Top 50 Core Java Interview Questions and Answers

In this article, we have listed 50 Core Java interview questions and answers to help you review key concepts and improve your confidence. The questions cover important areas like object-oriented programming, exception handling, collections, multithreading, and more.

This guide is designed to support your interview preparation by giving you clear and practical answers to common questions. Let’s get started.

1. What is Java?

Java is a high-level, object-oriented programming language that is designed to be platform-independent, meaning you can write code once and run it anywhere with the Java Virtual Machine (JVM).

It’s easy to learn for beginners and widely used for building web, mobile, and enterprise applications. Java is known for its robustness, security features, and versatility. It supports modular and reusable code through its object-oriented principles, and it comes with a rich set of libraries and frameworks to streamline development.

What is Java?
This post provides everything you'll need to know about getting started with the Java programming language.www.javaguides.net

You may also get a question like What are the key features of Java?

Refer to this article: Main Features of Java ( Explained with Examples )

2. What is the Java Virtual Machine (JVM)?

Answer:
The JVM is a virtual machine that executes Java bytecode. It converts bytecode into machine-specific code and handles tasks like memory management, garbage collection, and security. The JVM is what makes Java platform-independent.

Why is the JVM Important?

The JVM makes Java a platform-independent language. It allows developers to write code once and run it anywhere. The JVM also handles memory management, security, and performance optimization, making Java applications reliable and efficient.

What is JVM?
Blog about guides/tutorials on Java, Java EE, Spring, Spring Boot, Microservices, Hibernate, JPA, Interview, Quiz…www.javaguides.net

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

JDK (Java Development Kit)

🧑‍💻 JDK is the full toolbox for Java developers.

What it includes:

• JRE (which includes JVM)
• Java compiler (javac)
• Debugger (jdb)
• Other dev tools (like jar, javadoc, etc.)

✅ Use JDK when you want to WRITE, COMPILE, and RUN Java programs.

JRE (Java Runtime Environment)

⚙️ JRE is a software package that contains everything needed to run a Java program.

What it includes:

• JVM (Java Virtual Machine)
• Libraries and class files needed at runtime (like rt.jar)
• Other supporting files

What it doesn’t include:

• Compiler (javac)
• Development tools

JVM (Java Virtual Machine)

🧠 JVM is the brain behind Java.

What it does:

• It runs Java bytecode (compiled .class files).
• Converts bytecode into machine-specific instructions.
• Provides features like: Garbage collection, Memory management, Security and runtime optimization.

What is JDK, JRE and JVM in Java - Explained with Diagrams
In this post, we will discuss an important definition of JVM, JRE, and JDK in the Java programming language. We also…www.javaguides.net

4. What is a class and an object in Java?

Answer:

• A class is a blueprint or template for creating objects. It defines properties and behaviors.
• An object is an instance of a class. It holds real values for the properties defined in the class.

Example:

class Car {
    String color;
    void drive() {
        System.out.println("Driving...");
    }
}

Car myCar = new Car(); // object
myCar.color = "Red";
myCar.drive();

Object Oriented Programming in Java with Examples
This page contains a list of tutorials, and examples on important OOPS concepts and OOPS principles.www.javaguides.net

5. What is the main() method in Java?

Answer:
The main() method is the entry point of any Java application. It has a specific signature that the JVM looks for when starting a program.

public static void main(String[] args) {
    // code to run
}

• public – accessible from anywhere
• static – no need to create an object to call it
• void – does not return any value
• String[] args – receives command-line arguments

Java main() Method Interview Questions with Answers
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6. What is the difference between primitive and non-primitive data types?

7. What is a constructor in Java?

Answer:
A constructor is a special method used to initialize objects. It has the same name as the class and no return type. It is called automatically when an object is created.

public class Student {
    String name;
    Student(String n) {
        name = n;
    }
}

Java also provides a default constructor if none is defined.

8. What is method overloading?

Answer:
Method overloading means defining multiple methods with the same name but different parameters (number, type, or order).

void print(int a) { }
void print(String s) { }
void print(int a, String s) { }

It increases the readability and flexibility of the program.

Method Overloading in Java with Examples
In Java, it is possible to define two or more methods within the same class that share the same name, as long as their…www.javaguides.net

9. What is the use of the this keyword in Java?

The this keyword refers to the current object. It is used when instance variables are shadowed by method or constructor parameters.

class Employee {
    String name;
    Employee(String name) {
        this.name = name; // refers to instance variable
    }
}

10. What is garbage collection in Java?

Garbage collection is a process in which the JVM automatically removes unused or unreachable objects from memory to free up space. It helps manage memory efficiently and prevents memory leaks.

