Top 20 Java Interview Questions and Answers for 2025

This blog presents the Top 20 Java Interview Questions, curated to help you crack your next Java interview with confidence. Java remains one of the most sought-after skills in software development interviews. Whether you’re a fresher or an experienced developer, preparing for Java interviews requires a solid grasp of core concepts and real-world applications.

Java Interview Questions

1. What are the main features of Java?

Java is a popular, high-level programming language known for its simplicity, platform independence, and strong community support. Key features include:

Object-Oriented: Everything in Java is treated as an object.
Platform Independent: Write Once, Run Anywhere (WORA) via the Java Virtual Machine (JVM).
Robust and Secure: Exception handling, garbage collection, and security features.
Multithreaded: Supports concurrent execution through threads.
High Performance: Just-In-Time (JIT) compiler improves performance.

These features make Java ideal for enterprise-level applications, Android development, and web services.

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

JDK (Java Development Kit): Contains development tools like javac, debugger, and JRE. Used for developing Java applications.
JRE (Java Runtime Environment): Provides libraries and JVM to run Java applications. It does not include development tools.
JVM (Java Virtual Machine): Abstract machine that executes Java bytecode. Converts bytecode to machine-specific code.

Together, these components support the full cycle of Java application development and execution.

3. Explain the concept of OOPs in Java.

Java is built on the principles of Object-Oriented Programming (OOP), which include:

Encapsulation: Binding data and code into a single unit.
Inheritance: One class can inherit fields and methods from another.
Polymorphism: Methods behave differently based on the object that calls them.
Abstraction: Hiding implementation details from the user.

These principles promote code reuse, modularity, and scalability.

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

== operator compares object references (memory addresses).
.equals() method compares the actual content of objects.

For example:

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

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

Using .equals() is preferable when comparing the contents of strings or objects.

5. What is a constructor in Java?

A constructor is a special method used to initialize objects. Characteristics include:

Has the same name as the class.
Does not have a return type.
Called automatically when an object is created.

Types:

Default constructor: No parameters.
Parameterized constructor: Accepts arguments to initialize variables.

Constructors promote flexible and structured object initialization.

6. Explain method overloading and overriding.

Method Overloading: Same method name with different parameter lists in the same class.
Method Overriding: Subclass provides a specific implementation of a method already defined in its parent class.

These are key to achieving polymorphism in Java.

7. What is the difference between abstract class and interface?

Feature	Abstract Class	Interface
Methods	Can have abstract and concrete	Only abstract (Java 7)
Inheritance	Single inheritance	Multiple inheritance allowed
Constructor	Can have constructors	Cannot have constructors

From Java 8 onwards, interfaces can also have default and static methods.

8. What are access modifiers in Java?

Access modifiers define the visibility of classes, methods, and variables:

public: Accessible from everywhere.
protected: Accessible within the package and subclasses.
default: Accessible only within the same package.
private: Accessible only within the same class.

Using access modifiers helps in enforcing encapsulation and data hiding.

9. What is exception handling in Java?

Java provides a robust mechanism to handle runtime errors using:

try-catch block: Wrap risky code in try, handle exceptions in catch.
finally block: Executes regardless of exception occurrence.
throw and throws: Used to explicitly throw exceptions.

Proper exception handling ensures smoother execution and error logging.

10. What is multithreading in Java?

Multithreading allows concurrent execution of two or more threads for maximum CPU utilization.

Java provides the Thread class and Runnable interface.
Benefits include improved performance and responsiveness.
Threads can be controlled using methods like start(), sleep(), and join().

Used extensively in server-side and real-time applications.

11. What is the difference between Array and ArrayList?

Array: Fixed size, can hold both primitives and objects.
ArrayList: Dynamic size, part of the Collections Framework, only holds objects.

ArrayList is more flexible but slightly slower due to extra overhead.

12. What is garbage collection in Java?

Java handles memory management through garbage collection, which:

Automatically deletes unused objects.
Uses algorithms like Mark and Sweep, Generational GC.
Can be triggered with System.gc() (not recommended for regular use).

Reduces memory leaks and boosts application performance.

13. What are the different types of memory areas allocated by JVM?

JVM divides memory into:

Heap: Stores objects and class instances.
Stack: Stores method calls and local variables.
Method Area: Stores class-level data like static variables.
PC Register: Stores the address of the currently executing instruction.
Native Method Stack: For native methods in Java.

Understanding memory areas helps in performance tuning.

14. What is the final keyword in Java?

final can be used with:

Variables: Makes them constants.
Methods: Prevents method overriding.
Classes: Prevents class inheritance.

It ensures immutability and integrity of data.

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

String: Immutable, thread-safe.
StringBuilder: Mutable, not thread-safe, faster in single-threaded environments.
StringBuffer: Mutable, thread-safe, slightly slower.

Use based on application context and concurrency requirements.

16. What are lambda expressions in Java?

Introduced in Java 8, lambda expressions enable functional programming:

(param) -> expression

Used to implement functional interfaces.
Reduces boilerplate code in collections, streams, and event listeners.

Enhances readability and conciseness. Learn More on Lambda Expressions In Java 8.

17. What is the Stream API in Java 8?

Stream API processes data in a declarative and functional way:

Methods: filter(), map(), reduce(), collect().
Supports lazy evaluation.
Enhances performance with parallel streams.

Ideal for manipulating large datasets and performing bulk operations. Learn More on Stream API in Java 8

18. What is the difference between HashMap and Hashtable?

Feature	HashMap	Hashtable
Thread-safe	No	Yes
Performance	Faster	Slower
Nulls	Allows one null key	No null keys allowed

Use HashMap in non-threaded environments and Hashtable for legacy thread-safe use.

19. What is serialization in Java?

Serialization converts an object into a byte stream for:

Saving to a file
Transmitting over a network

Implemented using Serializable interface. Deserialization reconstructs the object.

Important for distributed systems and caching mechanisms.

20. What are functional interfaces in Java?

A functional interface has only one abstract method. Read More on Functional Interface and other Java 8 Features.

Annotated with @FunctionalInterface.
Used in lambda expressions and method references.

Examples include Runnable, Callable, and Comparator.

They are central to functional programming in Java 8 and beyond.

Conclusion

Mastering these Top 20 Java Interview Questions gives you an edge in technical interviews. Each answer covers key Java concepts with practical relevance. Use this guide to revise, prepare, and impress interviewers with your deep understanding of Java.

Whether you’re applying for a fresher role or a senior developer position, the Top 20 Java Interview Questions provide a solid foundation for your success. Download Java.