Java Backend Developer (3+ Yrs) :: Interview Questions Asked PayPal Microsoft Swiggy

 💡 Java Backend Developer (3+ Yrs) :: Interview Questions Asked @ PayPal 🚀

♦️ Core Java (OOPs, Collections, Concurrency)
1️⃣ Difference between HashMap and ConcurrentHashMap — and how thread-safety is achieved.
2️⃣ Explain how Java handles immutability and why it’s important for multithreading.
3️⃣ How does the Java Memory Model guarantee visibility between threads?
4️⃣ Difference between ThreadLocal and synchronized blocks.
5️⃣ Explain how to write a high-performance producer-consumer system using Java concurrency APIs.

♦️ Spring & Spring Boot
6️⃣ Explain how Spring Security works internally (filters, authentication manager).
7️⃣ Difference between @RequestParam, @PathVariable and @RequestBody — and when to use each.
8️⃣ How do you implement global exception handling in Spring Boot REST APIs?
9️⃣ How would you build idempotent APIs in Spring Boot (important for payment systems)?
🔟 Explain how to use @Transactional effectively to ensure data integrity.

♦️ JPA / Hibernate
1️⃣1️⃣ How do you handle high-volume inserts/updates in JPA?
1️⃣2️⃣ Explain difference between persist(), merge(), and save().
1️⃣3️⃣ How to avoid N+1 select problems in Hibernate?
1️⃣4️⃣ What’s the difference between optimistic and pessimistic locking in a payment system?
1️⃣5️⃣ How do you audit entity changes in JPA? (Envers or custom auditing)

♦️ Microservices & Architecture
1️⃣6️⃣ How do you implement distributed transactions in a payment system? (Saga, Outbox, 2PC)
1️⃣7️⃣ Explain how you’d design a secure API Gateway for microservices.
1️⃣8️⃣ How do you handle retries and idempotency when calling downstream services?
1️⃣9️⃣ Explain how you’d implement rate limiting and throttling for high-traffic APIs.
2️⃣0️⃣ How would you design a notification service that scales to millions of messages?

♦️ Security & Compliance (PayPal Focus)
2️⃣1️⃣ How do you securely store sensitive data (PCI DSS compliance)?
2️⃣2️⃣ Difference between OAuth2, OpenID Connect, and SAML.
2️⃣3️⃣ How do you handle token expiration and refresh in secure APIs?
2️⃣4️⃣ Explain data encryption at rest and in transit.
2️⃣5️⃣ How would you implement fraud detection / anomaly detection hooks in your service?

♦️ DSA & System Design (PayPal Special Focus)
3️⃣1️⃣ Design a service to process 100,000 transactions per second with idempotency guarantees.
3️⃣2️⃣ Implement a thread-safe rate limiter for API calls.
3️⃣3️⃣ Design a secure authentication system with multi-factor support.
3️⃣4️⃣ Given a stream of payments, detect duplicate or fraudulent transactions in real-time.
3️⃣5️⃣ Implement a Java program to validate credit card numbers (Luhn’s algorithm).

🚀 Swiggy SDE-2 Java Backend Interview Questions

💎 Core Java & Concurrency
💎 How would you implement a lock-free, thread-safe LRU cache in Java?
💎 Compare synchronized, ReentrantLock, StampedLock, and ReadWriteLock — when would you choose each?
💎 How would you handle high-concurrency read/write scenarios on a shared object without blocking threads?
💎 Explain memory consistency effects in Java; how does volatile differ from synchronized in guarantees?
💎 Implement rate-limiting for API requests at scale (Token Bucket / Leaky Bucket) across multiple microservice instances.

🚀 Data Structures & Algorithms
🚀 Design a high-performance in-memory queue supporting multiple producers and consumers, with priority scheduling.
🚀 How would you efficiently compute the top-k most frequently ordered items in a streaming system?
🚀 Solve dynamic window aggregation problems for real-time analytics (rolling order count per minute).
🚀 Implement deduplication of events in a distributed event stream ensuring idempotency.

🛰️ System Design & Distributed Systems
🛰️ Design a food delivery order tracking system for millions of concurrent users: API, DB schema, caching, eventual consistency.
🛰️ How would you implement distributed caching across multiple regions and handle invalidations?
🛰️ Design a Kafka-based microservice architecture; handle ordering guarantees, duplicates, and scaling consumers.
🛰️ Explain Saga vs 2-phase commit, and trade-offs for distributed transactions in microservices.
🛰️ How would you detect and prevent cascading failures in a microservices system?

🔥 High Performance & Optimization
🔥 Optimize a backend service that processes millions of requests per minute: caching, DB sharding, async processing.
🔥 How would you profile and reduce GC pause times in a low-latency Java service?
🔥 How would you avoid thread contention in a high-throughput service using Java concurrency utilities?
🔥 Explain strategies to reduce network latency between services deployed in multiple regions.

