Paytm Interview Questions

#1What is the difference between == and .equals() in Java?

#2What is the difference between abstract class and interface?

#3Explain the SOLID principles.

#4How does HashMap work internally?

#5What is the difference between HashMap and ConcurrentHashMap?

#6Explain Java Memory Model: Heap, Stack, and Metaspace.

#7What is the volatile keyword in Java?

#8What is the difference between synchronized and ReentrantLock?

#9What is a deadlock and how do you prevent it?

#10What are functional interfaces and lambda expressions in Java 8?

#11What is Optional and when should you use it?

#12Explain the Stream API and intermediate vs terminal operations.

#13What is the difference between checked and unchecked exceptions?

#14What is the difference between String, StringBuilder, and StringBuffer?

#15What is garbage collection? How does G1 differ from ZGC?

#16What is method overloading vs method overriding?

#17What is the Java ClassLoader hierarchy?

#18What is the difference between final, finally, and finalize?

#19What are Java records (Java 16+)?

#20What is the Executor framework and when would you use it over raw Threads?

#21What is encapsulation and how is it achieved in Java?

#22What is polymorphism? Explain runtime vs compile-time polymorphism.

#23What is the difference between composition and inheritance?

#24What is a marker interface? Give examples.

#25What are the design patterns most commonly asked in Java interviews?

#26How do you implement a Singleton in Java correctly?

#27What is the difference between ArrayList and LinkedList?

#28What is the difference between HashSet, LinkedHashSet, and TreeSet?

#29What is the difference between HashMap and TreeMap?

#30What is the contract between equals() and hashCode()?

#31What is the difference between fail-fast and fail-safe iterators?

#32What is CopyOnWriteArrayList and when should you use it?

#33How does PriorityQueue work and when would you use it?

#34What is the happens-before relationship in Java Memory Model?

#35What is the difference between sleep(), wait(), and yield()?

#36What are atomic classes (AtomicInteger, AtomicReference)? How do they work?

#37What is a CountDownLatch vs CyclicBarrier?

#38What is a ThreadLocal and what are its dangers?

#39What is a CompletableFuture and how does it improve async programming?

#40What are virtual threads (Project Loom, Java 21)?

#41What is the difference between JDK, JRE, and JVM?

#42How does the JIT (Just-In-Time) compiler work?

#43What is the difference between minor GC, major GC, and full GC?

#44What tools do you use to diagnose JVM memory issues?

#45What is the String pool and how does string interning work?

#46What is type erasure in Java generics?

#47What is the difference between <? extends T> and <? super T>?

#48What are sealed classes (Java 17)?

#49What is pattern matching for instanceof (Java 16)?

#50What are text blocks (Java 15)?

#51What is the difference between map() and flatMap() in streams?

#52What are method references in Java 8?

#53What is the difference between Comparable and Comparator?

#54What is Stream.collect() and what are common Collectors?

#55What is try-with-resources and how does it work?

#56What is the difference between throw and throws?

#57What is exception chaining and why is it important?

#58What is the difference between Java IO and NIO?

#59What is Java Serialization and what are its pitfalls?

#60What is reflection in Java and when is it used?

#61What is the difference between deep copy and shallow copy?

#62What is autoboxing and unboxing? What are the performance implications?

#63What are enums in Java and what advanced features do they support?

#64What is the difference between static and instance initializer blocks?

#65What is the difference between String.format() and String.formatted()?

#66What is the var keyword (Java 10) and when should you use it?

#67What are Java modules (Java 9 — JPMS)?

#68What is the difference between an inner class, static nested class, and anonymous class?

#69What are the new switch expressions (Java 14)?

#70What is the difference between Runnable, Callable, and Future?

#71What is the difference between String.equals() and String.equalsIgnoreCase()?

#72What is the difference between Iterator and ListIterator?

#73What is the difference between abstract class and concrete class?

#74How do you handle NullPointerException in Java?

#75What is the Collections utility class? What are its most important methods?

#76What is the difference between Deque and Queue?

#77What is a WeakHashMap and when would you use it?

#78What is the difference between checked and unchecked exceptions — best practices for APIs?

#79What is the ternary operator and the null-coalescing equivalent in Java?

#80What is the difference between == null check and Objects.isNull()?

#81What is the difference between interface default methods and abstract class methods?

#82What is the difference between Iterable and Iterator?

#83What are the best practices for writing clean Java code?

#84What is the difference between a process and a thread in Java?

#85What is a race condition and how do you prevent it?

#86What is the difference between HashMap.computeIfAbsent() and getOrDefault()?

#87What is the purpose of the transient keyword?

