ThoughtWorks Interview Questions

#1Explain the SOLID principles.

#2How does HashMap work internally?

#3What is the difference between HashMap and ConcurrentHashMap?

#4Explain Java Memory Model: Heap, Stack, and Metaspace.

#5What is the volatile keyword in Java?

#6What is the difference between synchronized and ReentrantLock?

#7What is a deadlock and how do you prevent it?

#8Explain the Stream API and intermediate vs terminal operations.

#9What is garbage collection? How does G1 differ from ZGC?

#10What is the Java ClassLoader hierarchy?

#11What are Java records (Java 16+)?

#12What is the Executor framework and when would you use it over raw Threads?

#13What is the difference between composition and inheritance?

#14What is a marker interface? Give examples.

#15What are the design patterns most commonly asked in Java interviews?

#16How do you implement a Singleton in Java correctly?

#17What is the contract between equals() and hashCode()?

#18What is the difference between fail-fast and fail-safe iterators?

#19What is CopyOnWriteArrayList and when should you use it?

#20How does PriorityQueue work and when would you use it?

#21What is the happens-before relationship in Java Memory Model?

#22What is the difference between sleep(), wait(), and yield()?

#23What are atomic classes (AtomicInteger, AtomicReference)? How do they work?

#24What is a CountDownLatch vs CyclicBarrier?

#25What is a ThreadLocal and what are its dangers?

#26What is a CompletableFuture and how does it improve async programming?

#27What are virtual threads (Project Loom, Java 21)?

#28How does the JIT (Just-In-Time) compiler work?

#29What is the difference between minor GC, major GC, and full GC?

#30What tools do you use to diagnose JVM memory issues?

#31What is the String pool and how does string interning work?

#32What is type erasure in Java generics?

#33What is the difference between <? extends T> and <? super T>?

#34What are sealed classes (Java 17)?

#35What is pattern matching for instanceof (Java 16)?

#36What is the difference between map() and flatMap() in streams?

#37What is the difference between Comparable and Comparator?

#38What is Stream.collect() and what are common Collectors?

#39What is try-with-resources and how does it work?

#40What is exception chaining and why is it important?

#41What is the difference between Java IO and NIO?

#42What is Java Serialization and what are its pitfalls?

#43What is reflection in Java and when is it used?

#44What is the difference between deep copy and shallow copy?

#45What is autoboxing and unboxing? What are the performance implications?

#46What are enums in Java and what advanced features do they support?

#47What is the difference between static and instance initializer blocks?

#48What are Java modules (Java 9 — JPMS)?

#49What is the difference between an inner class, static nested class, and anonymous class?

#50What are the new switch expressions (Java 14)?

#51What is the difference between Runnable, Callable, and Future?

#52How do you handle NullPointerException in Java?

#53What is the difference between Deque and Queue?

#54What is a WeakHashMap and when would you use it?

#55What is the difference between checked and unchecked exceptions — best practices for APIs?

#56What is the difference between interface default methods and abstract class methods?

#57What are the best practices for writing clean Java code?

#58What is a race condition and how do you prevent it?

#59What is the difference between HashMap.computeIfAbsent() and getOrDefault()?

#60What is the purpose of the transient keyword?

#61What is method hiding in Java?

#62What are the Java 8 Date/Time APIs and why were they introduced?

#63What is the difference between HashMap.put() and HashMap.putIfAbsent()?

#64What is an immutable class and how do you create one?

#65What is the covariant return type in Java?

#66What is the difference between Predicate.and(), Predicate.or(), and Predicate.negate()?

#67What is the Observer pattern and how is it implemented in Java?

#68What is the Strategy pattern in Java?

#69What is String.intern() and when should you use it?

#70What are the key differences between Java 8, 11, 17, and 21?

#71What is an API Gateway and what does it do?

#72How do microservices communicate?

#73What is service discovery in microservices?

#74What is the Circuit Breaker pattern and how does it prevent cascading failures?

#75What is distributed tracing and why is it important?

#76What is the Strangler Fig pattern for microservices migration?

#77What is a sidecar pattern?

#78How do you manage configuration in microservices?

#79What are the 12-Factor App principles?

#80What is event-driven architecture and when should you use it?

#81What is the difference between orchestration and choreography in Saga?

#82How do you test microservices effectively?

#83What is a service mesh and when should you use one?

#84How do you ensure data consistency across microservices?

#85What is the circuit breaker pattern?

#86What is service discovery and how does it work?

