Best Java Interview Questions 2025

A Java Backend Developer designs, develops, and maintains server-side logic, database interactions, and application architecture using Java technologies.

JDK (Java Development Kit) is a software development environment, JRE (Java Runtime Environment) is required to run Java applications, and JVM (Java Virtual Machine) is an abstract machine that provides runtime environment for Java bytecode.

Java is platform-independent, object-oriented, strongly typed, secure, distributed, multithreaded, and supports automatic memory management through garbage collection.

Java source code (.java) is compiled into bytecode (.class) by the Java compiler, which can then be executed by the JVM on any platform.

Java follows 'Write Once, Run Anywhere' principle, where compiled bytecode can run on any device with a JVM, regardless of the underlying hardware or operating system.

Main components include server-side logic, database interactions, API development, application architecture, security implementation, and performance optimization.

Java SE (Standard Edition) is the core Java platform, while Java EE (Enterprise Edition) provides additional features for enterprise-level application development, such as distributed computing and web services.

Application servers like Tomcat, JBoss, and WebSphere provide runtime environments for Java web applications, managing resources, handling requests, and supporting enterprise features.

Primary data storage options include relational databases (MySQL, PostgreSQL), NoSQL databases (MongoDB, Cassandra), and in-memory databases (Redis, Hazelcast).

Java supports concurrent programming through the java.util.concurrent package, thread synchronization, executor frameworks, atomic variables, and advanced concurrency utilities.

The four principles are Encapsulation (data hiding), Inheritance (creating new classes based on existing classes), Polymorphism (multiple forms of methods), and Abstraction (simplifying complex systems).

Abstract classes can have default method implementations and state, while interfaces (before Java 8) could only declare method signatures. Interfaces support multiple inheritance, while abstract classes do not.

Java supports single inheritance, multilevel inheritance, hierarchical inheritance, and interface-based inheritance. Multiple inheritance of classes is not supported directly.

Java has four access modifiers: public (accessible everywhere), protected (accessible within package and subclasses), default (package-private), and private (accessible only within the same class).

Method overloading occurs within a class with different parameter lists, while method overriding happens in inheritance where a subclass provides a specific implementation of a method defined in its superclass.

The 'final' keyword can be used with variables (constant), methods (cannot be overridden), and classes (cannot be inherited).

Java uses try-catch-finally blocks for exception handling. Exceptions are objects that represent error conditions, and they can be caught and handled to prevent program termination.

Checked exceptions are compiled-time exceptions that must be explicitly handled, while unchecked exceptions (runtime exceptions) do not require explicit handling.

Generics allow creation of classes, interfaces, and methods that can work with different data types while providing compile-time type safety.

Lambda expressions are anonymous functions that can be used to implement functional interfaces, enabling functional programming techniques in Java.

J2EE (Java 2 Enterprise Edition) is a platform for developing multi-tier enterprise applications, consisting of components like servlets, JSP, EJB, JPA, and web services.

Servlet lifecycle includes loading, initialization (init()), handling requests (service()), and destruction (destroy()). The web container manages this lifecycle.

forward() transfers request to another resource on the server-side without client involvement, while sendRedirect() creates a new request from the client to a different URL.

JSP is a server-side technology for creating dynamic web content, allowing Java code to be embedded in HTML. It simplifies web application development by separating presentation from business logic.

Session management techniques include HttpSession, cookies, URL rewriting, and hidden form fields to maintain user state across multiple requests.

JDBC (Java Database Connectivity) provides a standard interface for connecting to and executing queries on different databases, supporting database-independent code.

EJB types include Session Beans (stateless and stateful), Entity Beans (deprecated), and Message-Driven Beans for asynchronous processing.

Dependency injection is a design pattern where objects receive their dependencies from an external source rather than creating them internally, promoting loose coupling.

JNDI (Java Naming and Directory Interface) provides a unified interface for accessing different naming and directory services, enabling resource lookup and management.

Filters intercept and modify requests/responses, while listeners monitor lifecycle events of web objects like sessions, requests, and applications.

Spring is a comprehensive Java framework for enterprise application development, providing dependency injection, aspect-oriented programming, MVC architecture, and simplified configuration.

IoC is a principle where the framework controls object creation and lifecycle, with dependencies injected automatically, reducing tight coupling between components.

Spring supports constructor injection, setter injection, and field injection for managing object dependencies and promoting loose coupling.

Spring Beans have lifecycle methods like init() and destroy(). Scopes include singleton (default), prototype, request, session, and application.

AOP allows separating cross-cutting concerns like logging, security, and transaction management from business logic by creating modular aspects.

@Component is a generic stereotype, @Service is for service layer, and @Repository is for database access, each with specific semantic meaning.

Spring MVC is a model-view-controller framework for building web applications, with DispatcherServlet handling request routing, controllers processing requests, and views rendering response.

Spring provides declarative transaction management using @Transactional annotation, supporting programmatic and declarative transaction strategies.

Spring profiles allow configuring different beans and properties for different environments like development, testing, and production.

@Autowired automatically injects dependencies by type, reducing boilerplate code for dependency management and supporting loose coupling.

Spring Boot is a framework for creating standalone, production-grade Spring applications with auto-configuration, embedded servers, and minimal configuration.

Microservices is an architectural style where applications are composed of small, independent services that communicate via APIs, enabling scalability and flexibility.

Key components include service registry, load balancing, circuit breaker, configuration server, API gateway, and distributed tracing.

Spring Boot provides Netflix OSS libraries, Spring Cloud components, and starter dependencies for implementing microservices patterns.