🗃️ Database & Persistence
🗃️ Design a highly available and consistent database schema for order history with millions of reads/writes per day.
🗃️ How would you migrate a large database (billions of rows) without downtime?
🗃️ Compare SQL vs NoSQL for different services in a food delivery platform; justify your choices.
🗃️ How do you handle hot partitions / skewed keys in distributed databases like Cassandra or DynamoDB?

💡 Java Backend Developer (3+ Yrs) :: Interview Questions Asked @ Microsoft 🚀


♦️ Core Java (OOPs, Collections, Concurrency)

1️⃣ Difference between HashMap, LinkedHashMap and ConcurrentHashMap.
2️⃣ Explain Java ClassLoader hierarchy and how custom ClassLoaders work.
3️⃣ How to handle deadlock detection and prevention in Java?
4️⃣ Explain Fork/Join Framework and its use cases.
5️⃣ What are the advantages of using CompletableFuture over traditional Futures?


♦️ Spring & Spring Boot

6️⃣ How does Spring Boot auto-configuration choose which beans to create?
7️⃣ Explain difference between @ComponentScan, @EnableAutoConfiguration, @SpringBootApplication.
8️⃣ How do you implement rate limiting & throttling in Spring Boot APIs?
9️⃣ How do you implement distributed caching with Spring Boot (Redis/Azure Cache)?
🔟 Explain Spring Boot Actuator endpoints and how to secure them.


♦️ JPA / Hibernate

1️⃣1️⃣ Explain first-level vs second-level cache in Hibernate.
1️⃣2️⃣ Difference between FetchType.LAZY and FetchType.EAGER and how to optimize queries.
1️⃣3️⃣ How do you handle multi-tenant applications in JPA?
1️⃣4️⃣ Explain optimistic vs pessimistic locking with real examples.
1️⃣5️⃣ How does dirty checking and flush mode work in Hibernate?


♦️ Microservices & Architecture

1️⃣6️⃣ Explain difference between REST, gRPC, and GraphQL APIs.
1️⃣7️⃣ How do you achieve service discovery and load balancing in microservices?
1️⃣8️⃣ Explain Saga and Outbox patterns for distributed transactions.
1️⃣9️⃣ How do you design an event-driven system (Azure Service Bus/Kafka)?
2️⃣0️⃣ How do you ensure backward compatibility for APIs in microservices?


♦️ Cloud & DevOps (Azure Focus)

2️⃣1️⃣ Difference between Azure Functions and Azure App Service.
2️⃣2️⃣ How do you manage secrets/configurations securely in Azure?
2️⃣3️⃣ How do you achieve zero-downtime deployment on Azure Kubernetes Service?
2️⃣4️⃣ Explain how Azure Monitor or App Insights helps in monitoring Spring Boot apps.
2️⃣5️⃣ Difference between vertical and horizontal scaling in Azure with examples.


♦️ DSA & System Design (Microsoft Special Focus)

2️⃣6️⃣ Design a URL shortener service (like bit.ly) and discuss scaling it.
2️⃣7️⃣ Design a system to handle millions of chat messages per second.
2️⃣8️⃣ Implement an LRU Cache in Java with O(1) operations.
2️⃣9️⃣ Given a huge dataset distributed across servers, find the top K elements efficiently.
3️⃣0️⃣ Given a real-time stream of events, detect anomalies within a rolling window.


♦️ Behavioral / Cultural Fit (Microsoft Leadership Values)

3️⃣1️⃣ Tell me about a time when you had to deal with ambiguous requirements.
3️⃣2️⃣ Describe a situation when you worked across teams to deliver a project.
3️⃣3️⃣ Tell me about a time you mentored or helped another engineer grow.
3️⃣4️⃣ How do you prioritize conflicting requirements?
3️⃣5️⃣ Describe a challenging bug you fixed and how you approached it.

💡 Java Developer (4+ Yrs) :: Interview Questions Asked ICICI Prudential AMC Ltd

🔹 Core Java
1️⃣ Difference between HashMap, ConcurrentHashMap, and Hashtable?
2️⃣ Explain Garbage Collection (CMS vs G1 GC).
3️⃣ Difference between synchronized, ReentrantLock, and ReadWriteLock.
4️⃣ Difference between Comparable vs Comparator.
5️⃣ Explain Java memory model (Heap, Stack, Metaspace).
6️⃣ How does volatile work internally?
7️⃣ Difference between wait(), sleep(), join().
8️⃣ How to resolve deadlock, starvation, livelock?
9️⃣ How do you make a class immutable?
🔟 Best practices for checked vs unchecked exceptions.

🔹 Spring & Spring Boot
1️⃣1️⃣ Explain the Spring Bean lifecycle.
1️⃣2️⃣ Difference between @Component, @Service, @Repository, @Controller.
1️⃣3️⃣ How does Spring Boot auto-configuration work?
1️⃣4️⃣ Role of Spring Boot Starter dependencies.
1️⃣5️⃣ How to configure profiles in Spring Boot?
1️⃣6️⃣ Explain Spring Security (Authentication vs Authorization).
1️⃣7️⃣ How to implement JWT Authentication?
1️⃣8️⃣ Difference between @RestController vs @Controller.
1️⃣9️⃣ How to resolve circular dependencies in Spring?
2️⃣0️⃣ Explain @Transactional usage and propagation.