#87What is the Bulkhead pattern?

#88What is the Retry pattern and how do you implement it safely?

#89What is the difference between choreography and orchestration in Sagas?

#90What is the Backend for Frontend (BFF) pattern?

#91How do you implement distributed tracing?

#92What is the Strangler Fig pattern for migrating a monolith?

#93What is eventual consistency and how do you design for it?

#94How do you version microservice APIs?

#95What is the Sidecar pattern?

#96What is a dead letter queue and why is it important?

#97How do you implement health checks in microservices?

#98What is the Outbox pattern and how does it guarantee event delivery?

#99What is idempotency and how do you implement it in microservices?

#100What are the 12-Factor App principles?

#101How do you handle secrets management in microservices?

#102What is the Ambassador pattern in microservices?

#103How do you implement inter-service authentication?

#104What is the Throttling pattern and how does it differ from rate limiting?

#105What is CQRS and how does it apply to microservices?

#106How do you test microservices?

#107What is service isolation and why is it important?

#108What is an Event-Driven Architecture?

#109How do you handle database per service in practice?

#110What is the Role of API composition in microservices?

#111What is graceful degradation in microservices?

#112What is the Event Sourcing pattern?

#113How do you implement logging in a microservices environment?

#114What is the two-phase commit vs Saga for distributed transactions?

#115How do you handle timeouts in microservices?

#116What is the Scatter-Gather pattern?

#117What is the difference between a microservice and a nanoservice?

#118How do you implement canary deployments in microservices?

#119What is a Service Registry?

#120What is blue-green deployment and how does it work?

#121What is the Aggregator pattern?

#122How do microservices handle configuration management?

#123What is the Proxy pattern in microservices?

#124How do you implement distributed locking in microservices?

#125What is Kafka and why is it popular in microservices?

#126How do you manage service dependencies in microservices?

#127What is a bounded context?

#128How do you implement a request-reply pattern over async messaging?

#129What is the Token Bucket algorithm and how is it used for rate limiting?

#130How do you handle schema evolution in event-driven systems?

#131What is the difference between a monolith, microservices, and modular monolith?

#132What is the Choreography pattern in detail?

#133What is a composite microservice?

#134How do you deal with "chatty" microservices?

#135What is a process manager / workflow engine in microservices?

#136How do you implement zero-downtime deployments for microservices?

#137What is the Anti-Corruption Layer pattern?

#138What are the common pitfalls of microservices adoption?

#139What is the difference between orchestration and choreography?

#140How do you implement a consistent read across microservices?

#141What is load shedding?

#142What is the difference between gRPC and REST for inter-service communication?

#143What is the Gateway Aggregation pattern vs Gateway Routing?

#144What is the Inbox pattern?

#145How do you handle versioning of events in Kafka?

#146What is the Open Host Service pattern?

#147How do you implement service-level SLOs in microservices?

#148What is the Claim Check pattern?

#149What is service mesh traffic management?

#150What is the difference between horizontal and vertical scaling for microservices?

#151What is the role of a message schema registry?

#152How do you handle service discovery with Kubernetes?

#153How do you implement an API gateway from scratch?

#154What is consumer-driven contract testing (Pact)?

#155How do you use feature toggles in microservices?

#156What is polyglot persistence in microservices?

#157What are distributed caching strategies for microservices?

#158How do you design a CI/CD pipeline for microservices?

#159How do you implement observability with Micrometer, Prometheus, and Grafana?

#160What is the Retry pattern and when should you NOT retry?

#161How do you implement Saga compensation (rollback) in detail?

#162What is multi-tenancy in microservices and how do you implement it?

#163What are the steps to migrate a monolith to microservices using the Strangler Fig pattern?

#164How do you design microservices for failure?

#165What is data mesh and how does it relate to microservices data ownership?

#166What is a sidecar proxy and how does Envoy work?

#167How do you handle inter-service authorization (not just authentication)?

#168When should you NOT use microservices?

#169Explain the CAP theorem.

#170What is consistent hashing and why is it used?

#171What is database sharding and how does it work?

#172Explain cache invalidation strategies.

#173How would you design a URL shortener (like bit.ly)?

#174What is the Saga pattern in microservices?

#175What is CQRS and when should you use it?

#176What are rate limiting algorithms?

#177What is the Circuit Breaker pattern?

#178How do you design for high availability?

#179What is eventual consistency?

#180What is a message queue and when should you use one?

#181What is the difference between SQL and NoSQL databases?