#88What is method hiding in Java?

#89What is the difference between i++ and ++i in Java?

#90What are the Java 8 Date/Time APIs and why were they introduced?

#91What is the difference between HashMap.put() and HashMap.putIfAbsent()?

#92What is the difference between String.valueOf() and toString()?

#93What is an immutable class and how do you create one?

#94What is the covariant return type in Java?

#95What is the difference between static and instance methods?

#96What is the difference between Predicate.and(), Predicate.or(), and Predicate.negate()?

#97What is the Observer pattern and how is it implemented in Java?

#98What is the Strategy pattern in Java?

#99What is String.intern() and when should you use it?

#100What are the key differences between Java 8, 11, 17, and 21?

#101What is auto-configuration in Spring Boot?

#102What is @SpringBootApplication and what does it do?

#103What is IoC (Inversion of Control) and Dependency Injection?

#104What is the difference between @Component, @Service, and @Repository?

#105What is the difference between @Controller and @RestController?

#106Explain @Transactional and its propagation levels.

#107How does Spring Boot handle exceptions globally?

#108What is Spring Boot Actuator?

#109What is the difference between @Bean and @Component?

#110Explain Spring Bean scopes.

#111What is Spring AOP and where is it used?

#112What is circular dependency and how do you fix it?

#113How do you configure Spring profiles?

#114How does Spring Security authentication work?

#115What is @Async in Spring Boot and how does it work?

#116What is Spring Data JPA and how does it simplify data access?

#117How does Spring Boot externalized configuration work?

#118What is Spring Cache Abstraction and how do you use it?

#119What are Spring Boot starters and why are they useful?

#120How do you write integration tests in Spring Boot?

#121What is the Spring ApplicationContext and how does it differ from BeanFactory?

#122What is a BeanPostProcessor and when would you use it?

#123What is the @PostConstruct and @PreDestroy lifecycle?

#124What is @Qualifier and when do you need it?

#125How does Spring Boot auto-configuration work under the hood?

#126What is @ConfigurationProperties and how does it differ from @Value?

#127What is Spring Boot's embedded server and how do you switch from Tomcat to Undertow?

#128What is Spring's @EventListener and ApplicationEvent?

#129What is @Conditional and how do you create custom conditions?

#130What is Spring Boot DevTools?

#131What is the difference between @RequestParam, @PathVariable, and @RequestBody?

#132How do you implement request validation in Spring Boot?

#133What is ResponseEntity and when should you use it?

#134What is content negotiation in Spring MVC?

#135How do you implement pagination and sorting in Spring Boot?

#136What is HATEOAS and does Spring Boot support it?

#137How do you version a REST API in Spring Boot?

#138What is the difference between findById() and getById() in Spring Data JPA?

#139What are JPA entity states (Transient, Managed, Detached, Removed)?

#140What is the difference between @OneToMany with fetch LAZY vs EAGER?

#141What is Hibernate first-level cache and second-level cache?

#142What is the Open Session in View (OSIV) pattern and why is it controversial?

#143What is a Spring Data JPA Specification and when would you use it?

#144What is CSRF and how does Spring Security handle it?

#145How do you implement JWT authentication in Spring Boot?

#146What is the difference between @Secured, @PreAuthorize, and @RolesAllowed?

#147What is CORS and how do you configure it in Spring Boot?

#148What is OAuth2 and how does Spring Security support it?

#149What is MockMvc and how do you use it?

#150What is @MockBean vs @Mock in Spring Boot tests?

#151What is @DataJpaTest and how does it differ from @SpringBootTest?

#152What is Testcontainers and how does it improve Spring Boot tests?

#153What is Micrometer and how does it integrate with Spring Boot Actuator?

#154How do you implement custom health indicators in Spring Boot?

#155How do you implement distributed tracing in Spring Boot?

#156What is graceful shutdown in Spring Boot and how do you configure it?

#157What is a Filter vs HandlerInterceptor in Spring Boot?

#158What is @ControllerAdvice and how does it work?

#159What is Spring WebFlux and how does it differ from Spring MVC?

#160What is Spring's @Scheduled and how do you configure it?

#161What happens during Spring Boot startup?

#162What is CommandLineRunner and ApplicationRunner?

#163What is Spring Boot's banner and how can you customize it?

#164What is Spring Boot's fat JAR and how is it structured?

#165What is Spring Retry and how do you use it?

#166What is Spring Integration vs Spring Batch?

#167What is @Transactional readOnly = true and what does it optimize?

#168What is the difference between Spring @Repository exception translation and JDBC exceptions?

#169How do you handle file uploads in Spring Boot?