You can suggest garbage collection using:

System.gc();

But the JVM decides when to actually run it.

11. What are the four main principles of Object-Oriented Programming (OOP) in Java?

The four key principles of OOP are:

• Encapsulation: Hiding internal details and exposing only essential features using classes and access modifiers.
• Abstraction: Hiding complex implementation and showing only necessary details using abstract classes or interfaces.
• Inheritance: Reusing code by deriving a new class from an existing one.
• Polymorphism: Ability to take many forms — method overloading (compile-time) and method overriding (runtime).

OOPs Concepts in Java with Examples
Object-Oriented Programming (OOP) is a programming paradigm based on the concept of "objects", contains data and…www.javaguides.net

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

Answer:

• == checks reference equality — whether two references point to the same object.
• .equals() checks value equality — whether two objects have the same content (when overridden properly).

Example:

String a = new String("Java");
String b = new String("Java");

System.out.println(a == b);      // false
System.out.println(a.equals(b)); // true

13. What are access modifiers in Java?

Answer:
Access modifiers control the visibility of classes, methods, and variables. Java provides four main access levels:

Java Access Modifiers
In this article, we will discuss Java access modifiers - public, private, protected & default, which are used to…www.javaguides.net

14. What is the difference between static variables and instance variables?

Answer:

• Static variables belong to the class, not to any object. Only one copy exists.
• Instance variables belong to each object. Every object has its own copy.

Example:

class Counter {
    static int count = 0;
    int id;

    Counter() {
        count++;
        id = count;
    }
}

static variable vs instance variable in Java
A static variable is associated with the class itself. In contrast, an instance variable is associated with a specific…www.javaguides.net

15. What is method overriding?

Answer:
Method overriding is when a subclass provides a new implementation for a method that is already defined in its superclass.

• The method must have the same name, return type, and parameters.
• The method in the subclass should not have less access than the superclass method.
• Use the @Override annotation for clarity.

Method Overloading vs Method Overriding in Java
In this article, we will explore the differences between Method Overloading and Method Overriding in Java, understand…www.javaguides.net

16. Can we override a static method in Java?

Answer:
No, we cannot truly override a static method. Static methods belong to the class, not objects, so method hiding occurs instead of overriding.

Example:

class A {
    static void show() {
        System.out.println("Class A");
    }
}

class B extends A {
    static void show() {
        System.out.println("Class B");
    }
}

Calling B.show() calls B’s version, but it’s not true polymorphism.

Can You Override Private or Static Methods in Java?
Understand whether private or static methods can be overridden in Java. Learn the difference between method hiding and…medium.com

17. What is the purpose of the final keyword in Java?

Answer:
final is used to declare:

• Final variable: Value cannot be changed after assignment.
• Final method: Cannot be overridden.
• Final class: Cannot be extended (e.g., String class).

final Java Keyword with Examples
The final keyword in Java is used to restrict the user. The final keyword can be used with variable, method, and class.www.javaguides.net

18. What is a constructor overloading?

Answer:
Constructor overloading means defining multiple constructors in a class with different parameter lists. It allows objects to be initialized in different ways.

Example:

class Book {
    Book() { }
    Book(String title) { }
    Book(String title, String author) { }
}

19. What is super keyword in Java?

Answer:
The super keyword is used to refer to the immediate parent class. It can be used to:

• Call the parent class constructor: super()
• Access parent class methods or variables: super.methodName()

20. What is the difference between compile-time and run-time polymorphism?

Answer:

21. What is the difference between an abstract class and an interface in Java?

Answer:

22. What is the purpose of the interface keyword in Java?

Answer:
An interface defines a contract that a class must follow. It contains method declarations without implementations (except for default/static methods). A class implements an interface using the implements keyword.

Example:

interface Vehicle {
    void start();
}

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

23. What is exception handling in Java?

Answer:
Exception handling is a mechanism to handle runtime errors and prevent the program from crashing. Java provides:

• try block to write risky code
• catch block to handle exceptions
• finally block to execute code regardless of exceptions
• throw to manually throw an exception
• throws to declare exceptions

Example:

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
}

Java Exception Handling Tutorial
This is a complete tutorial to exception handling in Java. The source code examples of this guide are well tested with…www.javaguides.net

24. What is the difference between throw and throws?

Answer:

25. What is the difference between checked and unchecked exceptions?

Answer:

26. What is the use of finally block?

Answer:
The finally block is used to write clean-up code (like closing a file, releasing a database connection). It always executes whether an exception is thrown or not.

try {
    // code
} catch (Exception e) {
    // handle
} finally {
    System.out.println("Always runs");
}

27. What are wrapper classes in Java?

In Java, wrapper classes are used to convert primitive types into objects.

