Top 18 Java Backend Interview Questions

Top 18 Java Backend Interview Questions & Best Answers for 2025

Are you preparing for a Java Backend Developer interview, especially with experience in Spring Boot, Microservices, and Kafka? This post covers the most frequently asked interview questions — with detailed answers and relevant code samples — to help you ace your next technical interview!

1. Briefly Introduce Yourself and Your Tech Stack

Start with your experience, roles, and technologies:

I am a Java Backend Developer with X years of experience designing scalable services using Java, Spring Boot, Microservices, Hibernate, SQL, Kafka, Docker, and Maven.

Using key terms like "Java", "Spring Boot", "Kafka", and "Microservices" highlights your competency and matches common recruiter searches.

2. Describe Your Most Challenging Task

Discuss meaningful challenges. For example, building a fault-tolerant microservices architecture, or solving a performance bottleneck in a production system. Highlight technical skills, teamwork, and tools used.

3. Why Use Microservices?

• Scalability: Scale individual components independently.
• Flexibility: Different teams can use varied stacks.
• Easy Deployment: Deploy services separately for CI/CD.
• Fault Isolation: Issues in one microservice don't crash the whole system.

4. Key Considerations When Developing REST APIs

• Use clear resource modeling (endpoint naming).
• Apply correct HTTP methods (GET, POST, PUT, DELETE).
• Return proper HTTP status codes.
• Implement validation, authentication, and authorization.
• Support pagination and filtering.
• Use tools like Swagger/OpenAPI for documentation.

5. How to Fetch Employee Details in Spring Boot

@GetMapping("/employees/{id}")
public ResponseEntity<Employee> getEmployee(@PathVariable Long id) {
    Optional<Employee> emp = employeeRepository.findById(id);
    return emp.map(ResponseEntity::ok)
              .orElse(ResponseEntity.notFound().build());
}

This code demonstrates standard retrieve operations using Spring Boot’s REST controller.

6. Find 3rd Highest Salary Using Java 8 Streams

Optional<Integer> thirdHighest = salaries.stream()
    .distinct()
    .sorted(Comparator.reverseOrder())
    .skip(2)
    .findFirst();

This returns the third highest distinct salary using Java Streams.

7. Remove Duplicates from a List Using Java 8 Streams

List<Integer> uniques = list.stream()
    .distinct()
    .collect(Collectors.toList());

A simple, efficient Java 8 method to remove duplicates.

8. What Is a Singleton Class in Java?

A Singleton ensures a class has only one instance accessible globally within the application. It's useful for services like config, cache, etc.

• Private constructor
• Static instance holder
• Static getter method

9. What Is Thread Safety? How to Ensure It?

Thread safety means code performs reliably while accessed by multiple threads. Ensure thread safety using:

• synchronized blocks or methods


• Concurrent collections (ConcurrentHashMap)


• Immutable objects
• Atomic types (AtomicInteger)

10. How to Use groupingBy() in Java Streams

Map<String, List<Employee>> byDepartment = 
    employees.stream().collect(Collectors.groupingBy(Employee::getDepartment));

This groups employees by department — a common interview scenario.

11. What Are Functional Interfaces in Java?

A functional interface has one abstract method and enables lambda expressions. Annotated with @FunctionalInterface.
Examples: Runnable, Supplier, Function.

12. What Are Spring Boot Profiles?

Spring Profiles let you separate application configuration for different environments (dev/test/prod). Activate with -Dspring.profiles.active=dev or via environment variables and use application-dev.yml, etc.

13. Difference Between @RequestMapping and @GetMapping

• @RequestMapping: General, can map any HTTP method.


• @GetMapping: Specialization, maps only HTTP GET requests. Cleaner for REST APIs.

14. Experience Working with Kafka

Kafka is a distributed streaming platform. Typical interview focus:

• Publishing and consuming messages
• Integrating with microservices for real-time data pipelines
• Managing topics and partitions

15. Service Failure Impact in Microservices Architecture

Ideally, microservices are independent. If one service fails:

• Other services remain up if communication is handled with timeouts, retries, circuit breakers, and fallbacks.

16. What is Dependency Injection? How Is It Implemented in Spring?

Dependency Injection (DI) is a design pattern to provide required objects from the outside. In Spring, this is achieved with annotations like @Autowired, reducing tight coupling and improving testability.

17. Docker Experience: Benefits and Challenges

Benefits:

• Reproducible, isolated environments
• Easier scaling and deployment
• Efficient resource utilization

Challenges:

• Learning curve for Docker concepts
• Managing persistent data
• Security and image management

18. Common Maven Commands

• mvn clean: Remove compiled files/artifacts


• mvn install: Compile, test, and add build to local repo


• mvn package: Package project as JAR/WAR


• mvn test: Run tests


• mvn deploy: Deploy artifacts to remote repo

Final Tips for Java Backend Interviews

• Tailor your answers with specific examples and technologies used.
• Use code snippets or pseudocode when asked for implementation.
• Understand core concepts and be ready with real-world analogies.

This comprehensive list will help you prepare for your next Java Backend Developer interview with confidence, particularly roles involving Spring Boot, Microservices, and Kafka. Good luck!

For more interview guides, code snippets, and Java backend articles, stay tuned! Feel free to ask if you need more details or deeper explanations for any topic.

Keywords: Java Backend Interview Questions, Spring Boot Interview Questions, Kafka Interview, Java 8 Streams, Microservices Interview, Singleton, Thread Safety, Dependency Injection, Docker, Maven, REST API, Functional Interfaces, groupingBy Java.