JAVA Interview Questions 2025

Are you ready to excel in your next Java interview and land your dream job in 2025?The Java ecosystem is constantly evolving, with new features, frameworks, and architectural patterns emerging all the time. The Java ecosystem is constantly evolving with new features, frameworks, and architectural patterns. To stand out, you'll need more than just theoretical knowledge; you'll need to demonstrate a practical understanding and knowledge of modern development practices.

This comprehensive guide provides you with over 100 essential 2025 Java interview questions that have been carefully curated to cover everything from basic concepts to the hottest technologies. We dive into the heart of Java, explore the power of Spring Boot and microservices, and highlight key areas such as concurrency, cloud-native development, and the latest JDK features.

What you'll find in it:

Core Java Fundamentals: Strengthen your understanding of OOP principles, data structures, algorithms, collections, and the JVM. These timeless concepts remain the cornerstone of any Java interview. Spring Boot Savvy: Dive deeper into Spring Boot autoconfiguration, launchers, actuators, configuration files, and how Spring Boot simplifies building robust production-ready applications.

Microservices and Cloud Native: Explore the architectural shift to microservices, distributed systems, API gateways, service discovery, containerization (Docker, Kubernetes), and the challenges and patterns involved in building scalable, cloud-native Java applications.

Concurrency and Performance: Test your knowledge of multithreading, thread pooling, synchronization, virtual threads (Project Loom in Java 21), and common concurrency issues.

Modern Java Features (JDK 21+): Stay ahead of the curve with questions about the latest language enhancements such as switch pattern matching, sequence collections, record patterns, and more.

Databases and ORM: Understand the nuances of relational and NoSQL databases as well as key ORM concepts.

Testing Best Practices: Expect questions on a variety of testing methodologies, including unit testing, integration testing, and end-to-end testing.

Problem Solving and Design Patterns: Improve your analytical skills with common coding challenges and questions about widely used design patterns.

Whether you're a recent college graduate or a seasoned developer looking to make significant progress in your career, this guide is the perfect resource for you. Each question is designed not only to test your memory, but to push you to think critically, discuss tradeoffs, and explain real-world applications.

Don't just memorize the answers, but understand them. Be prepared to articulate why certain approaches are preferred, how you would implement the solution, and what considerations you would make in a production environment.

Be prepared to impress your interviewer.In 2025, your journey to Java success starts here!

Q.1. Explain the difference between == and the equals() method in Java.

\== Operator: For raw data types (int, char, boolean, etc.), == compares their actual values. For reference types (objects), == compares the memory addresses of objects. It checks to see if two references point to the exact same object in memory.

The equals() method: This is a method defined in the Object class (the root class of all Java classes). By default, the equals() method of an object class has the same behavior as == (comparing memory addresses). However, many classes (such as String, Integer, Date, and custom POJOs) often override equals() to provide meaningful content comparisons.

For example, two String objects are "equal" if they contain the same sequence of characters, even if they are different objects in memory.

Q.2. What is the purpose of the final keyword in Java?

The final keyword in Java is used to restrict modification. It can be applied to variables, methods, and classes for slightly different purposes depending on the context.

A final variable cannot be reassigned once it is initialized.

final int MAX_USERS = 100; 
MAX_USERS = 200; // ❌ Compile-time error

If it is a reference to an object, the reference cannot be changed, but the contents of the object can be modified (if mutable).

final List list = new ArrayList<>(); 
list.add("Hello"); // ✅ Allowed 

list = new ArrayList<>(); // ❌ Not allowed

• A final method cannot be overridden by subclasses.

class Parent {
    public final void greet() {
        System.out.println("Hello");
    }
}

class Child extends Parent {
    // public void greet() {} ❌ Compile-time error
}

Useful when you want to prevent altering core behavior in subclasses.

A final class cannot be extended (inherited from).

final class Utility {
    public static void log(String msg) { ... }
}

// class ExtendedUtility extends Utility {} ❌ Not allowed

Q.3. What are Java Generics?

Java generics are a language feature that allows you to write type-safe, reusable code by defining classes, interfaces, and methods with type parameters.

We use Generics for code reusability and eliminate type casting.

Here’s an example of JAVA Generics:

public class Box<T> {
    private T value;

    public void set(T value) {
        this.value = value;
    }

    public T get() {
        return value;
    }
}

Box<Integer> intBox = new Box<>();
intBox.set(123);
Integer val = intBox.get();

You can restrict generic types using extends.

Q.4. What is the difference between checked and unchecked exceptions in Java?

Checked exceptions must be handled with a throws declaration in the method signature, or with a try-catch. It must be checked at compile time. Checked exceptions usually represents a recoverable condition (e.g., file not found, network error).

public void readFile(String fileName) throws IOException {
    FileReader fr = new FileReader(fileName);
}

Common Checked Exceptions include:

• IOException

• SQLException

• FileNotFoundException

• ClassNotFoundException

Unchecked Exceptions do not need to be declared or handled explicitly. It’s checked at runtime. It represents programming bugs or logic errors (e.g., null access, divide by zero).

public void divide(int a, int b) {
    int result = a / b; // may throw ArithmeticException
}

Modern JAVA features

Q. What are Records in Java

Records are a new type of class in Java, intended to be a transparent carrier for immutable data. They provide a clean syntax for declaring classes that are primarily used to hold data.

Spring Boot and Microservices

Q. What is Spring Boot

Spring Boot is an insightful framework designed to simplify the development of production-ready, Spring-based, standalone applications. It builds on top of the Spring Framework to provide a faster, more efficient way to create microservices and web applications.