Java is an object-oriented language, but primitive types like int, double, char are not objects.

So Java provides wrapper classes for each primitive data type to help you use them like objects.

Example:

int x = 10;
Integer obj = Integer.valueOf(x); // Boxing
int y = obj.intValue();           // Unboxing

Wrapper Classes in Java: A Simple Guide
Learn what wrapper classes are in Java, why they’re important, and how to use them effectively. Includes code examples…rameshfadatare.medium.com

28. What is autoboxing and unboxing in Java?

Answer:

• Autoboxing: Automatic conversion of a primitive to a wrapper class object.
• Unboxing: Automatic conversion of a wrapper class object back to a primitive.

Example:

Integer a = 10; // Autoboxing
int b = a;      // Unboxing

Introduced in Java 5 to simplify code when using collections and generics.

Autoboxing and Unboxing in Java: A Simple Guide for Beginners
Learn what autoboxing and unboxing are in Java, why they matter, and how they simplify working with primitive types and…rameshfadatare.medium.com

29. What is the difference between String, StringBuilder, and StringBuffer?

Answer:

String vs StringBuilder vs StringBuffer in Java
String: Use when the text won't change, and thread safety is required. StringBuilder: Use in a single-threaded…www.javaguides.net

30. What is the difference between == and .equals() in the case of String?

Answer:

• == compares references (memory locations).
• .equals() compares values (actual characters in the string).

Example:

String a = new String("Java");
String b = new String("Java");

System.out.println(a == b);      // false
System.out.println(a.equals(b)); // true

Always use .equals() for value comparison in strings.

31. What is the Java Collections Framework?

Answer:
The Java Collections Framework is a set of classes and interfaces that provide data structures and algorithms to store, retrieve, and manipulate data efficiently. It includes:

• Interfaces like List, Set, Queue, Map
• Implementations like ArrayList, HashSet, LinkedList, HashMap, etc.

It supports both generic and non-generic types and provides operations like sorting, searching, and iteration.

Java Collections Tutorial
This tutorial is a one-stop shop for all the Java collections interfaces, implementation classes, interface questions…www.javaguides.net

32. What is the difference between ArrayList and LinkedList?

Answer:

Difference between ArrayList and LinkedList in Java
In this post, we will discuss the difference between ArrayList and LinkedList in Java.www.javaguides.net

33. What is the difference between HashSet and TreeSet?

Answer:

HashSet vs TreeSet: Difference Between HashSet and TreeSet in Java
In this article, we will discuss the difference between HashSet and TreeSet in Java with examples.www.javaguides.net

34. What is the difference between Iterator and ListIterator?

Answer:

35. What are Generics in Java?

Answer:
Generics allow you to define classes, interfaces, and methods with type parameters. They enable compile-time type checking and eliminate the need for type casting.

Example:

List<String> names = new ArrayList<>();
names.add("Ravi");
String first = names.get(0); // No casting needed

Generics improve code safety and readability.

Java Generics Tutorial
Generics were added in Java 5 to provide compile-time type checking and removing the risk of ClassCastException that…www.javaguides.net

36. What is the difference between Comparable and Comparator?

Answer:

37. What is a thread in Java?

Answer:
A thread is a lightweight unit of execution in a program. Java supports multithreading, which allows multiple threads to run concurrently, improving performance in CPU-bound or I/O-bound tasks.

You can create threads in two ways:

1. Extending Thread class
2. Implementing Runnable interface

Java Multithreading Tutorial
Multithreading in Java is a very important topic. In this tutorial, we will learn low-level APIs that have been part of…www.javaguides.net

38. What is the difference between start() and run() methods in threads?

Answer:

Difference Between start() and run() in Java
🔒 This is a Medium member-only article. If you’re not a Medium member, you can read the full article for free on my…rameshfadatare.medium.com

39. What is synchronization in Java?

Answer:
Synchronization is used to control access to shared resources in a multithreaded environment. It prevents race conditions by allowing only one thread to access a block of code or object at a time.

You can use:

• synchronized keyword on methods or code blocks
• synchronized blocks with object references

Example:

synchronized void increment() {
    count++;
}

40. What is a deadlock in Java?

Answer:
A deadlock is a situation where two or more threads are blocked forever, waiting for each other to release locks.

