💡 Verizon :: Java Backend Developer Interview Questions (3–6 Yrs) ➡️ Deep fundamentals + clean code + microservices + large-scale system understanding. --- 🧠 Core Java (Conceptual + Practical) 1️⃣ Difference between HashMap, Hashtable, ConcurrentHashMap — internal working. 2️⃣ How does Garbage Collection work? Explain CMS vs G1. 3️⃣ Explain volatile, synchronized, lock, and when to use which. 4️⃣ What happens internally when you call Object.wait() and notify()? 5️⃣ Difference between ArrayList and CopyOnWriteArrayList. 6️⃣ What is the Java Memory Model? 7️⃣ Explain immutability — how do you create an immutable class? 8️⃣ Difference between Stream API sequential vs parallel streams. 9️⃣ How do you reduce latency and GC pauses in a high-load backend? 🔟 Explain ExecutorService, ThreadPoolExecutor, and its parameters. --- 🌿 Spring / Spring Boot (Verizon heavily uses) 1️⃣1️⃣ What is Spring Boot Auto-configuration? How to customize it? 1️⃣2️⃣ Difference between @Component, @Service, @Repository. 1️⃣3️⃣ How does Spring Security work internally (Filter Chain)? 1️⃣4️⃣ What is OAuth2 + JWT authentication flow? 1️⃣5️⃣ Explain Spring AOP with real telco example (logging, billing). 1️⃣6️⃣ How to implement Global Exception Handling using @ControllerAdvice? 1️⃣7️⃣ Explain Bean Lifecycle and @PostConstruct / @PreDestroy. 1️⃣8️⃣ How does Spring Boot Actuator help in monitoring? 1️⃣9️⃣ How do you build custom Spring Boot starters? 2️⃣0️⃣ How do you handle schema migration with Flyway/Liquibase? --- 🧩 Microservices + Distributed Systems (Verizon focuses a LOT) 2️⃣1️⃣ Why microservices? Advantages over monolithic architecture. 2️⃣2️⃣ Explain service discovery — Eureka, Consul, Zookeeper. 2️⃣3️⃣ What is API Gateway? (Rate limiting, routing, auth). 2️⃣4️⃣ How do you ensure service-to-service security? (mTLS, OAuth2) 2️⃣5️⃣ Explain Circuit Breaker pattern — Resilience4j/Hystrix. 2️⃣6️⃣ How do you implement distributed logging & tracing? (Sleuth/Zipkin) 2️⃣7️⃣ What is idempotency in microservices? Provide examples. 2️⃣8️⃣ How do you ensure event consistency using Kafka? 2️⃣9️⃣ What is saga pattern? How to handle distributed transactions? 3️⃣0️⃣ How do you detect API latency issues in microservices? --- 💾 JPA / Hibernate / SQL (Used in telecom billing & provisioning) 3️⃣1️⃣ Difference between lazy and eager loading. 3️⃣2️⃣ What is N+1 problem? How to solve it? 3️⃣3️⃣ Difference between persist(), merge(), update(). 3️⃣4️⃣ Explain @OneToMany, @ManyToMany mappings. 3️⃣5️⃣ What is optimistic vs pessimistic locking? 3️⃣6️⃣ How do you optimize complex JOIN queries? 3️⃣7️⃣ What is connection pooling? How does HikariCP work? 3️⃣8️⃣ How to write dynamic queries with Criteria API? 3️⃣9️⃣ How do you handle batch insert/update? 4️⃣0️⃣ Explain ACID properties with real telecom billing examples.
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