Oracle 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?

#71How does indexing work in MySQL (InnoDB)?

#72What is a covering index?

#73What are MySQL transaction isolation levels?

#74What is MVCC (Multi-Version Concurrency Control) in InnoDB?

#75How do you use EXPLAIN to analyze a query?

#76What are window functions in SQL?

#77What are CTEs (Common Table Expressions) and when should you use them?

#78What is the N+1 query problem and how do you fix it?

#79What is a deadlock in MySQL and how does it occur?

#80How do you optimize a slow MySQL query?

#81What is database normalization?

#82What is MySQL replication and how does it work?

#83What is partitioning in MySQL?

#84What is a stored procedure and when should you use it?

#85How does the InnoDB storage engine handle locking?

#86How do you design a schema for a multi-tenant SaaS application?

#87What is the difference between a clustered and a non-clustered index?

#88What are covering indexes and how do they improve performance?

#89What is the EXPLAIN output and how do you read it?

#90What are database triggers and when should you avoid them?

#91What is a foreign key constraint and what are its performance implications?

#92How do you implement full-text search in MySQL?

#93How does MySQL replication work and what are its modes?

#94What is a deadlock in MySQL and how does InnoDB handle it?

#95What are MySQL stored procedures and when should you use them?

#96How do you perform a slow query analysis in MySQL?

#97What is InnoDB buffer pool and how do you size it?

#98What is MySQL's query cache and why was it removed?

#99How do you back up a MySQL database without downtime?

#100What is the difference between optimistic and pessimistic locking in MySQL?

#101What are MySQL partitioning types and when do you use them?

#102How do you handle large data migrations in production?

#103What is the difference between IN, EXISTS, and JOIN for subqueries?

#104What is the InnoDB redo log and undo log?

#105How do you implement pagination in MySQL efficiently?

#106What is group replication and InnoDB Cluster?

#107What is the difference between a view and a materialized view?

#108How does MySQL's optimizer choose an execution plan?

#109What are common MySQL performance anti-patterns?

#110What is a generated column in MySQL?

#111What are the best practices for MySQL schema design?

#112How do you use MySQL's JSON column type?

#113What is the difference between transaction isolation levels in MySQL?

#114How do you handle hierarchical data in MySQL?

#115What is ProxySQL and how is it used?

#116What is MVCC and how does InnoDB implement it?

#117How do you detect and fix index fragmentation in MySQL?

#118What are the MySQL 8.0 improvements over MySQL 5.7?

#119How do you design a schema for audit logging?

#120What is binlog and how is it used for CDC?

#121How do you use MySQL's performance_schema?

#122What is a composite index and how should you order columns?

#123How do you implement soft delete in MySQL?

#124How does MySQL handle concurrent writes to the same row?

#125What is the difference between schema-on-write and schema-on-read?

#126How do you handle time zones in MySQL?

#127How do you monitor MySQL in production?

#128What are window functions and how do you use them?

#129How do you implement row-level security in MySQL?

#130How do you use MySQL's event scheduler?

#131What is the EXPLAIN ANALYZE command?

#132How do you handle database connection failures in application code?

#133What is the MySQL query execution order?

#134How do you scale MySQL reads?

#135What is the difference between row-based and statement-based replication?

#136How do you use CTEs (Common Table Expressions) effectively?

#137How do you implement optimistic locking with versioning in MySQL?

#138What is the purpose of the information_schema?

#139What is innodb_flush_log_at_trx_commit and how does it affect durability?

#140How do you implement full-table search across millions of rows efficiently?

#141What is the max_connections parameter and how do you tune it?

#142What is a spatial index in MySQL and how is it used?

#143What is the difference between ROLLUP and CUBE in SQL?

#144How do you handle NULL values in MySQL queries?

#145What is the difference between a read and a write transaction?

#146How do you implement a queue in MySQL?

#147What is the mysql.user table and how does MySQL authentication work?

#148What is index merge optimization?

#149How do you detect and resolve replication lag?

#150What is the difference between MyISAM and InnoDB row format?

#151How do you implement rate limiting at the database level?

#152What is the difference between REPEATABLE READ and READ COMMITTED in practice?

#153What are MySQL's data types for storing money?

#154How do you handle schema evolution in a microservices environment?

#155Explain the CAP theorem.

#156What is consistent hashing and why is it used?

#157What is database sharding and how does it work?

#158Explain cache invalidation strategies.

