Core Java

Core Java Questions

Q1.What is the difference between HashMap and ConcurrentHashMap?+

HashMap is not thread-safe — concurrent modification causes ConcurrentModificationException. ConcurrentHashMap uses segment-level locking (Java 7) or CAS operations with node-level locking (Java 8+) for thread safety. ConcurrentHashMap does not allow null keys/values. For single-threaded code, HashMap is faster. For multi-threaded code, always use ConcurrentHashMap.

Q2.Explain Java Streams and when to use them.+

Streams process collections declaratively with operations like filter, map, reduce, collect. They support lazy evaluation (operations chain without executing until a terminal operation), parallel processing (parallelStream()), and functional style. Use for: transforming collections, filtering data, aggregating values. Avoid for: simple iterations, operations with side effects, or when index access is needed.

Q3.What is the difference between abstract class and interface in Java 8+?+

Since Java 8, interfaces can have default methods (with implementation) and static methods. Key differences: a class can implement multiple interfaces but extend only one abstract class. Abstract classes can have constructors, instance variables, and any access modifier. Interfaces have public abstract methods and public default methods. Use interfaces for contracts; abstract classes for shared implementation with state.

Q4.How does Java garbage collection work?+

JVM automatically reclaims memory from unreachable objects. Generations: Young (Eden + Survivor spaces) for short-lived objects, Old for long-lived objects. Minor GC collects Young generation. Major/Full GC collects both. GC algorithms: G1 (default, balanced), ZGC (low latency), Shenandoah (concurrent). Avoid creating unnecessary objects, prefer object pooling for expensive objects, and tune GC for your workload.

Q5.Explain the Optional class in Java and when to use it.+

Optional is a container that may or may not contain a value. Use instead of returning null to prevent NullPointerException. Methods: Optional.of(value), Optional.ofNullable(value), isEmpty(), orElse(default), orElseThrow(), map(), flatMap(). Use for return types, not for method parameters or class fields. It forces callers to explicitly handle the absence case.

Spring Boot

Spring Boot Questions

Q6.What is Spring Boot auto-configuration and how does it work?+

Auto-configuration automatically configures Spring beans based on classpath dependencies. If spring-boot-starter-web is on classpath, Spring Boot configures an embedded Tomcat, DispatcherServlet, and JSON converters. It uses @Conditional annotations — beans are created only if conditions are met (class exists, property set). Override auto-config by defining your own beans or using application.properties.

Q7.Explain dependency injection in Spring and its types.+

DI provides dependencies externally instead of components creating them. Constructor injection: dependencies in constructor (recommended — immutable, required dependencies clear). Setter injection: via setter methods (optional dependencies). Field injection: @Autowired on fields (not recommended — hard to test, hides dependencies). Spring resolves dependencies from the IoC container using @Autowired or constructor parameters.

Q8.What is the difference between @Component, @Service, @Repository, and @Controller?+

All are specializations of @Component that mark classes as Spring-managed beans. @Controller/@RestController: handles HTTP requests. @Service: business logic layer (no special behavior, semantic marker). @Repository: data access layer, translates persistence exceptions to Spring DataAccessException. Use the appropriate annotation for clarity — Spring treats them identically for component scanning.

Q9.How does Spring Data JPA simplify database operations?+

Spring Data JPA generates implementations of repository interfaces at runtime. Define an interface extending JpaRepository<Entity, ID> and Spring provides CRUD methods automatically. Custom queries: use method names (findByEmailAndStatus), @Query with JPQL/native SQL, or Specifications for dynamic queries. Pagination and sorting are built-in. This eliminates most boilerplate DAO code.

Q10.Explain Spring Security authentication flow.+

Request hits the SecurityFilterChain. AuthenticationFilter extracts credentials, creates an Authentication token. AuthenticationManager delegates to AuthenticationProvider(s). Provider uses UserDetailsService to load user and PasswordEncoder to verify password. On success, SecurityContextHolder stores the authenticated principal. For JWT: a custom filter validates the token and sets the security context on each request.

