1. What is Java?

Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle).

It follows the principle:

Write Once, Run Anywhere (WORA)

This means Java programs compiled into bytecode can run on any platform that has the Java Virtual Machine (JVM).

Java Program Example

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

Explanation:

public class HelloWorld → Defines the class.
main() → Entry point of the program.
System.out.println() → Prints output.

2. What are the main features of Java?

Important Java features include:

Platform Independent

Java programs run on any system with JVM.

Object-Oriented

Everything in Java revolves around objects and classes.

Secure

Java does not support explicit pointers, making it safer.

Multithreaded

Java supports concurrent execution using threads.

Robust

Strong memory management and exception handling.

3. What is JVM?

JVM (Java Virtual Machine) is the engine that runs Java bytecode.

Responsibilities of JVM

Load bytecode
Verify code
Execute instructions
Manage memory

JVM Architecture Components

Class Loader
Method Area
Heap
Stack
Execution Engine
Garbage Collector

4. What is JDK, JRE, and JVM?

Many interviews start with this classic question.

JVM

Executes Java bytecode.

JRE (Java Runtime Environment)

Provides libraries required to run Java programs.

JDK (Java Development Kit)

Contains tools needed to develop Java applications.

JDK = JRE + Development Tools
JRE = JVM + Libraries

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

OOP is a programming paradigm based on objects and classes.

Java follows four main OOP principles:

Encapsulation
Inheritance
Polymorphism
Abstraction

6. What is Encapsulation?

Encapsulation means binding data and methods together inside a class.

It protects data using private variables and public getter/setter methods.

Example

class Employee {    private int salary;    public void setSalary(int s){
        salary = s;
    }    public int getSalary(){
        return salary;
    }
}

Benefits:

Data hiding
Better security
Maintainability

7. What is Inheritance?

Inheritance allows one class to acquire properties of another class.

Example

class Animal {
    void sound(){
        System.out.println("Animal makes sound");
    }
}class Dog extends Animal {
    void bark(){
        System.out.println("Dog barks");
    }
}

Here:

Dog inherits from Animal.

Benefits:

Code reusability
Extensibility

8. What is Polymorphism?

Polymorphism means same method name with different behavior.

Two types:

Compile Time Polymorphism

(Method Overloading)

Runtime Polymorphism

(Method Overriding)

9. Method Overloading Example

Method overloading means multiple methods with same name but different parameters.

class Calculator {    int add(int a, int b){
        return a + b;
    }    int add(int a, int b, int c){
        return a + b + c;
    }
}

10. Method Overriding Example

Method overriding occurs when child class provides its own implementation of parent method.

class Animal {
    void sound(){
        System.out.println("Animal sound");
    }
}class Cat extends Animal {    @Override
    void sound(){
        System.out.println("Cat meows");
    }
}

11. What is Abstraction?

Abstraction hides implementation details and exposes only essential features.

Achieved using:

Abstract classes
Interfaces

12. Abstract Class Example

abstract class Shape {    abstract void draw();
}class Circle extends Shape {    void draw(){
        System.out.println("Drawing circle");
    }
}

13. Interface Example

interface Vehicle {    void start();
}class Car implements Vehicle {    public void start(){
        System.out.println("Car started");
    }
}

Difference:

Feature	Abstract Class	Interface
Methods	Can have implemented methods	Only abstract methods (before Java 8)
Multiple Inheritance	Not allowed	Allowed

14. What is a Constructor?

This is one of the common question interview will ask during java interview.
A constructor is a special method used to initialize objects.

Example

class Student {    Student(){
        System.out.println("Constructor called");
    }
}

Types:

Default Constructor
Parameterized Constructor

15. What is the `this` keyword?

this refers to current object instance.

Example

class Student {    int id;    Student(int id){
        this.id = id;
    }
}

16. What is the `super` keyword?

Used to call parent class constructor or method.

class Animal {
    Animal(){
        System.out.println("Animal created");
    }
}class Dog extends Animal {    Dog(){
        super();
        System.out.println("Dog created");
    }
}

17. What is the Java Collections Framework?

Collections framework provides data structures to store and manipulate data.

Important interfaces:

List
Set
Queue
Map

18. List Interface

List allows duplicate elements and maintains insertion order.