#182How do you handle distributed transactions without 2PC?

#183How would you design a notification system (push, email, SMS)?

#184What is the difference between REST and gRPC?

#185What is the Outbox Pattern?

#186How do you design a distributed cache layer?

#187What is the difference between synchronous and asynchronous communication in microservices?

#188How do you design a notification system that sends 100M notifications per day?

#189What are the different types of database indexes and when do you use each?

#190How would you design a URL shortener like bit.ly?

#191What is eventual consistency and how do you handle it in practice?

#192How do you design a search system at scale?

#193What is the difference between optimistic and pessimistic locking?

#194How would you design a real-time leaderboard?

#195What is a write-ahead log (WAL) and how is it used?

#196How do you handle distributed transactions across microservices?

#197How do you design a chat application like WhatsApp?

#198What is a service mesh and when should you use one?

#199How would you design a video streaming platform like YouTube?

#200What is the difference between a load balancer and an API gateway?

#201How do you prevent and handle cascading failures in distributed systems?

#202What is the difference between a message queue and a message broker?

#203How do you design a rate limiter for an API?

#204What is the difference between replication and partitioning in databases?

#205How would you design Twitter's news feed?

#206What is observability and what are its three pillars?

#207How do you implement idempotency in APIs?

#208How would you design a distributed job scheduler?

#209What is the Saga pattern vs 2PC for distributed transactions?

#210How do you design a file storage system like Dropbox?

#211What are the trade-offs of microservices vs monolithic architecture?

#212How does DNS work and how is it used in load balancing?

#213What is the strangler fig pattern?

#214How do you design an e-commerce inventory system that handles flash sales?

#215What is the difference between push and pull architectures for data?

#216How do you design a fraud detection system?

#217What is event sourcing and how does it differ from CRUD?

#218How would you design a geospatial service like Uber's nearby driver system?

#219What is API versioning and what are the different strategies?

#220What is a Bloom filter and where is it used in system design?

#221How do you implement zero-downtime deployments?

#222What is the two-generals problem and what does it tell us about distributed systems?

#223How do you design a payment processing system?

#224How would you design an autocomplete system?

#225What is backpressure and how do you handle it?

#226What is a time-series database and when should you use one?

#227What is a circuit breaker and how do you implement it?

#228How do you handle database migrations safely in production?

#229What is the CQRS pattern?

#230How do you secure an API at the architecture level?

#231What is the difference between synchronous replication and asynchronous replication?

#232How do you design a system for billions of events per day (analytics pipeline)?

#233What is chaos engineering and how do you implement it?

#234What is the database connection pool and how do you tune it?

#235How do you design an access control system?

#236How would you design an online code execution system like LeetCode?

#237How does the request flow work in a typical microservices architecture?

#238What is a hot partition problem and how do you solve it?

#239How do you design a multi-tenant SaaS architecture?

#240What is tail latency and why does it matter?

#241How do you design a global, low-latency key-value store?

#242What is the N+1 query problem and how do you fix it?

#243How would you design a booking system like Airbnb?

#244How do you design a recommendation system?

#245What is the read-your-own-writes consistency guarantee?

#246How do you design a search-as-you-type / typeahead feature?

#247What is the difference between gRPC unary, server streaming, client streaming, and bidirectional streaming?

#248How do you handle large file uploads efficiently?

#249What are the key considerations when designing for high write throughput?

#250What is the difference between monolithic and microservices deployment strategies?

#251How do you design for disaster recovery?

#252What is the thundering herd problem?

#253What is a vector clock and how does it work?

#254What is the difference between a monorepo and polyrepo?

#255How do you ensure data consistency across microservices?

#256What is the difference between a batch job and a stream processing job?

#257How do you design an API for mobile clients vs web clients?

#258What is the role of a message broker in event-driven architecture?

#259How do you approach capacity planning for a new system?

#260What is auto-configuration in Spring Boot?

#261Explain @Transactional and its propagation levels.

#262How does Spring Boot handle exceptions globally?

#263What is the difference between @Bean and @Component?

#264Explain Spring Bean scopes.

#265What is Spring AOP and where is it used?

#266What is circular dependency and how do you fix it?

#267How does Spring Security authentication work?

#268What is @Async in Spring Boot and how does it work?

#269What is Spring Data JPA and how does it simplify data access?

#270How does Spring Boot externalized configuration work?

#271What is Spring Cache Abstraction and how do you use it?

#272How do you write integration tests in Spring Boot?