Example Scenario:

• Thread A holds Lock 1 and waits for Lock 2
• Thread B holds Lock 2 and waits for Lock 1

To avoid deadlocks:

• Always acquire locks in the same order
• Use timeout with tryLock() from java.util.concurrent.locks

41. What is an immutable class in Java?

Answer:
An immutable class is one whose objects cannot be modified once they are created. All the fields of an immutable object are final and set only once through the constructor.

Example:

public final class Student {
    private final String name;

    public Student(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

The String class in Java is a common example of an immutable class.

🔒 How to Make an Immutable Class in Java (Step-by-Step Guide)
Learn how to create an immutable class in Java with real-world examples. Understand why immutability matters, and…rameshfadatare.medium.com

42. What is the Java Memory Model?

Answer:
The Java memory model divides memory into different regions:

• Heap: Stores objects and class instances
• Stack: Stores method call frames and local variables
• Method Area (MetaSpace): Stores class metadata
• Program Counter Register: Holds the address of the current executing instruction
• Native Method Stack: Used for native method calls

Understanding this model helps in optimizing memory usage and identifying memory leaks or performance bottlenecks.

43. Why is String immutable in Java?

Strings in Java are immutable by design. This design decision is about more than just simplicity.

Key reasons:

• Security: Strings are often used in sensitive areas like file paths and network connections. If a string could be changed, it could lead to vulnerabilities.
• Performance: Since immutable strings can be cached and reused, the JVM can optimize performance using a string pool.
• Thread safety: Immutable objects are naturally thread-safe, which avoids the need for synchronization.

Example:

String s = "Hello";
s.concat(" World");
System.out.println(s); // prints "Hello"

s = s.concat(" World");
System.out.println(s); // prints "Hello World"

44. What is class loading in Java?

Answer:
Java uses Class Loaders to load .class files into memory when needed.

The three main class loaders are:

• Bootstrap ClassLoader — loads core Java classes from the JDK
• Extension ClassLoader — loads JDK extension libraries
• Application ClassLoader — loads classes from the classpath

Java uses lazy loading, meaning classes are only loaded when they are first accessed.

45. What is the difference between compile-time and runtime errors?

Answer:

46. What are Lambda Expressions in Java?

Answer:
A lambda expression is simply a function without a name. It can even be used as a parameter in a function. Lambda Expressions facilitate functional programming and simplify development greatly.

The main use of Lambda expression is to provide an implementation for functional interfaces.

Syntax:

(parameter) -> expression

Example:

Java 8 Lambda Expressions
In this post, we will discuss the most important feature of Java 8 that is Lambda Expressions. We will learn Lambda…www.javaguides.net

47. What is a Functional Interface?

Answer:
A functional interface is an interface that has exactly one abstract method. It can have default or static methods.

Java 8 introduced the @FunctionalInterface annotation to ensure this rule.

Example:

@FunctionalInterface
interface MyFunc {
    void execute();
}

Common built-in functional interfaces: Runnable, Callable, Predicate, Function, Supplier, and Consumer.

Java 8 Functional Interfaces
In this post, we will learn the Java 8 the functional interface with examples. Key points about the functional…www.javaguides.net

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48. What is the Stream API in Java?

Answer:
Stream API is used to process collections (like List or Set) in a functional style. It supports operations like filter, map, reduce, collect, and more.

Example:

List<String> names = List.of("Ram", "Shyam", "Ravi");
names.stream()
     .filter(name -> name.startsWith("R"))
     .forEach(System.out::println);

Streams help write clean, readable, and concise code for data processing.

Java 8 Stream API Tutorial
This complete an in-depth tutorial, we will go through the practical usage of Java 8 Streams. Source code examples and…www.javaguides.net

49. What is Optional in Java?

Answer:
Optional is a container object used to avoid null checks and prevent NullPointerException.

Example:

Optional<String> name = Optional.ofNullable(getName());
name.ifPresent(System.out::println);

It encourages writing null-safe code in a more readable way.

Java 8 Optional Class with Examples
Java introduced a new class Optional in JDK 8. It is a public final class and used to deal with NullPointerException in…www.javaguides.net

50. What are the design principles in OOP?

Some key OOP design principles include:

1. Single Responsibility Principle (SRP)
2. Open/Closed Principle (OCP)
3. Liskov Substitution Principle (LSP)
4. Interface Segregation Principle (ISP)
5. Dependency Inversion Principle (DIP)
6. Encapsulate What Varies
7. DRY (Don’t Repeat Yourself)
8. YAGNI (You Aren’t Gonna Need It)
9. KISS (Keep It Simple, Stupid)
10. Composition over Inheritance
11. Dependency Injection

These principles guide developers toward creating clean, maintainable, and scalable systems.