#170What is the difference between @RequestMapping and specific mapping annotations?

#171What is Spring Cloud and its key components?

#172What is the difference between @Transactional on a class vs a method?

#173What is Spring Boot's connection pool? How do you tune HikariCP?

#174What is Spring Boot's auto-configuration for messaging (RabbitMQ / Kafka)?

#175What is Spring's @Lookup annotation?

#176What is Spring Security's method security and how is it different from URL security?

#177How does Spring Boot handle environment-specific properties?

#178What is the purpose of @EnableJpaAuditing?

#179How do you implement rate limiting in Spring Boot?

#180What is the difference between eager and lazy initialization in Spring Boot?

#181What is Spring Boot's support for caching with Redis?

#182What is Spring Boot's support for MongoDB?

#183What is @Transactional self-invocation problem and how do you fix it?

#184What is the difference between @Valid and @Validated in Spring?

#185What is Spring Boot's support for scheduled tasks with Quartz?

#186What is Spring Data REST and how does it differ from Spring MVC?

#187What is the difference between @Inject and @Autowired?

#188What is Flyway and how does it integrate with Spring Boot?

#189What is Spring Boot's support for WebSockets?

#190How do you implement a multi-tenant application in Spring Boot?

#191What are Spring Boot's management endpoints and how do you secure them?

#192What is Spring Boot's support for R2DBC (reactive database access)?

#193How do you implement idempotency in Spring Boot APIs?

#194What is Spring Boot's support for virtual threads (Java 21)?

#195What is Spring Boot's Docker image building support?

#196What is GraalVM native image and Spring Boot AOT support?

#197What is the difference between @SpringBootTest(webEnvironment) options?

#198What is Spring Boot's support for SSE (Server-Sent Events)?

#199What is Resilience4j and how does it integrate with Spring Boot?

#200What are best practices for Spring Boot application performance tuning?

#201What are the different types of SQL JOINs?

#202How does indexing work in MySQL (InnoDB)?

#203What is a covering index?

#204What are MySQL transaction isolation levels?

#205What is MVCC (Multi-Version Concurrency Control) in InnoDB?

#206How do you use EXPLAIN to analyze a query?

#207What are window functions in SQL?

#208What are CTEs (Common Table Expressions) and when should you use them?

#209What is the N+1 query problem and how do you fix it?

#210What is a deadlock in MySQL and how does it occur?

#211What is the difference between DELETE, TRUNCATE, and DROP?

#212How do you optimize a slow MySQL query?

#213What is database normalization?

#214What is MySQL replication and how does it work?

#215What is partitioning in MySQL?

#216What is GROUP BY vs HAVING in SQL?

#217What is a stored procedure and when should you use it?

#218How does the InnoDB storage engine handle locking?

#219What is the difference between UNION and UNION ALL?

#220How do you design a schema for a multi-tenant SaaS application?

#221What is the difference between a clustered and a non-clustered index?

#222How does MySQL handle AUTO_INCREMENT?

#223What are covering indexes and how do they improve performance?

#224What is the EXPLAIN output and how do you read it?

#225What are database triggers and when should you avoid them?

#226What is a foreign key constraint and what are its performance implications?

#227What is the difference between CHAR and VARCHAR?

#228How do you implement full-text search in MySQL?

#229What is the difference between MyISAM and InnoDB?

#230How does MySQL replication work and what are its modes?

#231What is a deadlock in MySQL and how does InnoDB handle it?

#232What are MySQL stored procedures and when should you use them?

#233How do you perform a slow query analysis in MySQL?

#234What is InnoDB buffer pool and how do you size it?

#235What is the difference between HAVING and WHERE?

#236What is MySQL's query cache and why was it removed?

#237How do you back up a MySQL database without downtime?

#238What is the difference between optimistic and pessimistic locking in MySQL?

#239What are MySQL partitioning types and when do you use them?

#240How do you handle large data migrations in production?

#241What is the difference between IN, EXISTS, and JOIN for subqueries?

#242What is the InnoDB redo log and undo log?

#243How do you implement pagination in MySQL efficiently?

#244What are the different types of JOINs in MySQL?

#245What is group replication and InnoDB Cluster?

#246What is the difference between a view and a materialized view?

#247How does MySQL's optimizer choose an execution plan?

#248What are common MySQL performance anti-patterns?

#249What is a generated column in MySQL?

#250What are the best practices for MySQL schema design?

#251How do you use MySQL's JSON column type?

#252What is the difference between transaction isolation levels in MySQL?

#253How do you handle hierarchical data in MySQL?

#254What is ProxySQL and how is it used?

