Ace Your Java Interview: Top 50 Java Interview Questions & Answers (2025)

 Introduction

Java continues to be a top programming language in 2025, widely used in enterprise applications, microservices, cloud computing, and AI-based systems. If you're preparing for a Java developer interview, you must be well-versed in Core Java, Java 8-21 features, Spring Boot, Microservices, and Cloud technologies.

This guide covers the top 50 Java interview questions for freshers and experienced professionals, ensuring you are ready for your next interview!

Table of Contents

1. Core Java Interview Questions
2. Java 8, Java 11, and Java 21 Features
3. OOP and Design Patterns
4. Multithreading and Concurrency
5. JVM, Garbage Collection & Memory Management
6. Collections Framework
7. Spring Boot & Microservices
8. Cloud Technologies (AWS, Azure, GCP)
9. Real-World Java Interview Scenarios

1. Core Java Interview Questions

Q1: What are the key features of Java?

Answer:

• Platform Independence – Java runs on JVM, making it OS-independent.
• Object-Oriented – Java follows OOP principles like Encapsulation, Inheritance, and Polymorphism.
• Multithreading – Supports parallel execution for high-performance applications.
• Automatic Garbage Collection – Handles memory management.
• Security – Provides robust security features like bytecode verification and sandboxing.

Q2: Difference between JDK, JRE, and JVM?

Q3: What are Wrapper Classes in Java?

Wrapper classes convert primitive data types into objects (e.g., int → Integer).

Example:

java

int num = 10;
Integer wrappedNum = Integer.valueOf(num); // Boxing
int unwrappedNum = wrappedNum.intValue(); // Unboxing

2. Java 8, Java 11, and Java 21 Features

Q4: What are the key features introduced in Java 8?

• Lambda Expressions – Enables functional programming.
• Stream API – Simplifies bulk operations on collections.
• Functional Interfaces – Defines interfaces with a single abstract method (@FunctionalInterface).
• Default & Static Methods in Interfaces – Allows adding methods without breaking existing implementations.

Q5: How does Java 21 improve concurrency?

Java 21 introduces Virtual Threads, which enhance thread management for high-performance applications.

Example:

java

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> System.out.println("Running in Virtual Thread"));
}

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3. OOP and Design Patterns

Q6: What is the SOLID principle in Java?

• S – Single Responsibility Principle (SRP)
• O – Open/Closed Principle (OCP)
• L – Liskov Substitution Principle (LSP)
• I – Interface Segregation Principle (ISP)
• D – Dependency Inversion Principle (DIP)

Q7: Explain the Singleton Design Pattern with Example

A Singleton ensures only one instance of a class exists.

java

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

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4. Multithreading and Concurrency

Q8: Difference between synchronized and Lock in Java?

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5. JVM, Garbage Collection & Memory Management

Q9: What are the different types of Garbage Collectors in Java?

• Serial GC – Suitable for small applications.
• Parallel GC – Uses multiple threads for minor GC.
• G1 GC – Balances throughput and latency.
• ZGC – Optimized for low-latency applications.

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6. Collections Framework

Q10: Difference between ArrayList and LinkedList?

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7. Spring Boot & Microservices

Q11: What is Dependency Injection in Spring Boot?

Dependency Injection (DI) allows loose coupling between components.

Example:

java

@Service
public class UserService {
    private final UserRepository userRepository;
    @Autowired
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
}

Q12: What is Circuit Breaker in Microservices?

A Circuit Breaker (Hystrix, Resilience4j) prevents cascading failures in distributed systems.

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8. Cloud Technologies (AWS, Azure, GCP)

Q13: What are the key AWS services used in Java applications?

• Amazon S3 – Storage service.
• Amazon RDS – Relational database.
• AWS Lambda – Serverless function execution.

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9. Real-World Java Interview Scenarios

Q14: How would you optimize a Java application running slow in production?

• Use JVM Profiling (JProfiler, YourKit).
• Enable Garbage Collection Logs.
• Optimize SQL Queries & Caching.
• Use Asynchronous Processing (CompletableFuture, Reactive Programming).

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Conclusion

These 50 Java interview questions will help you confidently prepare for Java developer interviews in 2025. Understanding Java fundamentals, latest features, multithreading, Spring Boot, Microservices, and Cloud technologies will give you an edge.