Top 30 Java Interview Questions for Experienced Candidates (2025 & 2026) — Complete Guide
Java remains one of the most in-demand programming languages in the world, powering enterprise systems, fintech platforms, big data frameworks, microservices, and modern cloud applications. As companies adopt AI-driven automation, distributed systems, and high-performance microservices, the demand for skilled Java professionals continues to grow in 2025 & 2026.
If you’re preparing for a Java developer or senior Java engineer role, this article covers the Top 30 Java Interview Questions for Experienced Candidates (2025 & 2026) with strong explanations, trending concepts, and high-volume SEO keywords.
⭐ Why Java Developers Are in High Demand in 2025 & 2026
- Growing adoption of microservices architecture
- Rise of Spring Boot, Spring Cloud, Kubernetes, Docker
- Expansion of FinTech & BFSI Java-based systems
- Cloud migration to AWS, Azure, GCP
- Use of Java in Big Data, Apache Kafka, Spark
- Enterprise-scale API development
- Java’s continuous upgrades (Java 17, Java 21, Java 23 LTS)
Top 30 Java Interview Questions for Experienced Candidates (With Answers)
1. What is the difference between JDK, JRE, and JVM?
- JDK → Development kit with compiler + tools
- JRE → Runtime environment
- JVM → Virtual machine executing bytecode
JDK contains JRE → JRE contains JVM.
2. What are the new features in Java 17, Java 21, and Java 23?
Common features:
- Sealed classes
- Pattern matching for switch
- Records
- Virtual threads (Java 21, Project Loom)
- Performance improvements
- Improved garbage collectors (ZGC, G1)
Trending question in 2025 interviews.
3. What is the difference between HashMap and ConcurrentHashMap?
- HashMap → Not thread-safe, allows nulls, unsafe under concurrency
- ConcurrentHashMap → Thread-safe, uses segmentation + CAS operations
4. Explain Java Memory Model (JMM).
Key parts:
- Heap
- Stack
- Method area
- Metaspace
- Garbage collection roots
Includes happens-before relationship and visibility guarantees.
5. What is the difference between Stack and Heap memory?
- Stack → Stores method calls, primitive data
- Heap → Stores objects, globally accessible
6. Explain Garbage Collection in Java.
GC types:
- Serial
- Parallel
- G1 (default)
- ZGC (low latency)
GC phases: - Mark
- Sweep
- Compact
7. What is the difference between composition and inheritance?
- Inheritance → “is-a” relationship
- Composition → “has-a” relationship (preferred for loose coupling)
8. What is a Java Stream API?
- Introduced in Java 8
- Supports functional programming
- Operations: map, filter, reduce
9. What is the difference between Lambda expressions and anonymous classes?
- Lambdas implement functional interfaces
- Anonymous classes can implement multiple methods
10. What is the difference between String, StringBuilder, and StringBuffer?
- String → Immutable
- StringBuilder → Mutable, not thread-safe
- StringBuffer → Mutable, thread-safe
11. Explain Spring Boot auto-configuration.
Spring Boot automatically configures components using:
- Classpath scanning
- Conditional beans
- @SpringBootApplication
12. What is Dependency Injection (DI) in Java?
- A design pattern to decouple components
Provided by Spring Framework via: - @Autowired
- @Inject
- @Qualifier
13. What are microservices, and why are they used?
Microservices:
- Independent deployable services
- Communicate via REST or messaging
Benefits: - Scalability
- Fault isolation
- Faster deployment
14. What is REST API in Java?
Representational State Transfer using:
- HTTP methods (GET, POST, PUT, DELETE)
- JSON/XML
Frameworks: - Spring Boot
- JAX-RS
15. What is the difference between REST and SOAP?
REST → Lightweight, JSON, stateless
SOAP → XML, strict standards
16. Explain exception hierarchy in Java.
- Throwable
- Error
- Exception
- Checked
- Unchecked
17. What is the difference between Checked and Unchecked exceptions?
- Checked → Must handle (IOException)
- Unchecked → Runtime exceptions (NullPointerException)
18. What is the SOLID principle?
- Single responsibility
- Open/closed
- Liskov substitution
- Interface segregation
- Dependency inversion
Frequently asked in senior Java roles.
19. What is Hibernate, and why is it used?
Hibernate is an ORM framework providing:
- Lazy loading
- Caching
- HQL
- Transaction management
20. Explain the difference between JPA and Hibernate.
- JPA → Specification
- Hibernate → Implementation
21. What is Kafka in Java systems?
Apache Kafka:
- Distributed messaging
- High throughput
- Used for event-driven systems
22. Explain the difference between synchronized and Lock API.
- synchronized → Block-level locking
- Lock API → More control, fairness policies, tryLock()
23. What is immutability in Java?
Immutable object:
- Cannot be changed after creation
String is the best example.
24. What is the use of final keyword?
- final variable → constant
- final method → cannot override
- final class → cannot extend
25. What is serialization in Java?
Serialization converts objects → byte stream
Used for:
- File storage
- Distributed systems
26. What is the Executor Framework?
Handles thread pooling using:
- Executor
- ExecutorService
- Future
- Callable
27. What are design patterns used in Java?
Common patterns:
- Singleton
- Factory
- Builder
- Strategy
- Observer
28. Explain CQRS and Event Sourcing (Trending 2025).
CQRS:
- Separate command & query models
Event Sourcing: - Application state stored as events
Used in modern Java microservices.
29. Explain Docker and Kubernetes for Java developers.
Docker → Containers
Kubernetes → Orchestration
Java apps use:
- Spring Boot Docker images
- K8s deployments, services, ingress
30. What is OAuth2 in Java Security?
Used for:
- Token-based authentication
- API security
Implemented via: - Spring Security
- Keycloak
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Conclusion
Preparing for Java interviews in 2025 & 2026 requires mastering:
- Core Java
- Multithreading
- Collections
- Spring Boot
- Microservices
- Cloud-native development
- REST APIs
- Docker & Kubernetes
The above Top 30 Java Interview Questions for Experienced Candidates will help you crack real-world interviews at companies hiring senior Java developers, Java backend engineers, and microservices architects.