Popular implementations:

ArrayList
LinkedList
Vector

Example

List<String> names = new ArrayList<>();names.add("John");
names.add("Mike");System.out.println(names);

19. Difference Between ArrayList and LinkedList

Feature	ArrayList	LinkedList
Structure	Dynamic array	Doubly linked list
Insertion	Slower	Faster
Access	Faster	Slower

20. What is HashMap?

HashMap stores key-value pairs.

Features:

No duplicate keys
Allows null values
Not synchronized

Example

HashMap<Integer,String> map = new HashMap<>();map.put(1,"Java");
map.put(2,"Python");System.out.println(map);

21. Difference Between HashMap and HashTable

Feature	HashMap	HashTable
Thread Safe	No	Yes
Performance	Faster	Slower

22. What is Exception Handling?

This is one of the common question interview will ask during java interview.
Exception handling allows handling runtime errors gracefully.

Example

try{
    int x = 10/0;
}
catch(Exception e){
    System.out.println("Error occurred");
}

23. Types of Exceptions

Checked Exceptions

Checked at compile time.

Example:

IOException
SQLException

Unchecked Exceptions

Occur at runtime.

Example:

NullPointerException
ArithmeticException

24. What is finally block?

Finally block always executes regardless of exception.

try{
    int a = 5;
}
finally{
    System.out.println("Always executed");
}

Below are couple of Java interview questions on Thread and Synchronization

25. What is Multithreading?

Multithreading allows multiple threads to execute simultaneously.

Benefits:

Better CPU utilization
Faster execution

26. Creating Thread using Thread class

class MyThread extends Thread {    public void run(){
        System.out.println("Thread running");
    }
}MyThread t = new MyThread();
t.start();

27. Creating Thread using Runnable Interface

class MyThread implements Runnable {    public void run(){
        System.out.println("Thread running");
    }
}Thread t = new Thread(new MyThread());
t.start();

28. What is Synchronization?

Synchronization prevents multiple threads from accessing shared resources simultaneously.

synchronized void print(){
    System.out.println("Thread safe");
}

29. What is String Pool?

Java maintains a String Constant Pool to optimize memory.

String a = "Java";
String b = "Java";

Both references point to same memory.

30. Difference Between String, StringBuilder, StringBuffer

Class	Mutable	Thread Safe
String	No	Yes
StringBuilder	Yes	No
StringBuffer	Yes	Yes

31. What is Garbage Collection?

Garbage Collector automatically removes unused objects from memory.

Example:

Student s = new Student();
s = null;

Object becomes eligible for GC.

32. What is the `final` keyword?

Used for:

Final Variable

Constant value

Final Method

Cannot be overridden

Final Class

Cannot be inherited

Example:

final int MAX = 100;

33. What is Static Keyword?

Static members belong to class rather than object.

class Counter {    static int count = 0;    Counter(){
        count++;
    }
}

34. What is the difference between `==` and `.equals()`?

Operator	Purpose
==	Compares memory reference
equals()	Compares values

Example:

String a = "Java";
String b = "Java";System.out.println(a == b);
System.out.println(a.equals(b));

35. What is a Singleton Class?

Singleton allows only one instance of class.

class Singleton {    private static Singleton instance = new Singleton();    private Singleton(){}    public static Singleton getInstance(){
        return instance;
    }
}

36. What is an Immutable Class in Java?

An immutable class is a class whose objects cannot be modified once created.

Example of immutable class in Java:

final class Employee {    private final int id;
    private final String name;    public Employee(int id, String name){
        this.id = id;
        this.name = name;
    }    public int getId(){
        return id;
    }    public String getName(){
        return name;
    }
}

Key rules to create an immutable class:

Declare class as final
Make fields private
Make fields final
Do not provide setters
Initialize fields using constructor

Example in Java library:

String class

is immutable.

This helps with:

Thread safety
Security
Performance

37. What is the difference between Comparable and Comparator?

Both are used for sorting objects in Java.

Comparable

Used for natural sorting order.

Example:

class Student implements Comparable<Student>{    int age;    public int compareTo(Student s){
        return this.age - s.age;
    }
}

Comparator

Used when we want custom sorting logic.

Example:

class AgeComparator implements Comparator<Student>{    public int compare(Student s1, Student s2){
        return s1.age - s2.age;
    }
}

Difference:

Comparable	Comparator
Used inside class	Used outside class
Single sorting logic	Multiple sorting logic

38. What is Java Stream API?

Java Streams were introduced in Java 8 to process collections using functional programming.