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Project / Practical

Project / Practical Questions

Q11.How would you implement a REST API with JWT authentication in Spring Boot?+

Add spring-boot-starter-security and a JWT library (jjwt). Create a JwtUtil for token generation/validation. Implement UserDetailsService to load users from the database. Create a JwtAuthenticationFilter that extracts JWT from Authorization header, validates it, and sets SecurityContext. Configure SecurityFilterChain to permit /auth/** and protect other endpoints. Return JWT on successful login.

Q12.How do you handle transactions in Spring Boot?+

Use @Transactional annotation on service methods. Spring creates a proxy that begins a transaction before the method and commits on success or rolls back on RuntimeException. Configure propagation (REQUIRED, REQUIRES_NEW, NESTED) and isolation levels. For read-only operations, use @Transactional(readOnly = true) for optimization. Test transactions with @Rollback in integration tests.

Q13.Describe how you would implement pagination and sorting in a Spring Boot API.+

Use Spring Data's Pageable parameter in repository methods. Controller accepts page, size, and sort query parameters — Spring auto-creates a Pageable. Repository: Page<User> findByStatus(String status, Pageable pageable). Return Page object which includes content, totalElements, totalPages, and navigation info. Frontend uses this for paginated tables and infinite scroll.

Problem Solving

Problem Solving Questions

Q16.How would you design a microservices architecture for an e-commerce platform?+

Services: User Service (auth, profiles), Product Service (catalog, inventory), Order Service (order management), Payment Service (payment processing), Notification Service (email, SMS). Communication: synchronous (REST/gRPC) for queries, asynchronous (Kafka/RabbitMQ) for events. API Gateway (Spring Cloud Gateway) for routing. Service discovery with Eureka. Circuit breaker with Resilience4j for fault tolerance.

Q17.How would you optimize a slow Spring Boot API endpoint?+

Profile: use Spring Boot Actuator and Micrometer for metrics. Check: N+1 queries (use @EntityGraph or JOIN FETCH), missing database indexes, large result sets (add pagination), slow external calls (add caching, async processing). Solutions: add @Cacheable for expensive operations, move heavy work to @Async methods or message queues, optimize JPA queries, and use connection pooling (HikariCP is default).

Frequently Asked Questions

Common Questions

What topics are asked in Java Spring Boot interviews?+

Core Java (OOP, collections, streams, concurrency), Spring Boot fundamentals, dependency injection, JPA/Hibernate, REST APIs, Spring Security, and microservices basics. Senior roles add system design and distributed systems.

Does CodingClave prepare students for Java Spring Boot interviews?+

Yes. The 6-month Java Spring Boot program at CodingClave in Lucknow includes mock interviews targeting enterprise companies that hire Java developers.

Are Java interviews harder than JavaScript interviews?+

Java interviews tend to be more rigorous on OOP and design patterns. Enterprise companies ask deeper theoretical questions. However, JavaScript interviews at startups may have harder DSA rounds.

What salary can I expect after clearing a Java Spring Boot interview?+

₹3.5-8 LPA for freshers. In Lucknow, ₹4-6.5 LPA. Enterprise companies like TCS, Infosys, and banks offer ₹4-7 LPA with good benefits.

Should I prepare DSA for Java interviews?+

Essential. Java interviews at product companies and service firms include DSA rounds. Practice in Java specifically — collections and streams knowledge helps with efficient solutions.

How important are design patterns for Java interviews?+

Important. Singleton, Factory, Observer, Strategy, and Builder patterns are commonly asked. Spring framework itself uses many patterns (Singleton beans, Template method, Proxy for AOP).

Is microservices knowledge required for fresher Java interviews?+

Basic understanding is increasingly expected. Know what microservices are, why they are used, and Spring Cloud basics. Deep microservices knowledge is more important for mid-level roles.

Can Java Spring Boot interview prep help for other Java frameworks?+

Yes. Core Java and Spring fundamentals transfer to other frameworks (Quarkus, Micronaut, Jakarta EE). The design patterns and OOP concepts are universal across Java ecosystem.

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Java Spring Boot Interview Questions and Answers 2026

Top 15 Java Spring Boot interview questions with answers. Core Java, Spring, microservices, and JPA questions for freshers in India.