Java Backend Developer Interview: A Comprehensive Guide
Are you preparing for a Senior Java Backend Developer role. The process can be challenging, but with the right preparation, you can stand out and land your dream job. This blog post dives into the key questions with clear, insightful answers and a focus on current enterprise practices. Whether you’re an experienced developer or seeking to upskill, read on for practical tips and sample code to help you succeed.
Java: Latest Features & Advanced Concepts
1. Sealed Classes in Java
Sealed classes restrict which classes can extend or implement them, providing better control over class hierarchies. They enhance safety and maintainability, especially in domain-driven or large-scale designs.
2. Difference Between Stream.peek() and forEach()
Stream.peek(): An intermediate operation mainly for debugging within the stream pipeline.Stream.forEach(): A terminal operation that consumes the stream and applies an action to each element.
3. Immutable Collections (Java 9+)
Immutable collections cannot be modified after creation. Methods like List.of() and Map.of() return collections that are thread-safe and prevent accidental changes.
4. List.of() vs. Arrays.asList()
| Feature | List.of() (Java 9+) |
Arrays.asList() |
|---|---|---|
| Mutability | Immutable | Fixed size, elements mutable |
| Null Values | Not allowed | Allowed |
| Backed By | Internal array | Provided array |
5. Virtual Threads (Project Loom)
Virtual threads are lightweight, managed by the JVM, and allow you to run millions of concurrent tasks efficiently. They provide better scalability in concurrent apps with minimal resources.
6. Switch Expressions with yield (Java 14+)
int result = switch(value) {
case 1 -> 100;
case 2 -> { yield 200; }
default -> 0;
};
7. When to Use CompletableFuture
CompletableFuture supports asynchronous, non-blocking code, allowing you to chain tasks, handle exceptions, and write more scalable code—ideal for API calls, DB operations, or parallel pipelines.
8. Instanceof Pattern Matching (Java 16+)
if (obj instanceof String s) {
// Use s directly
}
9. Useful Java 11+ String Methods
isBlank()lines()repeat(int count)strip(),stripLeading(),stripTrailing()indent(int n)translateEscapes()
10. Map.of() and Duplicate Keys
Using duplicate keys with Map.of() throws an IllegalArgumentException. All keys must be unique.
Spring & Spring Boot Essentials
11. Spring Boot Auto-Configuration
Spring Boot uses classpath scanning and @EnableAutoConfiguration to automatically configure beans based on detected libraries and settings.
12. Purpose of @ConditionalOnProperty
This annotation enables or disables beans based on configuration properties, letting you toggle features via configs.
13. Custom Validation with Annotations
- Define an annotation (
@MyConstraint). - Create a validator class implementing
ConstraintValidator. - Annotate fields/class.
- Spring enforces validation during data binding.
14. Nested Transactions with @Transactional
Nested transactions join the parent transaction by default. Use Propagation.REQUIRES_NEW for a new transaction. Rollbacks are handled according to the propagation type.
15. application.properties vs. application.yml
- Properties: Simple, flat key-value format.
- YAML: Hierarchical, supports lists and nested configs.
16. Externalizing Configuration
Use application-{profile}.yml or .properties files for different environments. Activate profiles with spring.profiles.active.
17. WebClient vs. RestTemplate
| Feature | WebClient | RestTemplate |
|---|---|---|
| Programming | Non-blocking/reactive | Blocking/sync |
| Use Case | Modern cloud apps | Legacy codebases |
18. Lazy Initialization in Spring Boot
Delay bean creation until first use to reduce startup time:
spring.main.lazy-initialization=true
19. Spring Profiles
Group configuration/beans for different environments. Switch profiles with:
spring.profiles.active=dev
20. Request Rate Limiting
Implement via:
- Custom filters/interceptors
- Libraries like Bucket4j or Resilience4j
- API gateways or reverse proxies
Microservices & Cloud Architecture
21. Service Discovery (Eureka, Consul)
Services register themselves with registries (Eureka, Consul). Clients query the registry to discover endpoints dynamically.
22. Centralized Logging
Aggregate logs using tools like the ELK stack (Elasticsearch, Logstash, Kibana) or EFK (Fluentd). Collect and analyze logs centrally for easier troubleshooting.
23. Maintaining API Backward Compatibility
- Use URI or header versioning
- Avoid breaking changes; add new features instead
- Deprecate old endpoints gradually
24. Fault Tolerance Strategies
- Circuit breakers
- Retries and timeouts
- Fallbacks and bulkheads
25. Producer-Consumer with BlockingQueue
BlockingQueue<Integer> queue = new ArrayBlockingQueue<>(100);
Runnable producer = () -> {
try { queue.put(1); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
};
Runnable consumer = () -> {
try { Integer val = queue.take(); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
};
26. Kafka + Avro: Role of Schema Registry
A schema registry centrally manages Avro schemas for Kafka topics, ensuring serialization/deserialization compatibility and supporting schema evolution.
27. Docker Compose for Microservices
Spin up environments locally using docker-compose.yml. Define all services, networks, and volumes, then run:
docker-compose up
28. AWS IAM Roles & Policies
IAM roles are assigned permissions (via policies) for users/resources, supporting principle of least privilege.
29. Kafka Partitions vs. Consumer Groups
| Kafka Concept | Purpose |
|---|---|
| Partition | Enables parallelism |
| Consumer Group | Distributes partitions among consumers for scalability and fault tolerance |
30. Kubernetes ConfigMaps and Secrets
- ConfigMaps: Store non-sensitive config, mounted as files or env vars.
- Secrets: Store sensitive data (base64-encoded), injected securely into pods.
31. SQL Query: Departments With More Than 5 Employees
SELECT department_id
FROM employees
GROUP BY department_id
HAVING COUNT(employee_id) > 5;
32. Blue-Green Deployment in CI/CD
Maintain two environments (blue and green). Deploy to green, test, then switch traffic over, reducing downtime and risk.
Final Tips
Preparing for an Java backend interview requires more than theoretical knowledge—you need to showcase deep understanding of new Java features, Spring Boot configurations, microservices patterns, and cloud-native practices. Use these detailed Q&As as a launchpad for your studies, and be ready to demonstrate not only your technical know-how but also your approach to solving real-world challenges. Good luck!