Streams allow operations like:

filter
map
reduce
collect

Example:

List<Integer> numbers = Arrays.asList(1,2,3,4,5);numbers.stream()
       .filter(n -> n % 2 == 0)
       .forEach(System.out::println);

Output:

2
4

Benefits:

Cleaner code
Parallel processing
Functional style programming

39. What is Lambda Expression?

Lambda expressions allow writing anonymous functions.

Syntax:

(parameters) -> expression

Example:

List<String> names = Arrays.asList("John","Alex","Mike");names.forEach(name -> System.out.println(name));

Before Java 8:

for(String name : names){
    System.out.println(name);
}

Lambda expressions make code:

Shorter
Cleaner
More readable

40. What is Functional Interface?

A functional interface contains exactly one abstract method.

Example:

@FunctionalInterface
interface MyInterface {    void display();
}

Example implementation:

MyInterface obj = () -> System.out.println("Hello Java");obj.display();

Common Java functional interfaces:

Predicate
Function
Consumer
Supplier

41. What is Optional Class in Java?

Optional is used to avoid NullPointerException.

Example without Optional:

String name = null;
System.out.println(name.length());

This causes:

NullPointerException

Using Optional:

Optional<String> name = Optional.ofNullable(null);System.out.println(name.orElse("Default"));

Output:

Default

42. What is Serialization in Java?

This is one of the common question interview will ask during java interview.
Serialization is the process of converting an object into byte stream.

It is mainly used for:

Saving objects to file
Sending objects over network

Example:

class Student implements Serializable {    int id;
    String name;
}

To serialize:

ObjectOutputStream out =
 new ObjectOutputStream(new FileOutputStream("file.txt"));out.writeObject(student);

43. What is Transient Keyword?

transient keyword prevents a variable from being serialized.

Example:

class Student implements Serializable {    int id;
    transient String password;
}

Here:

password will not be saved during serialization

Used for:

Security
Sensitive information

44. What is Volatile Keyword?

volatile ensures that changes made by one thread are visible to other threads immediately.

Example:

class Counter {    volatile int count = 0;
}

It prevents thread caching issues.

Common interview question for multithreading.

45. What is Deadlock?

One of the most common question in java interview. Deadlock occurs when two threads wait for each other indefinitely.

Example:

Thread 1 locks Resource A
Thread 2 locks Resource BThread 1 waits for B
Thread 2 waits for A

Both threads get stuck.

Deadlock prevention techniques:

Lock ordering
Timeout locks
Avoid nested locks

46. What is Executor Framework?

Executor framework simplifies thread management.

Example:

ExecutorService executor =
 Executors.newFixedThreadPool(3);executor.submit(() -> {
    System.out.println("Task executed");
});executor.shutdown();

Benefits:

Better thread control
Thread pooling
Improved performance

47. What is ConcurrentHashMap?

ConcurrentHashMap is a thread-safe version of HashMap.

Example:

ConcurrentHashMap<Integer,String> map =
 new ConcurrentHashMap<>();map.put(1,"Java");
map.put(2,"Python");

Advantages:

Thread-safe
Faster than Hashtable

48. What is Dependency Injection?

Dependency Injection means providing dependencies from outside instead of creating them inside class.

Example without DI:

class Car {    Engine engine = new Engine();
}

With DI:

class Car {    Engine engine;    Car(Engine engine){
        this.engine = engine;
    }
}

Benefits:

Loose coupling
Better testing
Used in Spring Framework

49. What are SOLID Principles?

SOLID principles improve software design and maintainability.

S → Single Responsibility Principle
O → Open Closed Principle
L → Liskov Substitution Principle
I → Interface Segregation Principle
D → Dependency Inversion Principle

These are important for:

Senior developer interviews
Automation framework design

50. What is JVM Memory Model?

JVM memory consists of several areas:

Heap

Stores objects.

Stack

Stores method calls.

Method Area

Stores class metadata.

PC Register

Tracks current instruction.

Diagram representation:

JVM
 ├── Heap
 ├── Stack
 ├── Method Area
 └── Native Method Stack

51. What is ClassLoader in Java?

ClassLoader loads Java classes into JVM.

Types:

Bootstrap ClassLoader
Extension ClassLoader
Application ClassLoader

Example process:

.java → compiled → .class → loaded by ClassLoader → JVM execution

52. What is Reflection API?

Reflection allows inspecting classes at runtime.