🔹 JPA / Hibernate
2️⃣1️⃣ Difference between Lazy vs Eager fetching.
2️⃣2️⃣ What is the N+1 select problem and how to fix it?
2️⃣3️⃣ Difference between merge(), persist(), saveOrUpdate().
2️⃣4️⃣ Explain 1st level vs 2nd level cache.
2️⃣5️⃣ How do you implement optimistic vs pessimistic locking?
2️⃣6️⃣ How to handle batch inserts/updates?
2️⃣7️⃣ How to write custom queries in Spring Data JPA?

🔹 Microservices & Architecture
2️⃣8️⃣ What are 12-factor app principles?
2️⃣9️⃣ How do microservices communicate (REST, gRPC, Messaging)?
3️⃣0️⃣ How to implement service discovery (Eureka, Consul)?
3️⃣1️⃣ Explain API Gateway pattern.
3️⃣2️⃣ How to handle distributed transactions?
3️⃣3️⃣ Difference between Monolithic vs Microservices.
3️⃣4️⃣ How to manage configurations (Spring Cloud Config, Vault)?
3️⃣5️⃣ How to implement circuit breaker & rate limiting (Resilience4j, Hystrix)?
3️⃣6️⃣ Difference between synchronous vs asynchronous communication.
3️⃣7️⃣ How do you ensure idempotency in REST APIs?

🔹 System Design
3️⃣8️⃣ Design a high-availability payment system.
3️⃣9️⃣ Design a low-latency stock trading API.
4️⃣0️⃣ How to isolate a frequently failing microservice?
4️⃣1️⃣ How to ensure backward compatibility in APIs?
4️⃣2️⃣ How to scale a Spring Boot service for 1M+ users?

💡 Java Developer (5+ Yrs) :: Interview Questions Asked @ JP Morgan Chase 🚀

🔹 Core Java
1️⃣ Difference between HashMap, ConcurrentHashMap, and Hashtable?
2️⃣ Explain Garbage Collection (CMS vs G1 GC).
3️⃣ Difference between synchronized, ReentrantLock, and ReadWriteLock.
4️⃣ Difference between Comparable vs Comparator.
5️⃣ Explain Java memory model (Heap, Stack, Metaspace).
6️⃣ How does volatile work internally?
7️⃣ Difference between wait(), sleep(), join().
8️⃣ How to resolve deadlock, starvation, livelock?
9️⃣ How do you make a class immutable?
🔟 Best practices for checked vs unchecked exceptions.

🔹 Spring & Spring Boot
1️⃣1️⃣ Explain the Spring Bean lifecycle.
1️⃣2️⃣ Difference between @Component, @Service, @Repository, @Controller.
1️⃣3️⃣ How does Spring Boot auto-configuration work?
1️⃣4️⃣ Role of Spring Boot Starter dependencies.
1️⃣5️⃣ How to configure profiles in Spring Boot?
1️⃣6️⃣ Explain Spring Security (Authentication vs Authorization).
1️⃣7️⃣ How to implement JWT Authentication?
1️⃣8️⃣ Difference between @RestController vs @Controller.
1️⃣9️⃣ How to resolve circular dependencies in Spring?
2️⃣0️⃣ Explain @Transactional usage and propagation.

🔹 JPA / Hibernate
2️⃣1️⃣ Difference between Lazy vs Eager fetching.
2️⃣2️⃣ What is the N+1 select problem and how to fix it?
2️⃣3️⃣ Difference between merge(), persist(), saveOrUpdate().
2️⃣4️⃣ Explain 1st level vs 2nd level cache.
2️⃣5️⃣ How do you implement optimistic vs pessimistic locking?
2️⃣6️⃣ How to handle batch inserts/updates?
2️⃣7️⃣ How to write custom queries in Spring Data JPA?

🔹 Microservices & Architecture
2️⃣8️⃣ What are 12-factor app principles?
2️⃣9️⃣ How do microservices communicate (REST, gRPC, Messaging)?
3️⃣0️⃣ How to implement service discovery (Eureka, Consul)?
3️⃣1️⃣ Explain API Gateway pattern.
3️⃣2️⃣ How to handle distributed transactions?
3️⃣3️⃣ Difference between Monolithic vs Microservices.
3️⃣4️⃣ How to manage configurations (Spring Cloud Config, Vault)?
3️⃣5️⃣ How to implement circuit breaker & rate limiting (Resilience4j, Hystrix)?
3️⃣6️⃣ Difference between synchronous vs asynchronous communication.
3️⃣7️⃣ How do you ensure idempotency in REST APIs?

🔹 System Design
3️⃣8️⃣ Design a high-availability payment system.
3️⃣9️⃣ Design a low-latency stock trading API.
4️⃣0️⃣ How to isolate a frequently failing microservice?
4️⃣1️⃣ How to ensure backward compatibility in APIs?
4️⃣2️⃣ How to scale a Spring Boot service for 1M+ users?