#159How would you design a URL shortener (like bit.ly)?

#160What is the Saga pattern in microservices?

#161What is CQRS and when should you use it?

#162What are rate limiting algorithms?

#163What is the Circuit Breaker pattern?

#164How do you design for high availability?

#165What is eventual consistency?

#166What is a message queue and when should you use one?

#167What is the difference between SQL and NoSQL databases?

#168How do you handle distributed transactions without 2PC?

#169How would you design a notification system (push, email, SMS)?

#170What is the difference between REST and gRPC?

#171What is the Outbox Pattern?

#172How do you design a distributed cache layer?

#173What is the difference between synchronous and asynchronous communication in microservices?

#174How do you design a notification system that sends 100M notifications per day?

#175What are the different types of database indexes and when do you use each?

#176How would you design a URL shortener like bit.ly?

#177What is eventual consistency and how do you handle it in practice?

#178How do you design a search system at scale?

#179What is the difference between optimistic and pessimistic locking?

#180How would you design a real-time leaderboard?

#181What is a write-ahead log (WAL) and how is it used?

#182How do you handle distributed transactions across microservices?

#183How do you design a chat application like WhatsApp?

#184What is a service mesh and when should you use one?

#185How would you design a video streaming platform like YouTube?

#186What is the difference between a load balancer and an API gateway?

#187How do you prevent and handle cascading failures in distributed systems?

#188What is the difference between a message queue and a message broker?

#189How do you design a rate limiter for an API?

#190What is the difference between replication and partitioning in databases?

#191How would you design Twitter's news feed?

#192What is observability and what are its three pillars?

#193How do you implement idempotency in APIs?

#194How would you design a distributed job scheduler?

#195What is the Saga pattern vs 2PC for distributed transactions?

#196How do you design a file storage system like Dropbox?

#197What are the trade-offs of microservices vs monolithic architecture?

#198How does DNS work and how is it used in load balancing?

#199What is the strangler fig pattern?

#200How do you design an e-commerce inventory system that handles flash sales?

#201What is the difference between push and pull architectures for data?

#202How do you design a fraud detection system?

#203What is event sourcing and how does it differ from CRUD?

#204How would you design a geospatial service like Uber's nearby driver system?

#205What is API versioning and what are the different strategies?

#206What is a Bloom filter and where is it used in system design?

#207How do you implement zero-downtime deployments?

#208What is the two-generals problem and what does it tell us about distributed systems?

#209How do you design a payment processing system?

#210How would you design an autocomplete system?

#211What is backpressure and how do you handle it?

#212What is a time-series database and when should you use one?

#213What is a circuit breaker and how do you implement it?

#214How do you handle database migrations safely in production?

#215What is the CQRS pattern?

#216How do you secure an API at the architecture level?

#217What is the difference between synchronous replication and asynchronous replication?

#218How do you design a system for billions of events per day (analytics pipeline)?

#219What is chaos engineering and how do you implement it?

#220What is the database connection pool and how do you tune it?

#221How do you design an access control system?

#222How would you design an online code execution system like LeetCode?

#223How does the request flow work in a typical microservices architecture?

#224What is a hot partition problem and how do you solve it?

#225How do you design a multi-tenant SaaS architecture?

#226What is tail latency and why does it matter?

#227How do you design a global, low-latency key-value store?

#228What is the N+1 query problem and how do you fix it?

#229How would you design a booking system like Airbnb?

#230How do you design a recommendation system?

#231What is the read-your-own-writes consistency guarantee?

#232How do you design a search-as-you-type / typeahead feature?

#233What is the difference between gRPC unary, server streaming, client streaming, and bidirectional streaming?

#234How do you handle large file uploads efficiently?

#235What are the key considerations when designing for high write throughput?

#236What is the difference between monolithic and microservices deployment strategies?

#237How do you design for disaster recovery?

#238What is the thundering herd problem?

#239What is a vector clock and how does it work?

#240What is the difference between a monorepo and polyrepo?

#241How do you ensure data consistency across microservices?

#242What is the difference between a batch job and a stream processing job?

#243How do you design an API for mobile clients vs web clients?

#244What is the role of a message broker in event-driven architecture?

#245How do you approach capacity planning for a new system?

#246What is an API Gateway and what does it do?

#247How do microservices communicate?

#248What is service discovery in microservices?

#249What is the Circuit Breaker pattern and how does it prevent cascading failures?

#250What is distributed tracing and why is it important?