Service discovery allows services to dynamically discover and communicate with each other using tools like Eureka, Consul, or Kubernetes service discovery.

API Gateway acts as a single entry point for client requests, handling routing, load balancing, authentication, and transforming requests/responses.

Circuit breaker prevents cascading failures by detecting and handling service failures, providing fallback mechanisms and improving system resilience.

Containerization uses technologies like Docker to package services with dependencies, ensuring consistent deployment across different environments.

Inter-service communication can be achieved through REST APIs, message queues (RabbitMQ, Kafka), and synchronous/asynchronous communication patterns.

Challenges include service coordination, data consistency, network latency, monitoring, debugging, and managing distributed system complexity.

Hibernate is an ORM tool that maps Java objects to database tables, simplifying database interaction by handling SQL generation and query execution.

Hibernate supports one-to-one, one-to-many, many-to-one, and many-to-many associations with various mapping strategies and fetch types.

Common JPA annotations include @Entity, @Table, @Column, @Id, @GeneratedValue, @OneToMany, @ManyToOne for defining object-relational mappings.

Hibernate sessions manage database connections, while transactions ensure data integrity by providing ACID (Atomicity, Consistency, Isolation, Durability) properties.

Lazy loading defers object initialization until accessed, while eager loading retrieves related objects immediately, affecting performance and memory usage.

Hibernate provides first-level (session), second-level (SessionFactory), and query cache to improve application performance by reducing database round trips.

HQL is an object-oriented query language similar to SQL, while Criteria API provides a type-safe, programmatic way to create database queries.

Database migrations can be managed using tools like Flyway or Liquibase, which version control database schema changes alongside application code.

Hibernate supports table per class hierarchy, table per subclass, and table per concrete class strategies for mapping inheritance relationships.

Best practices include using appropriate fetch types, limiting result sets, utilizing caching, batch processing, and using native SQL when necessary.

REST (Representational State Transfer) is an architectural style for designing networked applications, emphasizing stateless communication, uniform interface, and resource-based interactions.

HTTP methods include GET (retrieve), POST (create), PUT (update), DELETE (remove), PATCH (partial update), with each method having a specific semantic meaning.

REST is lightweight, uses HTTP methods, and supports multiple data formats, while SOAP is protocol-specific, XML-based, and more complex with strict standards.

API versioning can be implemented through URL path, query parameters, custom headers, or content negotiation to maintain backward compatibility.

Best practices include using nouns for resources, meaningful HTTP status codes, consistent error handling, pagination, filtering, and comprehensive documentation.

API documentation provides details about endpoints, request/response formats, authentication, and usage examples, crucial for developer understanding and adoption.

Spring Boot provides annotations like @RestController, @GetMapping for creating REST endpoints, with built-in support for serialization, content negotiation, and validation.

Authentication methods include JWT (JSON Web Tokens), OAuth 2.0, API keys, and basic authentication, with focus on securing endpoints and managing user access.

API testing includes unit testing, integration testing, performance testing, security testing, and contract testing to ensure reliability and functionality.

Rate limiting controls the number of API requests a client can make within a specific time frame, preventing abuse and ensuring fair resource allocation.

Key principles include defense in depth, least privilege, input validation, secure defaults, principle of complete mediation, and separation of privileges.

Spring Security provides comprehensive authentication, authorization, and protection against common security vulnerabilities like CSRF, XSS, and SQL injection.

JWT is a compact, URL-safe means of representing claims between two parties, commonly used for stateless authentication and information exchange.

OAuth 2.0 is an authorization framework supporting various grant types like authorization code, implicit, resource owner password, and client credentials.

Protection methods include input validation, parameterized queries, HTTPS, CSRF tokens, XSS prevention, secure password storage, and proper error handling.

Password hashing converts passwords to fixed-size strings using one-way algorithms, while salting adds random data to enhance security against rainbow table attacks.

RBAC manages user permissions by assigning roles with specific access rights, simplifying access management and improving security granularity.

Best practices include using secure, HttpOnly cookies, implementing proper logout, session timeout, token rotation, and protecting against session fixation.

Two-factor authentication adds an extra layer of security by requiring a second verification method beyond passwords, like SMS codes or authenticator apps.

Secure configuration involves using environment-specific properties, encrypting sensitive data, using secure configuration servers, and avoiding hardcoded credentials.

Techniques include efficient data structures, database query optimization, caching, connection pooling, lazy loading, and minimizing object creation.

Java automatically manages memory through garbage collection, with different collection algorithms like Serial, Parallel, CMS, and G1 for memory optimization.

Performance profiling involves using tools like JProfiler, VisualVM, JConsole, and monitoring JVM metrics, thread states, and memory usage.

Connection pooling reuses database connections, reducing overhead of creating new connections, improving performance, and managing resources efficiently.

Best practices include using immutable objects, avoiding unnecessary object creation, minimizing synchronization, leveraging streams, and choosing appropriate data structures.

Query optimization involves using indexes, avoiding N+1 queries, minimizing joins, denormalizing data, and using database-specific performance techniques.

Caching strategies include in-memory caches (Ehcache, Redis), distributed caching, cache-aside pattern, and considering cache invalidation and eviction policies.

Best practices include using appropriate thread pool sizes, avoiding thread creation overhead, using executor services, and minimizing shared mutable state.

Considerations include service granularity, efficient communication protocols, circuit breakers, bulk-heading, asynchronous processing, and service mesh technologies.

Reactive programming enables non-blocking, event-driven architectures, improving scalability and resource utilization through async processing and back-pressure mechanisms.