent constructor

15. What is String Pool?

Answer:
Java stores string literals in a special memory area called the String Constant Pool to improve memory efficiency.

Example:

String s1="Java";
String s2="Java";

Both refer to the same object.

16. Difference between `==` and `.equals()`

`==`	`.equals()`
Compares references	Compares content
Memory address	Actual value

17. String vs StringBuilder vs StringBuffer

String	StringBuilder	StringBuffer
Immutable	Mutable	Mutable
Slow	Fast	Thread-safe

18. What is Exception Handling?

Answer:
Java handles runtime errors using:

try
catch
finally
throw
throws

19. Checked vs Unchecked Exceptions

Checked

IOException
SQLException

Unchecked

NullPointerException
ArithmeticException
ArrayIndexOutOfBoundsException

20. What is Garbage Collection?

Answer:
Garbage Collector automatically removes unused objects from memory.

21. What is Multithreading?

Answer:
Multithreading allows multiple threads to execute simultaneously.

Advantages:

Faster execution
Better CPU utilization
Improved responsiveness

22. Difference between Process and Thread

Process	Thread
Independent	Part of process
Heavyweight	Lightweight
Separate memory	Shared memory

23. What is Synchronization?

Answer:
Synchronization prevents multiple threads from accessing shared resources simultaneously.

24. What is the Collection Framework?

Answer:
Java Collection Framework provides classes and interfaces to store and manipulate data.

Major Interfaces:

List
Set
Queue
Map

25. ArrayList vs LinkedList

ArrayList	LinkedList
Dynamic Array	Doubly Linked List
Faster search	Faster insertion/deletion

26. HashMap vs Hashtable

HashMap	Hashtable
Not synchronized	Synchronized
Allows one null key	No null key

27. HashMap vs TreeMap

HashMap	TreeMap
Unordered	Sorted
Faster	Slower

28. HashSet vs TreeSet

HashSet	TreeSet
Unordered	Sorted
Faster	Slower

29. What is Java 8?

Features

Lambda Expressions
Stream API
Functional Interface
Optional Class
Default Methods
Method References

30. What is Lambda Expression?

Example:

(a,b) -> a+b;

Used for writing concise functional code.

31. What is Stream API?

Example:

list.stream()
    .filter(x -> x > 10)
    .forEach(System.out::println);

32. What is Optional?

Answer:
Optional helps avoid NullPointerException.

Example:

Optional<String> name = Optional.of("Java");

33. What is Functional Interface?

Answer:
An interface containing exactly one abstract method.

Example:

@FunctionalInterface
interface Demo{
    void show();
}

34. What are Java 17 features?

Sealed Classes
Pattern Matching (enhanced in later releases)
Improved Switch Expressions
Records
Better Garbage Collectors
Long-Term Support (LTS)

35. What are Java 21 features?

Virtual Threads (Project Loom)
Sequenced Collections
String Templates (Preview)
Record Patterns
Pattern Matching Improvements
Scoped Values

36. What are Virtual Threads?

Answer:
Virtual Threads are lightweight threads introduced with Project Loom, enabling applications to handle a very large number of concurrent tasks with lower memory usage than traditional platform threads.

37. What is Spring Boot?

Answer:
Spring Boot is a Java framework for rapidly building production-ready applications with embedded servers, auto-configuration, and simplified dependency management.

38. What is Dependency Injection?

Answer:
Dependency Injection (DI) is a design pattern where dependencies are provided by a framework or container rather than being created inside the class, improving modularity and testability.

39. What is Hibernate?

Answer:
Hibernate is an ORM (Object-Relational Mapping) framework that maps Java objects to database tables and simplifies database operations.

40. What are REST APIs?

Answer:
REST APIs are web services that use HTTP methods (GET, POST, PUT, DELETE) to communicate between clients and servers, commonly exchanging JSON data.