#251What is the Strangler Fig pattern for microservices migration?

#252What is a sidecar pattern?

#253How do you manage configuration in microservices?

#254What are the 12-Factor App principles?

#255What is event-driven architecture and when should you use it?

#256What is the difference between orchestration and choreography in Saga?

#257How do you test microservices effectively?

#258What is a service mesh and when should you use one?

#259How do you ensure data consistency across microservices?

#260What is the circuit breaker pattern?

#261What is service discovery and how does it work?

#262What is the Bulkhead pattern?

#263What is the Retry pattern and how do you implement it safely?

#264What is the difference between choreography and orchestration in Sagas?

#265What is the Backend for Frontend (BFF) pattern?

#266How do you implement distributed tracing?

#267What is the Strangler Fig pattern for migrating a monolith?

#268What is eventual consistency and how do you design for it?

#269How do you version microservice APIs?

#270What is the Sidecar pattern?

#271What is a dead letter queue and why is it important?

#272How do you implement health checks in microservices?

#273What is the Outbox pattern and how does it guarantee event delivery?

#274What is idempotency and how do you implement it in microservices?

#275What are the 12-Factor App principles?

#276How do you handle secrets management in microservices?

#277What is the Ambassador pattern in microservices?

#278How do you implement inter-service authentication?

#279What is the Throttling pattern and how does it differ from rate limiting?

#280What is CQRS and how does it apply to microservices?

#281How do you test microservices?

#282What is service isolation and why is it important?

#283What is an Event-Driven Architecture?

#284How do you handle database per service in practice?

#285What is the Role of API composition in microservices?

#286What is graceful degradation in microservices?

#287What is the Event Sourcing pattern?

#288How do you implement logging in a microservices environment?

#289What is the two-phase commit vs Saga for distributed transactions?

#290How do you handle timeouts in microservices?

#291What is the Scatter-Gather pattern?

#292What is the difference between a microservice and a nanoservice?

#293How do you implement canary deployments in microservices?

#294What is a Service Registry?

#295What is blue-green deployment and how does it work?

#296What is the Aggregator pattern?

#297How do microservices handle configuration management?

#298What is the Proxy pattern in microservices?

#299How do you implement distributed locking in microservices?

#300What is Kafka and why is it popular in microservices?

#301How do you manage service dependencies in microservices?

#302What is a bounded context?

#303How do you implement a request-reply pattern over async messaging?

#304What is the Token Bucket algorithm and how is it used for rate limiting?

#305How do you handle schema evolution in event-driven systems?

#306What is the difference between a monolith, microservices, and modular monolith?

#307What is the Choreography pattern in detail?

#308What is a composite microservice?

#309How do you deal with "chatty" microservices?

#310What is a process manager / workflow engine in microservices?

#311How do you implement zero-downtime deployments for microservices?

#312What is the Anti-Corruption Layer pattern?

#313What are the common pitfalls of microservices adoption?

#314What is the difference between orchestration and choreography?

#315How do you implement a consistent read across microservices?

#316What is load shedding?

#317What is the difference between gRPC and REST for inter-service communication?

#318What is the Gateway Aggregation pattern vs Gateway Routing?

#319What is the Inbox pattern?

#320How do you handle versioning of events in Kafka?

#321What is the Open Host Service pattern?

#322How do you implement service-level SLOs in microservices?

#323What is the Claim Check pattern?

#324What is service mesh traffic management?

#325What is the difference between horizontal and vertical scaling for microservices?

#326What is the role of a message schema registry?

#327How do you handle service discovery with Kubernetes?

#328How do you implement an API gateway from scratch?

#329What is consumer-driven contract testing (Pact)?

#330How do you use feature toggles in microservices?

#331What is polyglot persistence in microservices?

#332What are distributed caching strategies for microservices?

#333How do you design a CI/CD pipeline for microservices?

#334How do you implement observability with Micrometer, Prometheus, and Grafana?

#335What is the Retry pattern and when should you NOT retry?

#336How do you implement Saga compensation (rollback) in detail?

#337What is multi-tenancy in microservices and how do you implement it?

#338What are the steps to migrate a monolith to microservices using the Strangler Fig pattern?

#339How do you design microservices for failure?

#340What is data mesh and how does it relate to microservices data ownership?

#341What is a sidecar proxy and how does Envoy work?

#342How do you handle inter-service authorization (not just authentication)?

#343When should you NOT use microservices?