Spring Boot provides sensible default configurations to get started quickly without extensive setup. It includes embedded servers such as embedded Tomcat, Jetty, or Undertow servers that package applications into a single executable JAR file (fat JAR) that can run standalone without a separate web server.

Q. Explain the concept of Microservices architecture

Microservices architecture is an architectural style that builds applications as collections of small, loosely coupled, independently deployable services. Each service typically focuses on a business capability, has its own codebase that can be developed and deployed independently, and communicates with other services, usually through lightweight mechanisms such as REST APIs or message queues.

Microservices comes with the following benefits:

• Scalability: Individual services can be scaled independently for their specific load requirements.

• Agility/Faster Development: Smaller code bases are easier to understand, develop and test, resulting in faster development cycles.

• Technical Heterogeneity: Different services can be developed using different programming languages, frameworks, and data stores that are best suited to their specific needs.

• Resilience: The failure of one service is unlikely to bring down the entire application. Services can be isolated and fail gracefully.

• Deployment independence: Services can be deployed independently without affecting the rest of the system, enabling continuous delivery.

• Team Autonomy: Smaller cross-functional teams can own and manage specific services end-to-end.

Testing and Database

Q. What is the difference between JUnit and Mockito?

JUnit is the Java's popular unit testing framework. It provides annotations (@Test, @BeforeEach, @AfterAll), assertions (assertEquals, assertTrue), and test runners to build and execute tests. We use JUnit to write actual test cases that define what a piece of code should do and verify its behavior. It is used to test the logic of classes and methods in isolation. Mockito:

Mockito is a mocking framework for Java. It allows you to create "mock" objects of dependencies. Mock objects are simulated objects that mimic the behavior of real objects, allowing you to isolate test code without relying on actual external components.

We use Mockito when our "unit under test" has a dependency (e.g., a service is dependent on a repository, or a controller is dependent on a service.) Instead of using a real dependency (which may involve a database call or an external API call), we can make use of Mockito.

Example If you are testing a UserService that depends on a UserRepository:

You would define the test method testCreateUser_Success() using JUnit. In this test, you would use Mockito to emulate the UserRepository and return a specific User object when its save() method is called. Then, verify that save() was called.

Q. Explain the concept of ORM (Object-Relational Mapping)

ORM (Object-Relational Mapping) is a programming technique that maps objects in an object-oriented programming language (such as Java) to tables in a relational database. Essentially, it creates a “virtual object database” that can be used in a programming language.

Q. What is Unit testing? Why is it important in JAVA development?

Unit testing is a software testing methodology in which individual units or components of an application are tested in isolation to determine if they are fit for purpose. In Java, “unit” usually refers to a single class or even a single method.

Unit testing catches bugs early in the development cycle, reducing the cost and effort of fixing them later.

Q. What are the key differences between Unit, Integration, and End-to-End (E2E) testing?

Unit Testing test individual components (e.g. individual methods or classes) in isolation. Unit tests runs very fast. Unit test focus on individual unit correctness.

Integration Testing tests the interaction and communication between two or more integrated units/modules (e.g. a service talking to a database, or two microservices interacting). Integrations tests runs slower than unit testing, faster than E2E.

End-to-End (E2E) Testing tests the entire application flow from start to finish, simulating real user scenarios. E2E testing runs very slow, often requiring complex setup and maintenance. It’s main focus is to verify the complete system behavior from the user's point of view.

Q. What are Mock Objects and Stub Objects?

Both types of test stand-ins are used to isolate the "unit under test" from its dependencies.

Stub Object: A simple test stand-in that provides pre-programmed, prefabricated answers to method calls during testing. It does not contain any dynamic behavioral logic and does not validate interactions. Use it when you only need to provide specific data to the code under test (e.g., when a call to findById always returns the root of the fixed user's inventory).

Mock Objects: A more sophisticated test stand-in that allows you to specify the expected interactions. You can program mock objects to return certain values and verify that certain methods are called with certain parameters or a certain number of times. Use it when you need to verify the behavior of the code under test or its interactions with its dependencies (e.g., to verify that a service calls repository.save() after processing data). Mockito is the most popular Java framework for creating mock objects:

Q: How would you test a REST API endpoint in a Spring Boot application without starting a full server?

You can use @WebMvcTest and MockMvc.

@WebMvcTest: focuses on testing Spring MVC components (controllers, filters, etc.). It automatically configures the Spring MVC infrastructure and limits the loaded beans to those required by the web tier.

MockMvc: MockMvc, provided by Spring Test, lets you perform requests to MVC controllers without actually starting an HTTP server. You can simulate an HTTP request (GET, POST, etc.) and then assert the response status, headers, and body.

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.test.web.servlet.MockMvc;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
import static org.hamcrest.Matchers.is;

@WebMvcTest(UserController.class) // Specify the controller to test
class UserControllerTest {

    @Autowired
    private MockMvc mockMvc;

    // You would typically mock the UserService dependency here if UserController uses it
    // @MockBean private UserService userService;

    @Test
    void testGetUserById() throws Exception {
        // When(userService.getUser("123")).thenReturn(new User("123", "Test User")); // Example mock setup

        mockMvc.perform(get("/users/123")) // Perform a GET request
               .andExpect(status().isOk()) // Expect HTTP 200 OK
               .andExpect(jsonPath("$.id", is("123"))) // Assert JSON response body
               .andExpect(jsonPath("$.name", is("Test User")));
    }
}