#273What is the Spring ApplicationContext and how does it differ from BeanFactory?

#274What is a BeanPostProcessor and when would you use it?

#275What is the @PostConstruct and @PreDestroy lifecycle?

#276What is @Qualifier and when do you need it?

#277How does Spring Boot auto-configuration work under the hood?

#278What is @ConfigurationProperties and how does it differ from @Value?

#279What is Spring Boot's embedded server and how do you switch from Tomcat to Undertow?

#280What is Spring's @EventListener and ApplicationEvent?

#281What is @Conditional and how do you create custom conditions?

#282How do you implement request validation in Spring Boot?

#283What is ResponseEntity and when should you use it?

#284What is content negotiation in Spring MVC?

#285How do you implement pagination and sorting in Spring Boot?

#286What is HATEOAS and does Spring Boot support it?

#287How do you version a REST API in Spring Boot?

#288What is the difference between findById() and getById() in Spring Data JPA?

#289What are JPA entity states (Transient, Managed, Detached, Removed)?

#290What is the difference between @OneToMany with fetch LAZY vs EAGER?

#291What is Hibernate first-level cache and second-level cache?

#292What is the Open Session in View (OSIV) pattern and why is it controversial?

#293What is a Spring Data JPA Specification and when would you use it?

#294What is CSRF and how does Spring Security handle it?

#295How do you implement JWT authentication in Spring Boot?

#296What is the difference between @Secured, @PreAuthorize, and @RolesAllowed?

#297What is CORS and how do you configure it in Spring Boot?

#298What is OAuth2 and how does Spring Security support it?

#299What is MockMvc and how do you use it?

#300What is @MockBean vs @Mock in Spring Boot tests?

#301What is @DataJpaTest and how does it differ from @SpringBootTest?

#302What is Testcontainers and how does it improve Spring Boot tests?

#303What is Micrometer and how does it integrate with Spring Boot Actuator?

#304How do you implement custom health indicators in Spring Boot?

#305How do you implement distributed tracing in Spring Boot?

#306What is graceful shutdown in Spring Boot and how do you configure it?

#307What is a Filter vs HandlerInterceptor in Spring Boot?

#308What is @ControllerAdvice and how does it work?

#309What is Spring WebFlux and how does it differ from Spring MVC?

#310What is Spring's @Scheduled and how do you configure it?

#311What happens during Spring Boot startup?

#312What is Spring Boot's fat JAR and how is it structured?

#313What is Spring Retry and how do you use it?

#314What is Spring Integration vs Spring Batch?

#315What is @Transactional readOnly = true and what does it optimize?

#316What is the difference between Spring @Repository exception translation and JDBC exceptions?

#317How do you handle file uploads in Spring Boot?

#318What is Spring Cloud and its key components?

#319What is the difference between @Transactional on a class vs a method?

#320What is Spring Boot's connection pool? How do you tune HikariCP?

#321What is Spring Boot's auto-configuration for messaging (RabbitMQ / Kafka)?

#322What is Spring's @Lookup annotation?

#323What is Spring Security's method security and how is it different from URL security?

#324How does Spring Boot handle environment-specific properties?

#325What is the purpose of @EnableJpaAuditing?

#326How do you implement rate limiting in Spring Boot?

#327What is the difference between eager and lazy initialization in Spring Boot?

#328What is Spring Boot's support for caching with Redis?

#329What is Spring Boot's support for MongoDB?

#330What is @Transactional self-invocation problem and how do you fix it?

#331What is the difference between @Valid and @Validated in Spring?

#332What is Spring Boot's support for scheduled tasks with Quartz?

#333What is Spring Data REST and how does it differ from Spring MVC?

#334What is Flyway and how does it integrate with Spring Boot?

#335What is Spring Boot's support for WebSockets?

#336How do you implement a multi-tenant application in Spring Boot?

#337What are Spring Boot's management endpoints and how do you secure them?

#338What is Spring Boot's support for R2DBC (reactive database access)?

#339How do you implement idempotency in Spring Boot APIs?

#340What is Spring Boot's support for virtual threads (Java 21)?

#341What is Spring Boot's Docker image building support?

#342What is GraalVM native image and Spring Boot AOT support?

#343What is the difference between @SpringBootTest(webEnvironment) options?

#344What is Spring Boot's support for SSE (Server-Sent Events)?

#345What is Resilience4j and how does it integrate with Spring Boot?

#346What are best practices for Spring Boot application performance tuning?