Example:

Class<?> c = Class.forName("Student");Method[] methods = c.getDeclaredMethods();for(Method m : methods){
    System.out.println(m.getName());
}

Used in:

Spring
Hibernate
Test frameworks

53. What is Java Annotation?

Annotations provide metadata about code.

Example:

@Override
public String toString(){
    return "Hello";
}

Custom annotation example:

@interface MyAnnotation {    String value();
}

Used heavily in:

Spring Boot
TestNG
JUnit

54. What is Method Reference?

Method reference is a short form of lambda expression.

Example:

Lambda:

names.forEach(name -> System.out.println(name));

Method reference:

names.forEach(System.out::println);

Cleaner and more readable.

55. What are the Best Practices in Java?

This is one of the most asked and expected to be answered in java interview which will display hands on experience.

Some important best practices:

Use meaningful variable names

Bad:

int x;

Good:

int employeeSalary;

Follow Java naming conventions

ClassName → PascalCase
methodName → camelCase
CONSTANT → UPPER_CASE

Avoid NullPointerException

Use:

Optional

Write clean and modular code

Follow:

SOLID principles

Conclusion

Preparing for Java interviews requires strong knowledge of:

OOP concepts
Collections framework
Multithreading
Exception handling
Java 8 features

The top 55 Java interview questions in this guide will help you prepare for:

Automation Testing Interviews
Java Developer Interviews
SDET Roles
Test Architect Positions

Practice these concepts with coding examples to master Java and confidently crack your next interview.

External Links :
Official Java Streams Documentation

Java Streams Tutorial

Java Collections Framework Tutorial

Have a look on Testng related Blog TestNG Automation Framework – Complete Architect Guide for Enterprise CI/CD & Parallel Execution

Have a look on Cucumber related Blog For a complete BDD implementation guide, read our Cucumber Automation Framework – Complete Beginner to Advanced Guide.

Have a look on API Authentication related Blog , read our The Ultimate API Authentication guide

Have a look on Playwright related Blog , read our Playwright-Interview-Questions-Guide

Tags:

Top 55 Best Java Interview Questions and Answers (With Code Examples) – Complete Guide for 2026

1. What is Java?

Java Program Example

2. What are the main features of Java?

Platform Independent

Object-Oriented

Secure

Multithreaded

Robust

3. What is JVM?

Responsibilities of JVM

JVM Architecture Components

4. What is JDK, JRE, and JVM?

JVM

JRE (Java Runtime Environment)

JDK (Java Development Kit)

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

6. What is Encapsulation?

Example

7. What is Inheritance?

Example

8. What is Polymorphism?

Compile Time Polymorphism

Runtime Polymorphism

9. Method Overloading Example

10. Method Overriding Example

11. What is Abstraction?

12. Abstract Class Example

13. Interface Example

14. What is a Constructor?

Example

15. What is the this keyword?

Example

16. What is the super keyword?

17. What is the Java Collections Framework?

18. List Interface

Example

19. Difference Between ArrayList and LinkedList

20. What is HashMap?

Example

21. Difference Between HashMap and HashTable

22. What is Exception Handling?

Example

23. Types of Exceptions

Checked Exceptions

Unchecked Exceptions

24. What is finally block?

25. What is Multithreading?

26. Creating Thread using Thread class

27. Creating Thread using Runnable Interface

28. What is Synchronization?

29. What is String Pool?

30. Difference Between String, StringBuilder, StringBuffer

31. What is Garbage Collection?

32. What is the final keyword?

Final Variable

Final Method

Final Class

33. What is Static Keyword?

34. What is the difference between == and .equals()?

35. What is a Singleton Class?

36. What is an Immutable Class in Java?

37. What is the difference between Comparable and Comparator?

Comparable

Comparator

38. What is Java Stream API?

39. What is Lambda Expression?

40. What is Functional Interface?

41. What is Optional Class in Java?

42. What is Serialization in Java?

43. What is Transient Keyword?

44. What is Volatile Keyword?

45. What is Deadlock?

46. What is Executor Framework?

47. What is ConcurrentHashMap?

48. What is Dependency Injection?

49. What are SOLID Principles?

50. What is JVM Memory Model?

Heap

Stack

15. What is the `this` keyword?

16. What is the `super` keyword?

32. What is the `final` keyword?

34. What is the difference between `==` and `.equals()`?