#255What is MVCC and how does InnoDB implement it?

#256How do you detect and fix index fragmentation in MySQL?

#257What are the MySQL 8.0 improvements over MySQL 5.7?

#258How do you design a schema for audit logging?

#259What is binlog and how is it used for CDC?

#260How do you use MySQL's performance_schema?

#261What is the difference between COUNT(*), COUNT(1), and COUNT(col)?

#262What is a composite index and how should you order columns?

#263How do you implement soft delete in MySQL?

#264What is the difference between CHAR, VARCHAR, TEXT, and BLOB?

#265How does MySQL handle concurrent writes to the same row?

#266What is the difference between schema-on-write and schema-on-read?

#267How do you handle time zones in MySQL?

#268What is the difference between a unique constraint and a unique index?

#269How do you monitor MySQL in production?

#270What are window functions and how do you use them?

#271How do you implement row-level security in MySQL?

#272What is the difference between a primary key and a unique key?

#273How do you use MySQL's event scheduler?

#274What is the EXPLAIN ANALYZE command?

#275How do you handle database connection failures in application code?

#276What is the MySQL query execution order?

#277How do you scale MySQL reads?

#278What is the difference between row-based and statement-based replication?

#279How do you use CTEs (Common Table Expressions) effectively?

#280What is the difference between TRUNCATE, DELETE, and DROP?

#281How do you implement optimistic locking with versioning in MySQL?

#282What are the different ways to copy a table in MySQL?

#283What is the purpose of the information_schema?

#284What is innodb_flush_log_at_trx_commit and how does it affect durability?

#285How do you implement full-table search across millions of rows efficiently?

#286What is the max_connections parameter and how do you tune it?

#287What is a spatial index in MySQL and how is it used?

#288What is the difference between ROLLUP and CUBE in SQL?

#289How do you handle NULL values in MySQL queries?

#290What is the difference between a read and a write transaction?

#291How do you implement a queue in MySQL?

#292What is the mysql.user table and how does MySQL authentication work?

#293What is index merge optimization?

#294How do you detect and resolve replication lag?

#295What is the difference between MyISAM and InnoDB row format?

#296How do you implement rate limiting at the database level?

#297What is the difference between REPEATABLE READ and READ COMMITTED in practice?

#298What are MySQL's data types for storing money?

#299How do you handle schema evolution in a microservices environment?

#300What is the difference between DATETIME and TIMESTAMP in MySQL?

#301What is the difference between horizontal and vertical scaling?

#302Explain the CAP theorem.

#303What is consistent hashing and why is it used?

#304What is database sharding and how does it work?

#305Explain cache invalidation strategies.

#306How would you design a URL shortener (like bit.ly)?

#307What is the Saga pattern in microservices?

#308What is CQRS and when should you use it?

#309What are rate limiting algorithms?

#310What is the Circuit Breaker pattern?

#311How do you design for high availability?

#312What is eventual consistency?

#313What is a message queue and when should you use one?

#314What is the difference between SQL and NoSQL databases?

#315What is a CDN and how does it improve performance?

#316How do you handle distributed transactions without 2PC?

#317How would you design a notification system (push, email, SMS)?

#318What is the difference between REST and gRPC?

#319What is the Outbox Pattern?

#320How do you design a distributed cache layer?

#321What is the difference between synchronous and asynchronous communication in microservices?

#322How do you design a notification system that sends 100M notifications per day?

#323What are the different types of database indexes and when do you use each?

#324How would you design a URL shortener like bit.ly?

#325What is eventual consistency and how do you handle it in practice?

#326How do you design a search system at scale?

#327What is the difference between optimistic and pessimistic locking?

#328How would you design a real-time leaderboard?

#329What is a write-ahead log (WAL) and how is it used?

#330How do you handle distributed transactions across microservices?

#331How do you design a chat application like WhatsApp?

#332What is a service mesh and when should you use one?

#333How would you design a video streaming platform like YouTube?

#334What is the difference between a load balancer and an API gateway?

#335How do you prevent and handle cascading failures in distributed systems?

#336What is the difference between a message queue and a message broker?

#337How do you design a rate limiter for an API?

#338What is the difference between replication and partitioning in databases?

#339How would you design Twitter's news feed?

#340What is observability and what are its three pillars?

#341What is the difference between SQL and NoSQL databases?

#342How do you implement idempotency in APIs?

#343How would you design a distributed job scheduler?

#344What is the Saga pattern vs 2PC for distributed transactions?

#345How do you design a file storage system like Dropbox?

#346What are the trade-offs of microservices vs monolithic architecture?