Core Java Difference between HashMap and ConcurrentHashMap Why is String immutable in Java? Difference between ArrayList and LinkedList Explain equals() vs hashCode() Difference between == and .equals() What happens when you use an object as a key in HashMap? Difference between checked and unchecked exceptions What is the difference between final, finally, and finalize()? Explain Java Memory Model (Heap, Stack, Metaspace) What causes memory leaks in Java? Difference between Comparable and Comparator What are immutable classes? How to create one? Explain method overloading vs overriding What is the use of volatile keyword? Difference between Runnable and Callable Explain synchronization and race conditions Difference between wait(), sleep(), and notify() What is deadlock? How do you prevent it? What are Java 8 Streams? Difference between map() and flatMap() Explain Optional in Java 8 Functional interfaces in Java What is lazy loading in streams? What is serialization/deserialization? What are marker interfaces? Multithreading & Concurrency How does thread pool work? What is ExecutorService? Explain CompletableFuture Difference between parallel stream and normal stream What is atomicity? Explain CountDownLatch vs CyclicBarrier What are concurrent collections? Spring Boot What happens internally when Spring Boot application starts? Explain dependency injection Difference between @Component, @Service, @Repository Difference between constructor and field injection What is Spring Bean lifecycle? Explain @Autowired Difference between @RestController and @Controller What is @Transactional? What is AOP in Spring? Explain profiles in Spring Boot How do you handle exceptions globally? What is Spring Security? How JWT authentication works? Difference between monolith and microservices How do microservices communicate? What is API Gateway? What is service discovery? Explain circuit breaker pattern What is Feign Client? Difference between PUT and PATCH Hibernate / JPA Difference between save() and persist() Explain lazy vs eager loading What is N+1 query problem? Difference between get() and load() What are cascading types? What is first-level cache? Difference between JPQL and native query Explain entity lifecycle states SQL & Database Difference between WHERE and HAVING Explain joins with examples Difference between clustered and non-clustered index What is normalization? What are ACID properties? Explain transactions Difference between DELETE, DROP, and TRUNCATE How to optimize slow queries? What is indexing? Microservices & System Design How do you secure microservices? How do you handle distributed transactions? What is idempotency? Explain Kafka basics What happens if one microservice is down? How do you monitor microservices? What is rate limiting? Explain caching strategies Redis use cases Design a URL shortener / notification service Coding Questions Often Asked Reverse a string without using built-in methods Find duplicate elements in array Find first non-repeated character Check palindrome LRU Cache implementation Producer-consumer problem Sort employees by salary using streams Group strings by length using streams Find second highest number Detect cycle in linked list Scenario-Based Questions (Very Important) Production issue due to high CPU — how will you debug? API response became slow after deployment — approach? Database connections exhausted — what could be wrong? How did you improve performance in your project? Explain one challenging bug you solved How do you handle backward compatibility in APIs? How do you deploy safely in production? For 3–5 years experience, interviewers focus heavily on: real project experience debugging ability concurrency basics API design Spring Boot internals database optimization communication/confidence